Solar System

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This article is about the Sun and its planetary system. For other systems, see planetary system and star system.

Planets and dwarf planets of the Solar System. Sizes are to scale, but relative distances from the Sun are not.

The Solar System[a] consists of the Sun and those celestial objects bound to it by gravity, all of which were formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. Of the many objects that orbit the Sun, most of the mass is contained within eight relatively solitary planets[e] whose orbits are almost circular and lie within a nearly flat disc called the ecliptic plane. The four smaller inner planets, Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, the gas giants, are substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are composed largely of ices, such as water, ammonia and methane, and are often referred to separately as "ice giants".

The Solar System is also home to two regions populated by smaller objects. The asteroid belt, which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed mainly of rock and metal. Beyond Neptune’s orbit lie trans-Neptunian objects composed mostly of ices such as water, ammonia and methane. Within these two regions, five individual objects, Ceres, Pluto, Haumea, Makemake and Eris, are recognized to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets.[e] In addition to thousands of small bodies[e] in those two regions, various other small body populations, such as comets, centaurs and interplanetary dust, freely travel between regions.

The solar wind, a flow of plasma from the Sun, creates a bubble in the interstellar medium known as the heliosphere, which extends out to the edge of the scattered disc. The hypothetical Oort cloud, which acts as the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere.

Six of the planets and three of the dwarf planets are orbited by natural satellites,[b] usually termed "moons" after Earth’s Moon. Each of the outer planets is encircled by planetary rings of dust and other particles.

Discovery and exploration

Main article: Discovery and exploration of the Solar System

For many thousands of years, humanity, with a few notable exceptions, did not recognize the existence of the Solar System. People believed the Earth to be stationary at the center of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos,[1] Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system. His 17th-century successors, Galileo Galilei, Johannes Kepler and Isaac Newton, developed an understanding of physics which led to the gradual acceptance of the idea that the Earth moves around the Sun and that the planets are governed by the same physical laws that governed the Earth. In more recent times, improvements in the telescope and the use of unmanned spacecraft have enabled the investigation of geological phenomena such as mountains and craters, and seasonal meteorological phenomena such asclouds, dust storms and ice caps on the other planets.


The orbits of the bodies in the Solar System to scale (clockwise from top left)

The principal component of the Solar System is the Sun, a main sequence G2 star that contains 99.86 percent of the system’s known mass and dominates it gravitationally.[2] The Sun’s four largest orbiting bodies, the gas giants, account for 99 percent of the remaining mass, with Jupiter and Saturn together comprising more than 90 percent.[c]

Most large objects in orbit around the Sun lie near the plane of Earth’s orbit, known as the ecliptic. The planets are very close to the ecliptic while comets and Kuiper belt objects are frequently at significantly greater angles to it.[3][4] All the planets and most other objects also orbit with the Sun’s rotation (counter-clockwise, as viewed from above the Sun’s north pole). There are exceptions, such as Halley’s Comet.

The overall structure of the charted regions of the Solar System consists of the Sun, four relatively small inner planets surrounded by a belt of rocky asteroids, and four gas giants surrounded by the outer Kuiper belt of icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the main asteroid belt. The outer Solar System is beyond the asteroids, including the four gas giant planets.[5] Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune.[6]

Kepler’s laws of planetary motion describe the orbits of objects about the Sun. According to Kepler’s laws, each object travels along an ellipse with the Sun at one focus. Objects closer to the Sun (with smaller semi-major axes) travel more quickly, as they are more affected by the Sun’s gravity. On an elliptical orbit, a body’s distance from the Sun varies over the course of its year. A body’s closest approach to the Sun is called its perihelion, while its most distant point from the Sun is called its aphelion. The orbits of the planets are nearly circular, but many comets, asteroids and Kuiper belt objects follow highly elliptical orbits.

Due to the vast distances involved, many representations of the Solar System show orbits the same distance apart. In reality, with a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between it and the previous orbit. For example, Venus is approximately 0.33 astronomical units (AU)[d] farther out from the Sun than Mercury, while Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a correlation between these orbital distances (for example, the Titius-Bode law),[7] but no such theory has been accepted.

Most of the planets in the Solar System possess secondary systems of their own, being orbited by planetary objects called natural satellites, or moons (two of which are larger than the planet Mercury), or, in the case of the four gas giants, by planetary rings; thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are insynchronous rotation, with one face permanently turned toward their parent.

The objects of the inner Solar System are composed mostly of rock,[8] the collective name for compounds with high melting points, such as silicates, iron or nickel, that remained solid under almost all conditions in the protoplanetary nebula.[9][9] Jupiter and Saturn are composed mainly of gases, the astronomical term for materials with extremely low melting points and high vapor pressure such as molecular hydrogen, helium, and neon, which were always in the gaseous phase in the nebula.[9] Ices, like water, methane, ammonia, hydrogen sulfide and carbon dioxide,[8] have melting points up to a few hundred kelvins, while their phase depends on the ambient pressure and temperature.[9] They can be found as ices, liquids, or gases in various places in the Solar System, while in the nebula they were either in the solid or gaseous phase.[9] Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "ice giants") and the numerous small objects that lie beyond Neptune’s orbit.[8][10] Together, gases and ices are referred to as volatiles.[11]


Main article: Sun

A transit of Venus

The Sun is the Solar System’s star, and by far its chief component. Its large mass (332,900 Earth masses)[12] produces temperatures and densities in its core great enough to sustain nuclear fusion,[13] which releases enormous amounts of energy, mostly radiated into space as electromagnetic radiation, peaking in the 400–to–700 nm band we call visible light.[14]

The Sun is classified as a type G2 yellow dwarf, but this name is misleading as, compared to the majority of stars in our galaxy, the Sun is rather large and bright.[15] Stars are classified by the Hertzsprung–Russell diagram, a graph which plots the brightness of stars with their surface temperatures. Generally, hotter stars are brighter. Stars following this pattern are said to be on the main sequence, and the Sun lies right in the middle of it. However, stars brighter and hotter than the Sun are rare, while substantially dimmer and cooler stars, known as red dwarfs, are common, making up 85 percent of the stars in the galaxy.[15][16]

It is believed that the Sun’s position on the main sequence puts it in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion. The Sun is growing brighter; early in its history it was 70 percent as bright as it is today.[17]

The Sun is a population I star; it was born in the later stages of the universe’s evolution, and thus contains more elements heavier than hydrogen and helium ("metals" in astronomical parlance) than older population II stars.[18] Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, while stars born later have more. This high metallicity is thought to have been crucial to the Sun’s developing a planetary system, because planets form from accretion of "metals".[19]

The heliospheric current sheet.

Interplanetary medium

Main article: Interplanetary medium

Along with light, the Sun radiates a continuous stream of charged particles (a plasma) known as the solar wind. This stream of particles spreads outwards at roughly 1.5 million kilometres per hour,[20] creating a tenuous atmosphere (the heliosphere) that permeates the Solar System out to at least 100 AU (see heliopause).[21] This is known as the interplanetary medium. Geomagnetic storms on the Sun’s surface, such assolar flares and coronal mass ejections, disturb the heliosphere, creating space weather.[22] The largest structure within the heliosphere is the heliospheric current sheet, a spiral form created by the actions of the Sun’s rotating magnetic field on the interplanetary medium.[23][24]

Earth’s magnetic field stops its atmosphere from being stripped away by the solar wind. Venus and Mars do not have magnetic fields, and as a result, the solar wind causes their atmospheres to gradually bleed away into space.[25] Coronal mass ejections and similar events blow magnetic field and huge quantities of material from the surface of the Sun. The interaction of this magnetic field and material with Earth’s magnetic field funnels charged particles into the Earth’s upper atmosphere, where its interactions create aurorae seen near the magnetic poles.

Cosmic rays originate outside the Solar System. The heliosphere partially shields the Solar System, and planetary magnetic fields (for those planets that have them) also provide some protection. The density of cosmic rays in the interstellar medium and the strength of the Sun’s magnetic field change on very long timescales, so the level of cosmic radiation in the Solar System varies, though by how much is unknown.[26]

The interplanetary medium is home to at least two disc-like regions of cosmic dust. The first, the zodiacal dust cloud, lies in the inner Solar System and causes zodiacal light. It was likely formed by collisions within the asteroid belt brought on by interactions with the planets.[27] The second extends from about 10 AU to about 40 AU, and was probably created by similar collisions within the Kuiper belt.[28][29]

Inner Solar System

The inner Solar System is the traditional name for the region comprising the terrestrial planets and asteroids.[30] Composed mainly of silicates and metals, the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is shorter than the distance between Jupiter and Saturn.

Inner planets

Main article: Terrestrial planet

The inner planets. From left to right:Mercury, Venus, Earth, and Mars (sizes to scale, interplanetary distances not)

The four inner or terrestrial planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of refractory minerals, such as the silicates which form their crusts andmantles, and metals such as iron and nickel, which form their cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather; all have impact craters andtectonic surface features such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets which are closer to the Sun than Earth is (i.e. Mercury and Venus).

Mercury (0.4 AU from the Sun) is the closest planet to the Sun and the smallest planet in the Solar System (0.055 Earth masses). Mercury has no natural satellites, and its only known geological features besides impact craters are lobed ridges or rupes, probably produced by a period of contraction early in its history.[31] Mercury’s almost negligible atmosphere consists of atoms blasted off its surface by the solar wind.[32] Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the young Sun’s energy.[33][34]
Venus (0.7 AU from the Sun) is close in size to Earth, (0.815 Earth masses) and like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere and evidence of internal geological activity. However, it is much drier than Earth and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C, most likely due to the amount of greenhouse gases in the atmosphere.[35] No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is regularly replenished by volcanic eruptions.[36]
Earth (1 AU from the Sun) is the largest and densest of the inner planets, the only one known to have current geological activity, and is the only place in the universe where life is known to exist.[37] Its liquid hydrosphere is unique among the terrestrial planets, and it is also the only planet where plate tectonics has been observed. Earth’s atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen.[38] It has one natural satellite, the Moon, the only large satellite of a terrestrial planet in the Solar System.
Mars (1.5 AU from the Sun) is smaller than Earth and Venus (0.107 Earth masses). It possesses an atmosphere of mostly carbon dioxide with a surface pressure of 6.1 millibars (roughly 0.6 percent that of the Earth’s).[39] Its surface, peppered with vast volcanoes such as Olympus Mons and rift valleys such as Valles Marineris, shows geological activity that may have persisted until as recently as 2 million years ago.[40] Its red colour comes from iron oxide (rust) in its soil.[41] Mars has two tiny natural satellites (Deimos and Phobos) thought to be captured asteroids.[42]
Asteroid belt

Main article: Asteroid belt

Image of the main asteroid belt and the Trojan asteroids

Asteroids are mostly small Solar System bodies[e] composed mainly of refractory rocky and metallic minerals.[43]

The main asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be remnants from the Solar System’s formation that failed to coalesce because of the gravitational interference of Jupiter.[44]

Asteroids range in size from hundreds of kilometres across to microscopic. All asteroids save the largest, Ceres, are classified as small Solar System bodies, but some asteroids such as Vesta andHygieia may be reclassed as dwarf planets if they are shown to have achieved hydrostatic equilibrium.[45]

The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter.[46] Despite this, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth.[47] The main belt is very sparsely populated; spacecraft routinely pass through without incident. Asteroids with diameters between 10 and 10−4 m are called meteoroids.[48]

Ceres (2.77 AU) is the largest body in the asteroid belt and is classified as a dwarf planet.[e] It has a diameter of slightly under 1000 km, and a mass large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in the 19th century, but was reclassified as an asteroid in the 1850s as further observation revealed additional asteroids.[49] It was again reclassified in 2006 as a dwarf planet.
Asteroid groups

Asteroids in the main belt are divided into asteroid groups and families based on their orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comets which may have been the source of Earth’s water.[50]

Trojan asteroids are located in either of Jupiter’s L4 or L5 points (gravitationally stable regions leading and trailing a planet in its orbit); the term "Trojan" is also used for small bodies in any other planetary or satellite Lagrange point. Hilda asteroids are in a 2:3 resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.[51]

The inner Solar System is also dusted with rogue asteroids, many of which cross the orbits of the inner planets.[52]

Outer Solar System

The outer region of the Solar System is home to the gas giants and their large moons. Many short period comets, including the centaurs, also orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain more ices (such as water, ammonia, methane, often called ices in planetary science) than the rocky denizens of the inner Solar System, as the colder temperatures allow these compounds to remain solid.

Outer planets

Main article: Gas giant

From top to bottom: Neptune, Uranus,Saturn, and Jupiter (not to scale)

The four outer planets, or gas giants (sometimes called Jovian planets), collectively make up 99 percent of the mass known to orbit the Sun.[c] Jupiter and Saturn are each many tens of times the mass of the Earth and consist overwhelmingly of hydrogen and helium; Uranus and Neptune are far less massive (<20 Earth masses) and possess more ices in their makeup. For these reasons, some astronomers suggest they belong in their own category, “ice giants.”[53] All four gas giants have rings, although only Saturn’s ring system is easily observed from Earth. The term outer planet should not be confused with superior planet, which designates planets outside Earth’s orbit and thus includes both the outer planets and Mars.

Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. It is composed largely of hydrogen and helium. Jupiter’s strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot.
Jupiter has 63 known satellites. The four largest, Ganymede, Callisto, Io, and Europa, show similarities to the terrestrial planets, such as volcanism and internal heating.[54] Ganymede, the largest satellite in the Solar System, is larger than Mercury.
Saturn (9.5 AU), distinguished by its extensive ring system, has several similarities to Jupiter, such as its atmospheric composition and magnetosphere. Although Saturn has 60% of Jupiter’s volume, it is less than a third as massive, at 95 Earth masses, making it the least dense planet in the Solar System. The rings of Saturn are made up of small ice and rock particles.
Saturn has 62 confirmed satellites; two of which, Titan and Enceladus, show signs of geological activity, though they are largely made of ice.[55] Titan, the second largest moon in the Solar System, is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.
Uranus (19.6 AU), at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt is over ninety degrees to the ecliptic. It has a much colder core than the other gas giants, and radiates very little heat into space.[56]
Uranus has 27 known satellites, the largest ones being Titania, Oberon, Umbriel, Ariel and Miranda.
Neptune (30 AU), though slightly smaller than Uranus, is more massive (equivalent to 17 Earths) and therefore more dense. It radiates more internal heat, but not as much as Jupiter or Saturn.[57]
Neptune has 13 known satellites. The largest, Triton, is geologically active, with geysers of liquid nitrogen.[58] Triton is the only large satellite with a retrograde orbit. Neptune is accompanied in its orbit by a number of minor planets, termed Neptune Trojans, that are in 1:1 resonance with it.

Main article: Comet

Comet Hale-Bopp

Comets are small Solar System bodies,[e] typically only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye.

Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are believed to originate in the Kuiper belt, while long-period comets, such as Hale-Bopp, are believed to originate in the Oort cloud. Many comet groups, such as the Kreutz Sungrazers, formed from the breakup of a single parent.[59] Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult.[60] Old comets that have had most of their volatiles driven out by solar warming are often categorised as asteroids.[61]


Main article: Centaur (minor planet)

The centaurs are icy comet-like bodies with a semi-major axis greater than Jupiter (5.5 AU) and less than Neptune (30 AU). The largest known centaur, 10199 Chariklo, has a diameter of about 250 km.[62] The first centaur discovered, 2060 Chiron, has also been classified as comet (95P) since it develops a coma just as comets do when they approach the Sun.[63]

Trans-Neptunian region

The area beyond Neptune, or the "trans-Neptunian region", is still largely unexplored. It appears to consist overwhelmingly of small worlds (the largest having a diameter only a fifth that of the Earth and a mass far smaller than that of the Moon) composed mainly of rock and ice. This region is sometimes known as the "outer Solar System", though others use that term to mean the region beyond the asteroid belt.

Kuiper belt

Main article: Kuiper belt

Plot of all known Kuiper belt objects, set against the four outer planets

The Kuiper belt, the region’s first formation, is a great ring of debris similar to the asteroid belt, but composed mainly of ice.[64] It extends between 30 and 50 AU from the Sun. Though it contains at least three dwarf planets, it is composed mainly of small Solar System bodies. However, many of the largest Kuiper belt objects, such as Quaoar, Varuna, and Orcus, may be reclassified as dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km, but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of the Earth.[65] Many Kuiper belt objects have multiple satellites,[66] and most have orbits that take them outside the plane of the ecliptic.[67]

The Kuiper belt can be roughly divided into the "classical" belt and the resonances.[64] Resonances are orbits linked to that of Neptune (e.g. twice for every three Neptune orbits, or once for every two). The first resonance begins within the orbit of Neptune itself. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 AU to 47.7 AU.[68] Members of the classical Kuiper belt are classified as cubewanos, after the first of their kind to be discovered, (15760) 1992 QB1, and are still in near primordial, low-eccentricity orbits.[69]

Pluto and Charon

Comparison of Eris, Pluto, Makemake, Haumea, Sedna,Orcus, 2007 OR10, Quaoar, and Earth (all to scale)

Pluto (39 AU average), a dwarf planet, is the largest known object in the Kuiper belt. When discovered in 1930, it was considered to be the ninth planet; this changed in 2006 with the adoption of a formal definition of planet. Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion.
Charon, Pluto’s largest moon, is sometimes described as part of a binary system with Pluto, as the two bodies orbit a barycenter of gravity above their surfaces (i.e., they appear to "orbit each other"). Beyond Charon, two much smaller moons, Nix and Hydra, orbit within the system.
Pluto has a 3:2 resonance with Neptune, meaning that Pluto orbits twice round the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called plutinos.[70]
Haumea and Makemake
Haumea (43.34 AU average), and Makemake (45.79 AU average), while smaller than Pluto, are the largest known objects in the classical Kuiper belt (that is, they are not in a confirmed resonancewith Neptune). Haumea is an egg-shaped object with two moons. Makemake is the brightest object in the Kuiper belt after Pluto. Originally designated 2003 EL61 and 2005 FY9 respectively, they were given names and designated dwarf planets in 2008.[71] Their orbits are far more inclined than Pluto’s, at 28° and 29°.[72]
Scattered disc

Main article: Scattered disc

The scattered disc, which overlaps the Kuiper belt but extends much further outwards, is thought to be the source of short-period comets. Scattered disc objects are believed to have been ejected into erratic orbits by the gravitational influence of Neptune’s early outward migration. Most scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia as far as 150 AU from the Sun. SDOs’ orbits are also highly inclined to the ecliptic plane, and are often almost perpendicular to it. Some astronomers consider the scattered disc to be merely another region of the Kuiper belt, and describe scattered disc objects as "scattered Kuiper belt objects."[73] Some astronomers also classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.[74]

Eris (68 AU average) is the largest known scattered disc object, and caused a debate about what constitutes a planet, since it is at least 5% larger than Pluto with an estimated diameter of 2400 km (1500 mi). It is the largest of the known dwarf planets.[75] It has one moon, Dysnomia. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto’s distance from the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane.

Farthest regions

The point at which the Solar System ends and interstellar space begins is not precisely defined, since its outer boundaries are shaped by two separate forces: the solar wind and the Sun’s gravity. The outer limit of the solar wind’s influence is roughly four times Pluto’s distance from the Sun; this heliopause is considered the beginning of the interstellar medium.[21] However, the Sun’s Roche sphere, the effective range of its gravitational dominance, is believed to extend up to a thousand times farther.[76]


The Voyagers entering the heliosheath

The heliosphere is divided into two separate regions. The solar wind travels at roughly 400 km/s until it collides with the interstellar wind; the flow of plasma in the interstellar medium. The collision occurs at the termination shock, which is roughly 80–100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind.[77] Here the wind slows dramatically, condenses and becomes more turbulent,[77] forming a great oval structure known as the heliosheath. This structure is believed to look and behave very much like a comet’s tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind; but evidence from the Cassini and Interstellar Boundary Explorer spacecraft has suggested that it is in fact forced into a bubble shape by the constraining action of the interstellar magnetic field.[78] Both Voyager 1 and Voyager 2 are reported to have passed the termination shock and entered the heliosheath, at 94 and 84 AU from the Sun, respectively.[79][80] The outer boundary of the heliosphere, the heliopause, is the point at which the solar wind finally terminates and is the beginning of interstellar space.[21]

The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics of interactions with the interstellar medium[77] as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU (roughly 900 million miles) farther than the southern hemisphere. Beyond the heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it travels through the Milky Way.[81]

No spacecraft have yet passed beyond the heliopause, so it is impossible to know for certain the conditions in local interstellar space. It is expected that NASA‘s Voyager spacecraft will pass the heliopause some time in the next decade and transmit valuable data on radiation levels and solar wind back to the Earth.[82] How well the heliosphere shields the Solar System from cosmic rays is poorly understood. A NASA-funded team has developed a concept of a "Vision Mission" dedicated to sending a probe to the heliosphere.[83][84]

Oort cloud

Main article: Oort cloud

An artist’s rendering of the Oort Cloud, the Hills Cloud, and the Kuiper belt (inset)

The hypothetical Oort cloud is a spherical cloud of up to a trillion icy objects that is believed to be the source for all long-period comets and to surround the Solar System at roughly 50,000 AU (around 1 light-year (LY)), and possibly to as far as 100,000 AU (1.87 LY). It is believed to be composed of comets which were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events such as collisions, the gravitational effects of a passing star, or the galactic tide, the tidal force exerted by the Milky Way.[85][86]


90377 Sedna (525.86 AU average) is a large, reddish Pluto-like object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 928 AU at aphelion and takes 12,050 years to complete. Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper belt as its perihelion is too distant to have been affected by Neptune’s migration. He and other astronomers consider it to be the first in an entirely new population, which also may include the object 2000 CR105, which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3,420 years.[87] Brown terms this population the "Inner Oort cloud," as it may have formed through a similar process, although it is far closer to the Sun.[88] Sedna is very likely a dwarf planet, though its shape has yet to be determined with certainty.


See also: Vulcanoid asteroid, Planets beyond Neptune, and Nemesis (star)

Much of our Solar System is still unknown. The Sun’s gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000 AU.[89] Despite discoveries such as Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually unmapped. There are also ongoing studies of the region between Mercury and the Sun.[90] Objects may yet be discovered in the Solar System’s uncharted regions.

Galactic context

Location of the Solar System within ourgalaxy

The Solar System is located in the Milky Way galaxy, a barred spiral galaxy with a diameter of about 100,000 light-years containing about 200 billion stars.[91] Our Sun resides in one of the Milky Way’s outer spiral arms, known as the Orion Arm or Local Spur.[92] The Sun lies between 25,000 and 28,000 light years from the Galactic Centre,[93] and its speed within the galaxy is about 220 kilometres per second, so that it completes one revolution every 225–250 million years. This revolution is known as the Solar System’s galactic year.[94] The solar apex, the direction of the Sun’s path through interstellar space, is near the constellation of Hercules in the direction of the current location of the bright star Vega.[95] The plane of the Solar System’s ecliptic lies at an angle of about 60° to the galactic plane.[f]

The Solar System’s location in the galaxy is very likely a factor in the evolution of life on Earth. Its orbit is close to being circular and is at roughly the same speed as that of the spiral arms, which means it passes through them only rarely. Since spiral arms are home to a far larger concentration of potentially dangerous supernovae, this has given Earth long periods of interstellar stability for life to evolve.[96] The Solar System also lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could perturb bodies in the Oort Cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. The intense radiation of the galactic centre could also interfere with the development of complex life.[96] Even at the Solar System’s current location, some scientists have hypothesised that recent supernovae may have adversely affected life in the last 35,000 years by flinging pieces of expelled stellar core towards the Sun as radioactive dust grains and larger, comet-like bodies.[97]


The immediate galactic neighbourhood of the Solar System is known as the Local Interstellar Cloud or Local Fluff, an area of denser cloud in an otherwise sparse region known as the Local Bubble, an hourglass-shaped cavity in the interstellar medium roughly 300 light years across. The bubble is suffused with high-temperature plasma that suggests it is the product of several recent supernovae.[98]

There are relatively few stars within ten light years (95 trillion km) of the Sun. The closest is the triple star system Alpha Centauri, which is about 4.4 light years away. Alpha Centauri A and B are a closely tied pair of Sun-like stars, while the small red dwarfAlpha Centauri C (also known as Proxima Centauri) orbits the pair at a distance of 0.2 light years. The stars next closest to the Sun are the red dwarfs Barnard’s Star (at 5.9 light years), Wolf 359 (7.8 light years) and Lalande 21185 (8.3 light years). The largest star within ten light years is Sirius, a bright main sequence star roughly twice the Sun’s mass and orbited by a white dwarf called Sirius B. It lies 8.6 light years away. The remaining systems within ten light years are the binary red dwarf systemLuyten 726-8 (8.7 light years) and the solitary red dwarf Ross 154 (9.7 light years).[99] Our closest solitary sun-like star is Tau Ceti, which lies 11.9 light years away. It has roughly 80 percent the Sun’s mass, but only 60 percent its luminosity.[100] The closest known extrasolar planet to the Sun lies around the star Epsilon Eridani, a star slightly dimmer and redder than the Sun, which lies 10.5 light years away. Its one confirmed planet, Epsilon Eridani b, is roughly 1.5 times Jupiter’s mass and orbits its star every 6.9 years.[101]

A series of five star maps that show from left to right our location in the Solar System, in the Sun's neighborhood of stars, in the local area of the Milky Way galaxy, in the Local Group of galaxies, and in the Supercluster of galaxies

A diagram of our location in the Local Superclusterclick here to view more detail

Formation and evolution

Projected timeline of the Sun's life.

Main article: Formation and evolution of the Solar System

The Solar System formed from the gravitational collapse of a giant molecular cloud 4.568 billion years ago.[102] This initial cloud was likely several light-years across and probably birthed several stars.[103]

As the region that would become the Solar System, known as the pre-solar nebula,[104] collapsed, conservation of angular momentum made it rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc.[103] As the contracting nebula rotated, it began to flatten into a spinning protoplanetary disc with a diameter of roughly 200 AU[103] and a hot, dense protostar at the centre.[105][106] At this point in its evolution, the Sun is believed to have been a T Tauri star. Studies of T Tauri stars show that they are often accompanied by discs of pre-planetary matter with masses of 0.001–0.1 solar masses, with the vast majority of the mass of the nebula in the star itself.[107] The planets formed by accretion from this disk.[108]

Within 50 million years, the pressure and density of hydrogen in the centre of the protostar became great enough for it to begin thermonuclear fusion.[109] The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved, with the thermal energy countering the force of gravitational contraction. At this point the Sun became a full-fledged main sequence star.[110]

The Solar System as we know it today will last until the Sun begins its evolution off of the main sequence of the Hertzsprung-Russell diagram. As the Sun burns through its supply of hydrogen fuel, the energy output supporting the core tends to decrease, causing it to collapse in on itself. This increase in pressure heats the core, so it burns even faster. As a result, the Sun is growing brighter at a rate of roughly ten percent every 1.1 billion years.[111]

Around 5.4 billion years from now, the hydrogen in the core of the Sun will have been entirely converted to helium, ending the main sequence phase. As the hydrogen reactions shut down, the core will contract further, increasing pressure and temperature, causing fusion to commence via the helium process. Helium in the core burns at a much hotter temperature, and the energy output will be much greater than during the hydrogen process. At this time, the outer layers of the Sun will expand to roughly up to 260 times its current diameter; the Sun will become a red giant. Because of its vastly increased surface area, the surface of the Sun will be considerably cooler than it is on the main sequence (2600 K at the coolest).[112]

Eventually, helium in the core will exhaust itself at a much faster rate than the hydrogen, and the Sun’s helium burning phase will be but a fraction of the time compared to the hydrogen burning phase. The Sun is not massive enough to commence fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will fall away into space, leaving a white dwarf, an extraordinarily dense object, half the original mass of the Sun but only the size of the Earth.[113] The ejected outer layers will form what is known as a planetary nebula, returning some of the material that formed the Sun to the interstellar medium.

See also

Objects in the Solar System



Book:Solar System

Books are collections of articles that can be downloaded or ordered in print.

Solar system.jpg
Solar System portal


  1. ^ Capitalization of the name varies. The IAU, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects (Solar System). However, the name is commonly rendered in lower case (solar system) – as, for example, in the Oxford English Dictionary, Merriam-Webster’s 11th Collegiate Dictionary, and Encyclopædia Britannica.
  2. ^ See List of natural satellites for the full list of natural satellites of the eight planets and five dwarf planets.
  3. ^ The mass of the Solar System excluding the Sun, Jupiter and Saturn can be determined by adding together all the calculated masses for its largest objects and using rough calculations for the masses of the Oort cloud (estimated at roughly 3 Earth masses),[114] the Kuiper belt (estimated at roughly 0.1 Earth mass)[65] and the asteroid belt (estimated to be 0.0005 Earth mass)[47] for a total, rounded upwards, of ~37 Earth masses, or 8.1 percent the mass in orbit around the Sun. With the combined masses of Uranus and Neptune (~31 Earth masses) subtracted, the remaining ~6 Earth masses of material comprise 1.3 percent of the total.
  4. ^ Astronomers measure distances within the Solar System in astronomical units (AU). One AU equals the average distance between the centers of Earth and the Sun, or 149,598,000 km. Pluto is about 38 AU from the Sun and Jupiter is about 5.2 AU from the Sun. One light-year is 63,240 AU.
  5. ^ According to current definitions, objects in orbit around the Sun are classed dynamically and physically into three categories: planets, dwarf planets and small Solar System bodies. A planet is any body in orbit around the Sun that has enough mass to form itself into a spherical shape and has cleared its immediate neighbourhood of all smaller objects. By this definition, the Solar System has eight known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto does not fit this definition, as it has not cleared its orbit of surrounding Kuiper belt objects.[115] A dwarf planet is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but which has not cleared its neighbouring region of planetesimals and is not a satellite.[115] By this definition, the Solar System has five known dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris.[71] Other objects may be classified in the future as dwarf planets, such as Sedna, Orcus, and Quaoar.[116] Dwarf planets that orbit in the trans-Neptunian region are called "plutoids".[117] The remainder of the objects in orbit around the Sun are small Solar System bodies.[115]
  6. ^ If ψ is the angle between the north pole of the ecliptic and the north galactic pole then:
    cosψ = cos(βg)cos(βe)cos(αg − αe) + sin(βg)sin(βe),
    where βg = 27° 07′ 42.01″ and αg = 12h 51m 26.282 are the declination and right ascension of the north galactic pole,[118] while βe = 66° 33′ 38.6″ and αe = 18h 0m 00 are those for the north pole of the ecliptic. (Both pairs of coordinates are for J2000 epoch.) The result of the calculatgion is 60.19°.


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  111. ^ Reid, M.J.; Brunthaler, A. (2004 2004). "The Proper Motion of Sagittarius A*". The Astrophysical Journal (The American Astronomical Society) 616 (2): 883. doi:10.1086/424960. Retrieved 2008-02-07.

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Categories: Solar System | Planetary science | Space science | Planetary systems

Our Cosmic Neighborhood

From our small world we have gazed upon the cosmic ocean for thousands of years. Ancient astronomers observed points of light that appeared to move among the stars. They called these objects "planets," meaning wanderers, and named them after Roman deities—Jupiter, king of the gods; Mars, the god of war; Mercury, messenger of the gods; Venus, the goddes of love and beauty, and Saturn, father of Jupiter and god of agriculture. The stargazers also observed comets with sparkling tails, and meteors or shooting stars apparently falling from the sky.
Since the invention of the telescope, three more planets have been discovered in our solar system: Uranus (1781), Neptune (1846), and, now downgraded to a dwarf planet, Pluto (1930). In addition, there are thousands of small bodies such as asteroids and comets. Most of the asteroids orbit in a region between the orbits of Mars and Jupiter, while the home of comets lies far beyond the orbit of Pluto, in the Oort Cloud.
The four planets closest to the sun—Mercury, Venus, Earth, and Mars—are called the terrestrial planetsbecause they have solid rocky surfaces. The four large planets beyond the orbit of Mars—Jupiter, Saturn, Uranus, and Neptune—are called gas giants. Tiny, distant, Pluto has a solid but icier surface than the terrestrial planets.
Nearly every planet—and some of the moons—has an atmosphere. Earth’s atmosphere is primarily nitrogen and oxygen. Venus has a thick atmosphere of carbon dioxide, with traces of poisonous gases such as sulfur dioxide. Mars’s carbon dioxide atmosphere is extremely thin. Jupiter, Saturn, Uranus, and Neptune are primarily hydrogen and helium. When Pluto is near the sun, it has a thin atmosphere, but when Pluto travels to the outer regions of its orbit, the atmosphere freezes and collapses to the planet’s surface. In that way, Pluto acts like a comet.
Moons, Rings, and Magnetospheres
There are 140 known natural satellites, also called moons, in orbit around the various planets in our solar system, ranging from bodies larger than our own moon to small pieces of debris.
From 1610 to 1977, Saturn was thought to be the only planet with rings. We now know that Jupiter, Uranus, and Neptune also have ring systems, although Saturn’s is by far the largest. Particles in these ring systems range in size from dust to boulders to house-size, and may be rocky and/or icy.
Most of the planets also have magnetic fields, which extend into space and form a magnetosphere around each planet. These magnetospheres rotate with the planet, sweeping charged particles with them. The sun has a magnetic field, the heliosphere, which envelops our entire solar system.
Ancient astronomers believed that the Earth was the center of the universe, and that the sun and all the other stars revolved around the Earth. Copernicus proved that Earth and the other planets in our solar system orbit our sun. Little by little, we are charting the universe, and an obvious question arises: Are there other planets where life might exist? Only recently have astronomers had the tools to indirectly detect large planets around other stars in nearby solar systems.
—Text courtesy NASA/JPL

Animation of asteroid flying by the Earth

Two Small Asteroids to Pass Close by Earth on September 8, 2010
September 7, 2010

Two asteroids, several meters in diameter and in unrelated orbits, will pass within the Moon’s distance of Earth on Wednesday, September 8th. The Catalina Sky Survey near Tucson Arizona discovered both objects on the morning of September 5 during their routine monitoring of the skies.

Full Story

Trajectory of Asteroid 2010 KQ - March-Oct 2010Asteroid 2010 KQ: Probably a Rocket Body
May 27, 2010

A small asteroid-like object has been discovered in an orbit about the Sun that is so similar to the Earth’s orbit that scientists strongly suspect it to be a rocket stage that escaped years ago from the Earth-Moon system.

Full Story

Trajectory of Asteroid 2010 GA6 Past EarthAsteroid 2010 GA6 To Fly Within Moon’s Orbit on April 8, 2010
April 6, 2010

A newly discovered asteroid, 2010 GA6, will safely fly by Earth this Thursday at 4:06 p.m. Pacific (23:06 U.T.C.). At time of closest approach 2010 GA6 will be about 359,000 kilometers (223,000 miles) away from Earth – about 9/10ths the distance to the moon. The asteroid, approximately 22 meters (71 feet) wide, was discovered by the Catalina Sky Survey, Tucson, Az.

Full Story

Trajectory of Asteroid 2010 AL30 Past Earth on January 12/13, 2010Small Asteroid 2010 AL30 To Fly Past The Earth
January 12, 2010

Asteroid 2010 AL30, discovered by the LINEAR survey of MIT’s Lincoln Laboratories on Jan. 10, will make a close approach to the Earth’s surface to within 76,000 miles on Wednesday January 13 at 12:46 pm Greenwich time (7:46 EST, 4:46 PST).

Full Story

Trajectory of Asteroid 2009 VA Past Earth on November 6, 2009Small Asteroid 2009 VA Whizzes By The Earth
November 9, 2009

A newly discovered asteroid designated 2009 VA, which is only about 7 meters in size, passed about 2 Earth radii (14,000 km) from the Earth’s surface Nov. 6 at around 16:30 EST. This is the third-closest known (non-impacting) Earth approach on record for a cataloged asteroid.

Full Story

Image for amateur video of the smoke trail of the fireballAsteroid Impactor Reported over Indonesia
October 23, 2009

On October 8, 2009 about 03:00 Greenwich time, an atmospheric fireball blast was observed and recorded over an island region of Indonesia. The blast is thought to be due to the atmospheric entry of a small asteroid about 10 meters in diameter that, due to atmospheric pressure, detonated in the atmosphere with an energy of about 50 kilotons (the equivalent of 110 million pounds of TNT explosives). The blast was recorded visually and reported upon by local media representatives. See the YouTube video here

A report from Elizabeth Silber and Peter Brown at the University of Western Ontario indicates that several international very-long wavelength infrasound detectors recorded the blast and fixed the position near the coastal city of Bone in South Sulawesi, island of Sulewesi. They note that the blast was in the 10 to 50 kT range with the higher end of this range being more likely. Assuming an estimated size of about 5-10 meters in diameter, we would expect a fireball event of this magnitude about once every 2 to 12 years on average. As a rule, the most common types of stony asteroids would not be expected to cause ground damage unless their diameters were about 25 meters in diameter or larger. A more extensive report by Elizabeth Silber and Peter Brown of the University of Western Ontario is here.


The Sun and Nine Planets
The Solar System
For I dipped into the Future, far as human eye could see; saw the vision of the world, and all the wonder that would be. -Alfred Lord Tennyson, 1842

Our solar system consists of an average star we call the Sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. It includes: the satellites of the planets; numerous comets,asteroids, and meteoroids; and the interplanetary medium. The Sun is the richest source of electromagnetic energy (mostly in the form of heat and light) in the solar system. The Sun’s nearest known stellar neighbor is a red dwarf star called Proxima Centauri, at a distance of 4.3 light years away. The whole solar system, together with the local stars visible on a clear night, orbits the center of our home galaxy, a spiral disk of 200 billion stars we call the Milky Way. The Milky Way has two small galaxies orbiting it nearby, which are visible from the southern hemisphere. They are called the Large Magellanic Cloud and the Small Magellanic Cloud. The nearest large galaxy is the Andromeda Galaxy. It is a spiral galaxy like the Milky Way but is 4 times as massive and is 2 million light years away. Our galaxy, one of billions of galaxies known, is traveling through intergalactic space.

The planets, most of the satellites of the planets and the asteroids revolve around the Sun in the same direction, in nearly circular orbits. When looking down from above the Sun’s north pole, the planets orbit in a counter-clockwise direction. The planets orbit the Sun in or near the same plane, called the ecliptic. Pluto is a special case in that its orbit is the most highly inclined (18 degrees) and the most highly elliptical of all the planets. Because of this, for part of its orbit, Pluto is closer to the Sun than is Neptune. The axis of rotation for most of the planets is nearly perpendicular to the ecliptic. The exceptions are Uranus and Pluto, which are tipped on their sides.

Composition Of The Solar System

The Sun contains 99.85% of all the matter in the Solar System. The planets, which condensed out of the same disk of material that formed the Sun, contain only 0.135% of the mass of the solar system. Jupiter contains more than twice the matter of all the other planets combined. Satellites of the planets, comets, asteroids, meteoroids, and the interplanetary medium constitute the remaining 0.015%. The following table is a list of the mass distribution within our Solar System.

  • Sun: 99.85%
  • Planets: 0.135%
  • Comets: 0.01% ?
  • Satellites: 0.00005%
  • Minor Planets: 0.0000002% ?
  • Meteoroids: 0.0000001% ?
  • Interplanetary Medium: 0.0000001% ?
Interplanetary Space

Nearly all the solar system by volume appears to be an empty void. Far from being nothingness, this vacuum of "space" comprises the interplanetary medium. It includes various forms of energy and at least two material components: interplanetary dust and interplanetary gas. Interplanetary dust consists of microscopic solid particles. Interplanetary gas is a tenuous flow of gas and charged particles, mostly protons and electrons — plasma — which stream from the Sun, called the solar wind.

Interplanetary Wind

The solar wind can be measured by spacecraft, and it has a large effect on comet tails. It also has a measurable effect on the motion of spacecraft. The speed of the solar wind is about 400 kilometers (250 miles) per second in the vicinity of Earth’s orbit. The point at which the solar wind meets the interstellar medium, which is the "solar" wind from other stars, is called the heliopause. It is a boundary theorized to be roughly circular or teardrop-shaped, marking the edge of the Sun’s influence perhaps 100 AU from the Sun. The space within the boundary of the heliopause, containing the Sun and solar system, is referred to as the heliosphere.

The solar magnetic field extends outward into interplanetary space; it can be measured on Earth and by spacecraft. The solar magnetic field is the dominating magnetic field throughout the interplanetary regions of the solar system, except in the immediate environment of planets which have their own magnetic fields.

Terrestrial Planets
The Terrestrial Planets

The terrestrial planets are the four innermost planets in the solar system, Mercury, Venus, Earth and Mars. They are called terrestrial because they have a compact, rocky surface like the Earth’s. The planets, Venus, Earth, and Mars have significant atmospheres while Mercury has almost none. The following diagram shows the approximate distance of the terrestrial planets to the Sun.

approximate distance of the terrestrial planets to the Sun

The Jovian Planets

Jupiter, Saturn, Uranus, and Neptune are known as the Jovian (Jupiter-like) planets, because they are all gigantic compared with Earth, and they have a gaseous nature like Jupiter’s. The Jovian planets are also referred to as the gas giants, although some or all of them might have small solid cores. The following diagram shows the approximate distance of the Jovian planets to the Sun.

Distance of the Jovian Planets to the Sun

Solar System Animation

Views of the Solar System


Our Milkyway Galaxy
This image of our galaxy, the Milky Way, was taken with NASA’s Cosmic Background Explorer’s (COBE) Diffuse Infrared Background Experiment (DIRBE). This never-before-seen view shows the Milky Way from an edge-on perspective with the galactic north pole at the top, the south pole at the bottom and the galactic center at the center. The picture combines images obtained at several near-infrared wavelengths. Stars within our galaxy are the dominant source of light at these wavelengths. Even though our solar system is part of the Milky Way, the view looks distant because most of the light comes from the population of stars that are closer to the galactic center than our own Sun. (Courtesy NASA)
converted PNM file

Our Milky Way Gets a Makeover
Like early explorers mapping the continents of our globe, astronomers are busy charting the spiral structure of our galaxy, the Milky Way. Using infrared images from NASA’s Spitzer Space Telescope, scientists have discovered that the Milky Way’s elegant spiral structure is dominated by just two arms wrapping off the ends of a central bar of stars. Previously, our galaxy was thought to possess four major arms.

This artist’s concept illustrates the new view of the Milky Way, along with other findings presented at the 212th American Astronomical Society meeting in St. Louis, Mo. The galaxy’s two major arms (Scutum-Centaurus and Perseus) can be seen attached to the ends of a thick central bar, while the two now-demoted minor arms (Norma and Sagittarius) are less distinct and located between the major arms. The major arms consist of the highest densities of both young and old stars; the minor arms are primarily filled with gas and pockets of star-forming activity.

The artist’s concept also includes a new spiral arm, called the "Far-3 kiloparsec arm," discovered via a radio-telescope survey of gas in the Milky Way. This arm is shorter than the two major arms and lies along the bar of the galaxy.

Our sun lies near a small, partial arm called the Orion Arm, or Orion Spur, located between the Sagittarius and Perseus arms. (Courtesy NASA/JPL-Caltech)

Spiral Galaxy, NGC 4414
The majestic galaxy, NGC 4414, is located 60 million light-years away. Like the Milky Way, NGC 4414 is a giant spiral-shaped disk of stars, with a bulbous central hub of older yellow and red stars. The outer spiral arms are considerably bluer due to ongoing formation of young, blue stars, the brightest of which can be seen individually at the high resolution provided by the Hubble camera. The arms are also very rich in clouds of interstellar dust, seen as dark patches and streaks silhouetted against the starlight. (Courtesy NASA/STSCI)
Obliquity of the Nine Planets

Obliquity of the Eight Planets
This illustration shows the obliquity of the eight planets. Obliquity is the angle between a planet’s equatorial plane and its orbital plane. By International Astronomical Union (IAU) convention, a planet’s north pole lies above the ecliptic plane. By this convention, Venus, Uranus, and Pluto have a retrograde rotation, or a rotation that is in the opposite direction from the other planets. (Copyright 2008 by Calvin J. Hamilton)

The Solar System
During the past three decades a myriad of space explorers have escaped the confines of planet Earth and have set out to discover our planetary neighbors. This picture shows the Sun and all nine planets of the solar system as seen by the space explorers. Starting at the top-left corner is the Sun followed by the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn,Uranus, Neptune, and Pluto. (Copyright 1998 by Calvin J. Hamilton)
The Sun and Nine Planets

Sun and Planets
This image shows the Sun and nine planets approximately to scale. The order of these bodies are: Sun,Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. (Copyright Calvin J. Hamilton)

Jovian Planets
This image shows the Jovian planets Jupiter, Saturn, Uranus and Neptune approximately to scale. The Jovian planets are named because of their gigantic Jupiter-like appearance. (Copyright Calvin J. Hamilton)

The Largest Moons and Smallest Planets
This image shows the relative sizes of the largest moons and the smallest planets in the solarsystem. The largest satellites pictured in this image are: Ganymede (5262 km), Titan (5150 km), Callisto (4806 km), Io (3642 km), the Moon (3476 km),Europa (3138 km), Triton (2706 km), and Titania (1580 km). Both Ganymede and Titan are larger than planet Mercuryfollowed by Io, the Moon, Europa, and Triton which are larger than the planet Pluto. (Copyright Calvin J. Hamilton)

Diagram of Portrait Frames
On February 14, 1990, the cameras of Voyager 1 pointed back toward the Sun and took a series of pictures of the Sun and the planets, making the first ever "portrait" of our solar system as seen from the outside. This image is a diagram of how the frames for the solar system portrait were taken. (Courtesy NASA/JPL)

All Frames from the Family Portrait
This image shows the series of pictures of the Sun and the planets taken on February 14, 1990, for the solar system family portrait as seen from the outside. In the course of taking this mosaic consisting of a total of 60 frames, Voyager 1 made several images of the inner solar system from a distance of approximately 6.4 billion kilometers (4 billion miles) and about 32° above the ecliptic plane. Thirty-nine wide angle frames link together six of the planets of our solar system in this mosaic. Outermost Neptune is 30 times further from the Sun than Earth. Our Sun is seen as the bright object in the center of the circle of frames. The insets show the planets magnified many times. (Courtesy NASA/JPL)

Portrait of the Solar System
These six narrow-angle color images were made from the first ever "portrait" of the solar system taken by Voyager 1, which was more than 6.4 billion kilometers (4 billion miles) from Earth and about 32° above the ecliptic. Mercury is too close to the Sun to be seen. Mars was not detectable by the Voyager cameras due to scattered sunlight in the optics, and Pluto was not included in the mosaic because of its small size and distance from the Sun. These blown-up images, left to right and top to bottom are Venus, Earth, Jupiter, Saturn, Uranus, and Neptune. (Courtesy NASA/JPL)

Sun and Planet Summary

The following table lists statistical information for the Sun and planets:

Statistical Information for the Sun and Planets

* The Sun’s period of rotation at the surface varies from approximately 25 days at the equator to 36 days at the poles. Deep down, below theconvective zone, everything appears to rotate with a period of 27 days.


A Decade of Studying the Earth’s Magnetic Shield, in 3-D

ScienceDaily (Sep. 1, 2010) — Space scientists around the world are celebrating ten years of ground-breaking discoveries by ‘Cluster’, a mission that is illuminating the mysteries of the magnetosphere, the northern lights and the solar wind.

Cluster is a European Space Agency mission, launched in summer 2000. It consists of a unique constellation of four spacecraft flying in formation around Earth, studying the interaction between the solar wind and the magnetosphere. The spacecraft each carry an identical set of 11 scientific instruments, which together capture 3D information about the magnetosphere — Earth’s ‘magnetic shield’. A key instrument — PEACE — was designed by a team led by space scientists at UCL.

The solar wind is a continuous outflow of hot, magnetised, electrified gas from the Sun. Earth is shielded from the solar wind by its magnetic field, which surrounds the planet in a zone called the magnetosphere, many times larger than the Earth.

The magnetosphere prevents the solar wind from stripping away the atmosphere and protects Earth from deadly energetic particles produced by storms on the Sun. However the magnetosphere is not a perfect shield. Energy and material from solar wind can get inside, to cause the northern lights, ionospheric disturbances, the generation of radiation belts and disturbances to the ground-level magnetic field. These "space weather effects" are important because they interfere with spacecraft operations, communications, GPS signals and electrical power systems on the ground. Cluster is being used to find out how transfer of solar wind energy to the magnetosphere leads to these diverse effects.

PEACE measures electrons and electric currents in the solar wind, magnetosphere and aurora. During Cluster’s mission PEACE has been used to study huge bubbles of plasma three times the size of Earth jetting through the magnetosphere, very thin sheets of electric current flowing through space where explosive magnetic reconnection occurs, and grand waves on the edge of the magnetosphere, formed by the solar wind ‘blowing’ over the surface before breaking and forming tornado-like vortices.

Dr Andrew Fazakerley, from UCL’s Mullard Space Science Laboratory, and Principal Investigator for PEACE, said: "Cluster is revolutionising the study of the solar wind and the magnetosphere because it is the first space mission to reveal what plasmas are like in 3D, which is crucial to testing our theoretical models."

Cluster is also the first multi-spacecraft mission to study the northern lights or aurora. The aurora are caused when electrons from the magnetosphere smash into the upper atmosphere, but it’s a mystery how these electrons are accelerated to high enough energies. Cluster’s simultaneous measurements at different locations have given scientists the first opportunity to test ideas about what could be the cause.

"Cluster was not designed to visit the aurora, but luckily the orbit of the four spacecraft has naturally evolved to allow us to explore the unexplained auroral acceleration region which is the key to the formation of the aurora," said Dr Forsyth.

"We are very excited at the coming opportunity to investigate how the magnetosphere responds in the near future, as solar activity increases to solar maximum," said Dr Fazakerley.

Solar System May Be 2 Million Years Older Than We Thought, Meteorite Analysis Suggests

ScienceDaily (Aug. 25, 2010) — Timescales of early Solar System processes rely on precise, accurate and consistent ages obtained with radiometric dating. However, recent advances in instrumentation now allow scientists to make more precise measurements, some of which are revealing inconsistencies in the ages of samples. Seeking better constraints on the age of the Solar System, Arizona State University researchers Audrey Bouvier and Meenakshi Wadhwa analyzed meteorite Northwest Africa (NWA) 2364 and found that the age of the Solar System predates previous estimates by up to 1.9 million years.

ASU researcher Audrey Bouvier works in the lab. Bouvier and fellow ASU researcher Meenakshi Wadhwa analyzed meteorite Northwest Africa (NWA) 2364 and found that the age of the Solar System predates previous estimates by up to 1.9 million years. (Credit: Audrey Bouvier)

By using a dating technique known as lead-lead dating, Bouvier and Wadhwa were able to calculate the age of a calcium-aluminum-rich inclusion (CAI) contained within the Northwest Africa 2364 chondritic meteorite. These CAIs are thought to be the first solids to condense from the cooling protoplanetary disk during the birth of the Solar System.

The study’s findings, published online on August 22 in Nature Geoscience, fix the age of the Solar System at 4.5682 billion years old, between 0.3 and 1.9 million years older than previous estimates. This relatively small revision to the currently accepted age of about 4.56 billion years is significant since some of the most important events that shaped the Solar System occurred within the first ~10 million years of its formation.

"This relatively small age adjustment means that there was as much as twice the amount of iron-60, a certain short-lived isotope of iron, in the early Solar System than previously determined. This higher initial abundance of this isotope in the Solar System can only be explained by supernova injection," said Bouvier, a faculty research associate in the School of Earth and Space Exploration (SESE) in ASU’s College of Liberal Arts and Sciences. "This supernova event, and possibly others, could have triggered the formation of the Solar System. By studying meteorites and their isotopic characteristics, we bring new clues about the stellar environment of our Sun at birth."

According to Meenakshi Wadhwa, professor in SESE and director of the Center for Meteorite Studies, "This work also helps to resolve some long-standing inconsistencies in early Solar System time scales as obtained by different high-resolution chronometers. However, there is certainly room for future studies. In particular, it will be important to conduct high precision chronologic investigations of CAIs from other pristine meteorites. We also need to understand the reasons for why the CAIs measured previously from two other chondritic meteorites, Allende and Efremovka, have yielded younger ages."

One significant aspect of this study is that it is the first published lead-lead isotopic investigation that takes into account the possible variation of the uranium isotope composition. Earlier work conducted in Wadhwa’s laboratory by ASU graduate student Gregory Brennecka, in collaboration with SESE professor Ariel Anbar, has shown that the uranium isotope composition of CAIs, long assumed to be constant, can in fact be highly variable and this has important implications for the calculation of the precise lead-lead ages of these objects.

Using the relationship demonstrated by Brennecka and colleagues between the uranium isotope composition and other geochemical indicators in CAIs, Bouvier and Wadhwa inferred a uranium isotope composition for the CAI for which they reported the lead-lead age. Future work at ASU will focus on development of analytical techniques for the direct measurement of the precise uranium isotope composition of CAIs for which lead-lead isotopic investigations are being conducted.

"Our work can help researchers better understand the sequence of events that took place within the first few million years of the Solar system formation, such as the accretion and melting of planetary bodies," Bouvier said. "All these processes happened extremely rapidly, and only by reaching such a precision on isotopic measurements and chronology can we find out about these processes of planetary formation."


Solar System Similar to Ours? Richest Planetary System Discovered

ScienceDaily (Aug. 24, 2010) — Astronomers using ESO’s world-leading HARPS instrument have discovered a planetary system containing at least five planets, orbiting the Sun-like star HD 10180. The researchers also have tantalising evidence that two other planets may be present, one of which would have the lowest mass ever found. This would make the system similar to our Solar System in terms of the number of planets (seven as compared to the Solar System’s eight planets). Furthermore, the team also found evidence that the distances of the planets from their star follow a regular pattern, as also seen in our Solar System.

The planetary system around the Sun-like star HD 10180 (artist’s impression). (Credit: ESO/L. Calçada)

"We have found what is most likely the system with the most planets yet discovered," says Christophe Lovis, lead author of the paper reporting the result. "This remarkable discovery also highlights the fact that we are now entering a new era in exoplanet research: the study of complex planetary systems and not just of individual planets. Studies of planetary motions in the new system reveal complex gravitational interactions between the planets and give us insights into the long-term evolution of the system."

The team of astronomers used the HARPS spectrograph, attached to ESO’s 3.6-metre telescope at La Silla, Chile, for a six-year-long study of the Sun-like star HD 10180, located 127 light-years away in the southern constellation of Hydrus (the Male Water Snake). HARPS is an instrument with unrivalled measurement stability and great precision and is the world’s most successful exoplanet hunter.

Thanks to the 190 individual HARPS measurements, the astronomers detected the tiny back and forth motions of the star caused by the complex gravitational attractions from five or more planets. The five strongest signals correspond to planets with Neptune-like masses — between 13 and 25 Earth masses [1] — which orbit the star with periods ranging from about 6 to 600 days. These planets are located between 0.06 and 1.4 times the Earth-Sun distance from their central star.

"We also have good reasons to believe that two other planets are present," says Lovis. One would be a Saturn-like planet (with a minimum mass of 65 Earth masses) orbiting in 2200 days. The other would be the least massive exoplanet ever discovered, with a mass of about 1.4 times that of the Earth. It is very close to its host star, at just 2 percent of the Earth-Sun distance. One "year" on this planet would last only 1.18 Earth-days.

"This object causes a wobble of its star of only about 3 km/hour — slower than walking speed — and this motion is very hard to measure," says team member Damien Ségransan. If confirmed, this object would be another example of a hot rocky planet, similar to Corot-7b (eso0933).

The newly discovered system of planets around HD 10180 is unique in several respects. First of all, with at least five Neptune-like planets lying within a distance equivalent to the orbit of Mars, this system is more populated than our Solar System in its inner region, and has many more massive planets there [2]. Furthermore, the system probably has no Jupiter-like gas giant. In addition, all the planets seem to have almost circular orbits.

So far, astronomers know of fifteen systems with at least three planets. The last record-holder was 55 Cancri, which contains five planets, two of them being giant planets. "Systems of low-mass planets like the one around HD 10180 appear to be quite common, but their formation history remains a puzzle," says Lovis.

Using the new discovery as well as data for other planetary systems, the astronomers found an equivalent of the Titius-Bode law that exists in our Solar System: the distances of the planets from their star seem to follow a regular pattern [3]. "This could be a signature of the formation process of these planetary systems," says team member Michel Mayor.

Another important result found by the astronomers while studying these systems is that there is a relationship between the mass of a planetary system and the mass and chemical content of its host star. All very massive planetary systems are found around massive and metal-rich stars, while the four lowest-mass systems are found around lower-mass and metal-poor stars [4]. Such properties confirm current theoretical models.

The discovery was announced Aug. 24 at the international colloquium "Detection and dynamics of transiting exoplanets," at the Observatoire de Haute-Provence, France.


[1] Using the radial velocity method, astronomers can only estimate a minimum mass for a planet as the mass estimate also depends on the tilt of the orbital plane relative to the line of sight, which is unknown. From a statistical point of view, this minimum mass is however often close to the real mass of the planet.

[2] On average the planets in the inner region of the HD 10180 system have 20 times the mass of the Earth, whereas the inner planets in our own Solar System (Mercury, Venus, Earth and Mars) have an average mass of half that of the Earth.

[3] The Titius-Bode law states that the distances of the planets from the Sun follow a simple pattern. For the outer planets, each planet is predicted to be roughly twice as far away from the Sun as the previous object. The hypothesis correctly predicted the orbits of Ceres and Uranus, but failed as a predictor of Neptune’s orbit.

[4] According to the definition used in astronomy, "metals" are all the elements other than hydrogen and helium. Such metals, except for a very few minor light chemical elements, have all been created by the various generations of stars. Rocky planets are made of "metals."

More information

This research was presented in a paper submitted to Astronomy and Astrophysics ("The HARPS search for southern extra-solar planets. XXVII. Up to seven planets orbiting HD 10180: probing the architecture of low-mass planetary systems" by C. Lovis et al.).

The team is composed of C. Lovis, D. Ségransan, M. Mayor, S. Udry, F. Pepe, and D. Queloz (Observatoire de Genève, Université de Genève, Switzerland), W. Benz (Universität Bern, Switzerland), F. Bouchy (Institut d’Astrophysique de Paris, France), C. Mordasini (Max-Planck-Institut für Astronomie, Heidelberg, Germany), N. C. Santos (Universidade do Porto, Portugal), J. Laskar (Observatoire de Paris, France), A. Correia (Universidade de Aveiro, Portugal), and J.-L. Bertaux (Université Versailles Saint-Quentin, France) and G. Lo Curto (ESO).

Searching For Extrasolar Planets
Astronomers Need Your Help To Find Planets Outside Our Solar System

May 1, 2007 — A new distributed computing program analyzes data to characterize new planetary systems by computing the light reflected from nearby extrasolar planets and the wobble the planets cause in their stars and wobble combinations and compares them to known planet systems.

Systemic Space Search

SYSTEMIC Console Homepage

Systemic Console Download Page

Systemic Console Tutorial Pages




There are 209 known planets outside our solar system, but there are even more out there, and astronomers need help finding them.

Greg Laughlin, Ph.D., an astronomer at the University of California in Santa Cruz, says anyone can help. He explains, "You don’t have to have any experience or knowledge of astronomy, just an interest is all that you really require to help us out."

University of California student Rion Parsons is happy to help look for other worlds. All he needs is a computer, the Internet, and some spare time. Parsons told DBIS, "The whole process of looking is really fun."

The out of this world project is called Systemic. It’s a free web program that lets anyone hunt outer-space data to find our cosmic neighbors, which is not a one man job. Dr. Laughlin explains, "The kinds of computations that we need to do are computations that require a lot of computer power. They can be farmed out to a large number of individual computers."

The process works like this: stars, like our sun, reflect light off orbiting planets. Planets also tug on stars, causing them to wobble. Software measures the light and wobble combinations and compares them to known planet systems. Users submit the information to confirm a new planet.

Laughlin explained why it is fun for users. He says, "If you find a planet that hasn’t been announced and then later find that the data supports your planet, then you have a real thrill of discovery."

More than 4,000 possible planets have been submitted, and four are awaiting confirmation. Parsons is still searching. He says, "I haven’t found anything too amazing yet, but, you know, keep trying."

The American Astronomical Society contributed to the information contained in the TV portion of this report.

BACKGROUND: Astronomers at the University of California, Santa Cruz are seeking the public’s help to find and understand planets outside our solar system. No advanced degree or fancy equipment is needed: just a computer, Internet access, and an interest in astronomy. The project is called Systemic, and it enlists volunteers to identify and explore other planetary systems in the Milky Way. This will help create a virtual database of extrasolar planetary systems. Several hundred people have already volunteered in the project’s introductory phase.

HOW IT WORKS: Systemic is modeled on other successful public participation projects, such as SETI@home, where users download a screensaver that uses their personal computerýs processing power to analyze radio telescope data. But instead of just a screensaver, the astronomers wanted something that would more fully engage the user. The project involves a sophisticated simulation of the search for planets by creating a data set of 100,000 stars.

Participants can analyze this virtual galaxy themselves with freely available software. They can analyze the data for a target star. They can change planetary properties like mass, shape of the planet’s orbit, and the time it takes for the planet to orbit its sun to find a configuration that best fits the data. Complicated large-scale simulation systems with multiple planets require a human eye and patience to arrive at an accurate description, which is a data-intensive, time-consuming process. By comparing the simulated observations with the real observations, the researchers hope to better understand how well, or how poorly, the search process collects a census of extrasolar planets.

WHY IT’S NEEDED: Albert Einstein’s theory of general relativity says that gravity occurs because the mass of a celestial object, like the sun, warps the surrounding space-time. Planets orbiting the star follow that curvature. Astronomers find planets outside our own Solar system by measuring slight wobbles in a star’s motion caused by the gravitational tug of an orbiting planet. Nearly 200 planets have been found orbiting other stars in our galaxy. However, this technique tends to locate planets that are both very large — on a par with Jupiter — and also close to their star. To make the process even slower, astronomers must share time on the few very large telescopes and are limited to observations lasting only a few days. This also limits what parts of the sky astronomers can observe, and thus the current data on planets outside our solar system is incomplete. Systemicýs simulated search uses the same kind of planetary wobble data that astronomers measure, as well as incorporating the observational biases that occur as they collect real data.


First Superstorm on Exoplanet Detected

ScienceDaily (June 23, 2010) — Astronomers have measured a superstorm for the first time in the atmosphere of an exoplanet, the well-studied "hot Jupiter" HD209458b. The very high-precision observations of carbon monoxide gas show that it is streaming at enormous speed from the extremely hot day side to the cooler night side of the planet. The observations also allow another exciting "first" — measuring the orbital speed of the exoplanet itself, providing a direct determination of its mass.

Astronomers have measured a superstorm for the first time in the atmosphere of an exoplanet, the well-studied "hot Jupiter" HD209458b. The very high-precision observations of carbon monoxide gas show that it is streaming at enormous speed from the extremely hot day side to the cooler night side of the planet. This artist’s impression shows the Jupiter-like transiting planet around its solar-like host star. (Credit: ESO/L. Calçada)

The results appear in the journalNature.

"HD209458b is definitely not a place for the faint-hearted. By studying the poisonous carbon monoxide gas with great accuracy we found evidence for a super wind, blowing at a speed of 5000 to 10 000 km per hour," says Ignas Snellen, who led the team of astronomers.

HD209458b is an exoplanet of about 60% the mass of Jupiter orbiting a solar-like star located 150 light-years from Earth towards the constellation of Pegasus (the Winged Horse). Circling at a distance of only one twentieth the Sun-Earth distance, the planet is heated intensely by its parent star, and has a surface temperature of about 1000 degrees Celsius on the hot side. But as the planet always has the same side to its star, one side is very hot, while the other is much cooler. "On Earth, big temperature differences inevitably lead to fierce winds, and as our new measurements reveal, the situation is no different on HD209458b," says team member Simon Albrecht.

HD209458b was the first exoplanet to be found transiting: every 3.5 days the planet moves in front of its host star, blocking a small portion of the starlight during a three-hour period. During such an event a tiny fraction of the starlight filters through the planet’s atmosphere, leaving an imprint. A team of astronomers from the Leiden University, the Netherlands Institute for Space Research (SRON), and MIT in the United States, have used ESO’s Very Large Telescope and its powerful CRIRES spectrograph to detect and analyse these faint fingerprints, observing the planet for about five hours, as it passed in front of its star. "CRIRES is the only instrument in the world that can deliver spectra that are sharp enough to determine the position of the carbon monoxide lines at a precision of 1 part in 100 000," says another team member Remco de Kok. "This high precision allows us to measure the velocity of the carbon monoxide gas for the first time using the Doppler effect."

The astronomers achieved several other firsts. They directly measured the velocity of the exoplanet as it orbits its home star. "In general, the mass of an exoplanet is determined by measuring the wobble of the star and assuming a mass for the star, according to theory. Here, we have been able to measure the motion of the planet as well, and thus determine both the mass of the star and of the planet," says co-author Ernst de Mooij.

Also for the first time, the astronomers measured how much carbon is present in the atmosphere of this planet. "It seems that H209458b is actually as carbon-rich as Jupiter and Saturn. This could indicate that it was formed in the same way," says Snellen. "In the future, astronomers may be able to use this type of observation to study the atmospheres of Earth-like planets, to determine whether life also exists elsewhere in the Universe."


Prospects for Finding New Earths Boosted by Brand New Planet-Finding Technique

ScienceDaily (July 9, 2010) — A team of astronomers from Germany, Bulgaria and Poland have used a completely new technique to find an exotic extrasolar planet. The same approach is sensitive enough to find planets as small as the Earth in orbit around other stars. The group, led by Dr Gracjan Maciejewski of Jena University in Germany, used Transit Timing Variation to detect a planet with 15 times the mass of the Earth in the system WASP-3, 700 light years from the Sun in the constellation of Lyra.

This image shows the faint star WASP-3 (magnitude 10.5 or about 60 times fainter than can be seen with the unaided eye) in the centre of the image, made using the 90-cm telescope of the University Observatory Jena. The star is enlarged with better sensitivity and resolution in the inlay in the lower left. WASP-3 is at a distance of 700 light years and is located in the constellation Lyra. North is up, east to the left. The large image is a composite of three images taken using different filters (blue, visual and red) and the small inlay only uses a red filter. (Credit: Gracan Maciejewski, Dinko Dimitrov, Ralph Neuhäuser, Andrzej Niedzielski et al.)

They publish their work in the journalMonthly Notices of the Royal Astronomical Society.

Transit Timing Variation (TTV) was suggested as a new technique for discovering planets a few years ago. Transits take place where a planet moves in front of the star it orbits, temporarily blocking some of the light from the star. So far this method has been used to detect a number of planets and is being deployed by the Kepler and Corot space missions in its search for planets similar to the Earth.

If a (typically large) planet is found, then the gravity of additional smaller planets will tug on the larger object, causing deviations in the regular cycle of transits. The TTV technique compares the deviations with predictions made by extensive computer-based calculations, allowing astronomers to deduce the makeup of the planetary system.

For this search, the team used the 90-cm telescopes of the University Observatory Jena and the 60-cm telescope of the Rohzen National Astronomical Observatory in Bulgaria to study transits of WASP-3b, a large planet with 630 times the mass of the Earth.

"We detected periodic variations in the transit timing of WASP-3b. These variations can be explained by an additional planet in the system, with a mass of 15 Earth-mass (i.e. one Uranus mass) and a period of 3.75 days," said Dr Maciejewski.

"In line with international rules, we called this new planet WASP-3c." This newly discovered planet is among the least massive planets known to date and also the least massive planet known orbiting a star which is more massive than our Sun.

This is the first time that a new extra-solar planet has been discovered using this method. The new TTV approach is an indirect detection technique, like the previously successful transit method.

A team of astronomers from Germany, Bulgaria and Poland have used a completely new technique to find an exotic extrasolar planet. The same approach is sensitive enough to find planets as small as the Earth in orbit around other stars. The group, led by Dr Gracjan Maciejewski of Jena University in Germany, used Transit Timing Variation to detect a planet with 15 times the mass of the Earth in the system WASP-3, 700 light years from the Sun in the constellation of Lyra. They publish their work in the journal Monthly Notices of the Royal Astronomical Society.

The discovery of the second, 15 Earth-mass planet makes the WASP-3 system very intriguing. The new planet appears to be trapped in an external orbit, twice as long as the orbit of the more massive planet. Such a configuration is probably a result of the early evolution of the system.

The TTV method is very attractive, because it is particularly sensitive to small perturbing planets, even down to the mass of the Earth. For example, an Earth-mass planet will pull on a typical gas giant planet orbiting close to its star and cause deviations in the timing of the larger objects’ transits of up to 1 minute.

This is a big enough effect to be detected with relatively small 1-m diameter telescopes and discoveries can be followed up with larger instruments. The team are now using the 10-m Hobby-Eberly Telescope in Texas to study WASP-3c in more detail.


Fifth Dwarf Planet Named Haumea

ScienceDaily (Sep. 22, 2008) — The International Astronomical Union (the IAU) has announced that the object previously known as 2003 EL61 is to be classified as the fifth dwarf planet in the Solar System and named Haumea.


The decision was made after discussions by members of the International Astronomical Union’s Committee on Small Body Nomenclature (CSBN) and the IAU Working Group for Planetary System Nomenclature (WGPSN). This now means that the family of dwarf planets in the Solar System is up to five. They are now Ceres, Pluto, Haumea, Eris and Makemake.

The discovery of Haumea was announced in mid-2005, and the object was initially given the provisional designation of 2003 EL61. It is a bizarre object with a shape resembling a plump cigar. Its diameter is approximately the same as that of the dwarf planet Pluto; however, its odd shape means that it is much thinner. It is also known to be spinning very fast, making one rotation in about four hours. Some have suggested that this rapid rotation could be the reason Haumea came to look as it does – the dwarf planet has been drawn out and elongated by its swift spin.

Haumea sits among the trans-Neptunian objects, a vast ring of distant cold and rocky bodies in the outer Solar System. At this moment it is roughly 50 times the Sun-Earth distance from the Sun, but at its closest the elliptical orbit of Haumea brings it 35 times the Sun-Earth distance from our star.


Haumea is the name of the goddess of childbirth and fertility in Hawaiian mythology. The name is particularly apt as the goddess Haumea also represents the element of stone and observations of Haumea hint that, unusually, the dwarf planet is almost entirely composed of rock with a crust of pure ice.

Hawaiian mythology says that the goddess Haumea’s children sprang from different parts of her body. The dwarf planet Haumea has a similar history, as it is joined in its orbit by two satellites that are thought to have been created by impacts with it in the past. During these impacts, parts of Haumea’s icy surface were blasted off. The debris from these impacts is then thought to have gone onto form the two moons.

After their discovery, in 2005, the moons were also given provisional designations, but have now too been given names by the CSBN and the WGPSN. The first and largest moon is to be called Hi’iaka, after the Hawaiian goddess who is said to have been born from the mouth of Haumea and the patron goddess of the island of Hawai’i. The second moon of Haumea is named Namaka, a water spirit who is said to have been born from Haumea’s body.


Gas giant

Our solar system’s four gas giants against the Sun, to scale

A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in our Solar System: Jupiter, Saturn, Uranus, andNeptune. Many extrasolar gas giants have been identified orbiting other stars.

Planets above 10 Earth masses are termed giant planets. Below 10 Earth masses they are called super earths or, sometimes probably more accurately for the higher mass examples, "Gas Dwarfs" eg. as suggested by MIT Professor Sara Seager for Gliese 581c using a model where that exoplanet was mostly composed of hydrogen and helium.

Objects above 13 Jupiter masses are called brown dwarfs and these occupy the mass range between that of large gas giant planets and the lowest mass stars.


This cut-away illustrates a model of the interior of Jupiter, with a rocky core overlaid by a deep layer of metallic hydrogen.

A gas giant is a massive planet with a thick atmosphere and a solid core. The "traditional" gas giants, Jupiter and Saturn, are composed primarily ofhydrogen and helium. Uranus and Neptune are sometimes called ice giants, as they are mostly composed of water, ammonia, and methane ices. Among extrasolar planets, Hot Jupiters are gas giants that orbit very close to their stars and thus have a very high surface temperature. Hot Jupiters are currently the most common form of extrasolar planet known, perhaps due to the relative ease of detecting them.

Gas giants are commonly said to lack solid surfaces, but it is closer to the truth to say that they lack surfaces altogether since the gases that make them up simply become thinner and thinner with increasing distance from the planets’ centers, eventually becoming indistinguishable from the interstellar medium. Therefore landing on a gas giant may or may not be possible, depending on the size and composition of its core.

Belt-zone circulation

The bands seen in the Jovian atmosphere are due to counter-circulating streams of material called zones and belts, encircling the planet parallel to its equator. The zones are the lighter bands, and are at higher altitudes in the atmosphere. They have an internal updraft, and are high-pressure regions. The belts are the darker bands. They are lower in the atmosphere, and have an internal downdraft. They are low-pressure regions. These structures are somewhat analogous to high- and low-pressure cells in Earth’s atmosphere, but they have a very different structure — latitudinal bands that circle the entire planet, as opposed to small confined cells of pressure. This appears to be a result of the rapid rotation and underlying symmetry of the planet. There are no oceans or landmasses to cause local heating, and the rotation speed is much faster than it is on Earth. There are smaller structures as well: spots of different sizes and colors. On Jupiter, the most noticeable of these features is the Great Red Spot, which has been present for at least 300 years. These structures are huge storms. Some such spots are thunderheads as well.

Relative masses of the gas giants of the Solar System

Jupiter and Saturn

Jupiter and Saturn consist mostly of hydrogen and helium, with heavier elements making up between 3 and 13 percent of the mass.Their structures are thought to consist of an outer layer of molecular hydrogen, surrounding a layer of liquid metallic hydrogen, with a probable rocky core. The outermost portion of the hydrogen atmosphere is characterized by many layers of visible clouds that are mostly composed of water and ammonia. The metallic hydrogen layer makes up the bulk of each planet, and is described as "metallic" because the great pressure turns hydrogen into an electrical conductor. The core is thought to consist of heavier elements at such high temperatures (20,000 K) and pressures that their properties are poorly understood.

Uranus and Neptune

Uranus and Neptune have distinctly different interior compositions from Jupiter and Saturn. Models of their interior begin with a hydrogen-rich atmosphere that extends from the cloud-tops down to about 85% of Neptune’s radius and 80% of Uranus’. Below this point is predominantly "icy", composed of water, methane and ammonia. There is also some rock and gas but various proportions of ice/rock/gas could mimic pure ice so the exact proportions are unknown.

Very hazy atmosphere layers with a small amount of methane gives them aquamarine colors such as baby blue and ultramarine colors respectively. Both have magnetic fields that are sharply inclined to their axes of rotation.

Unlike the other gas giants, Uranus has an extreme tilt that causes its seasons to be severely pronounced.

Extrasolar gas giants

An artist’s conception of 79 Ceti b, the first extrasolar gas giant found with a mass less than the mass of Saturn

See also: Hot Jupiter

Because of the limited techniques currently available to detect extrasolar planets, many of those found to date have been of a size associated, in our solar system, with gas giants. Because these large planets are inferred to share more in common with Jupiter than with the other gas giant planets some have claimed that "Jovian planet" is a more accurate term for them. Many of the extrasolar planets are much closer to their parent stars and hence much hotter than gas giants in the solar system, making it possible that some of those planets are a type not observed in our solar system. Considering the relative abundances of the elements in the universe (approximately 98% hydrogen and helium) it would be surprising to find a predominantly rocky planet more massive than Jupiter. On the other hand previous models of planetary system formation suggested that gas giants would be inhibited from forming as close to their stars as have many of the new planets that have been observed.

Smaller gas planets

Comparison of sizes of planets with different compositions

Although the words "gas" and "giant" are often combined, hydrogen planets need not be as large as the familiar gas giants from our solar system. However smaller gas planets and planets closer to their star will lose atmospheric mass more quickly viahydrodynamic escape than larger planets or planets farther out.


The term gas giant was coined in 1952 by the science fiction writer James Blish. Arguably it is something of a misnomer, since throughout most of the volume of these planets all the components (other than solid materials in the core) are above the critical point and therefore there is no distinction between liquids and gases. Fluid planet would be a more accurate term. Jupiter is an exceptional case, having metallic hydrogen near the center, but much of its volume is hydrogen, helium and traces of other gases above their critical points. The observable atmospheres of any of these planets (at less than unit optical depth) are quite thin compared to the planetary radii, only extending perhaps one percent of the way to the center. Thus the observable portions are gaseous (in contrast to Mars and Earth, which have gaseous atmospheres through which the crust may be seen).

The rather misleading term has caught on because planetary scientists typically use "rock", "gas", and "ice" as shorthands for classes of elements and compounds commonly found as planetary constituents, irrespective of what phase they appear in. In the outer solar system, hydrogen and helium are "gases"; water, methane, and ammonia are "ices"; and silicates and metals are rock. When deep planetary interiors are considered, it may not be far off to say that, by "ice" astronomers mean oxygen and carbon, by "rock" they mean silicon, and by "gas" they mean hydrogen and helium.

The alternative term Jovian planet refers to the Roman god Jupiter—the genitive form of which is Jovis, hence Jovian—and was intended to indicate that all of these planets were similar to Jupiter. However, the many ways in which Uranus and Neptune differ from Jupiter and Saturn have led some to use the term only for the inner two.[who?]

With this terminology in mind, some astronomers are starting to refer to Uranus and Neptune as "ice giants" to indicate the apparent predominance of the "ices" (in liquid form) in their interior composition.