Solar System

See also: List of Solar System bodies in hydrostatic equilibrium

The terrestrial planets: Mercury, Venus, Earth, Mars (Sizes to scale)

The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune (Sizes to scale)

According to the IAU's current definitions, there are eight planets in the Solar System. In increasing distance from the Sun, they are:

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune

Jupiter is the largest, at 318 Earth masses, while Mercury is smallest, at 0.055 Earth masses.

The planets of the Solar System can be divided into categories based on their composition:

• Terrestrials: Planets that are similar to Earth, with bodies largely composed of rock: Mercury, Venus, Earth and Mars.
• Gas giants: Planets with a composition largely made up of gaseous material and are significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. Ice giants, comprising Uranus and Neptune, are a sub-class of gas giants, distinguished from gas giants by their significantly lower mass, and by depletion in hydrogen and helium in their atmospheres together with a significantly higher proportion of rock and ice.

Planetary attributes

	Name	Equatorial diameter[a]	Mass[a]	Orbital radius (AU)	Orbital period (years)	Inclination to Sun's equator (°)	Orbital eccentricity	Rotation period (days)	Named moons	Rings	Atmosphere
Terrestrials	Mercury	0.382	0.06	0.39	0.24	3.38	0.206	58.64	—	no	minimal
	Venus	0.949	0.82	0.72	0.62	3.86	0.007	-243.02	—	no	CO2, N2
	Earth[b]	1.00	1.00	1.00	1.00	7.25	0.017	1.00	1	no	N2, O2
	Mars	0.532	0.11	1.52	1.88	5.65	0.093	1.03	2	no	CO2, N2
Gas giants	Jupiter	11.209	317.8	5.20	11.86	6.09	0.048	0.41	49	yes	H2, He
	Saturn	9.449	95.2	9.54	29.46	5.51	0.054	0.43	52	yes	H2, He
	Uranus	4.007	14.6	19.22	84.01	6.48	0.047	-0.72	27	yes	H2, He
	Neptune	3.883	17.2	30.06	164.8	6.43	0.009	0.67	13	yes	H2, He
a Measured relative to the Earth. b See Earth article for absolute values.

Dwarf planets

Main article: Dwarf planet

Before the August 2006 decision, several objects were proposed by astronomers, including at one stage by the IAU, as planets. However in 2006 several of these objects were reclassified as dwarf planets, objects distinct from planets. Currently five dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto, Haumea, Makemake and Eris. Several other objects in both the Asteroid belt and the Kuiper belt are under consideration, with as many as 50 that could eventually qualify. There may be as many as 200 that could be discovered once the Kuiper belt has been fully explored. Dwarf planets share many of the same characteristics as planets, although notable differences remain – namely that they are not dominant in their orbits. Their attributes are:

Dwarf planetary attributes

Name	Equatorial diameter[c]	Mass[c]	Orbital radius (AU)	Orbital period (years)	Inclination to ecliptic (°)	Orbital eccentricity	Rotation period (days)	Moons	Rings	Atmosphere
Ceres	0.08	0.000 2	2.5–3.0	4.60	10.59	0.080	0.38	0	no	none
Pluto	0.19	0.002 2	29.7–49.3	248.09	17.14	0.249	−6.39	3	no	temporary
Haumea	0.37×0.16	0.000 7	35.2–51.5	285.38	28.19	0.189	0.16	2
Makemake	~0.12	0.000 7	38.5–53.1	309.88	28.96	0.159	?	0	?	? [d]
Eris	0.19	0.002 5	37.8–97.6	~557	44.19	0.442	~0.3	1	?	? [d]
c Measured relative to the Earth. d A temporary atmosphere is suspected but has not yet been directly observed by stellar occultation.

By definition, all dwarf planets are members of larger populations. Ceres is the largest body in the asteroid belt, while Pluto and Makemake are members of the Kuiper belt and Eris is a member of the scattered disc. Scientists such as Mike Brown believe that there may soon be over forty trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition.^[58]

Extrasolar planets

Main article: Extrasolar planet

HR 8799, the first extrasolar planetary system to be directly imaged

The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi. Of the 306 extrasolar planets discovered by August 2008, most have masses which are comparable to or larger than Jupiter's, though masses ranging from just below that of Mercury to many times Jupiter's mass have been observed.^[59] The smallest extrasolar planets found to date have been discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12.^[60] There have been roughly a dozen extrasolar planets found of between 10 and 20 Earth masses,^[59] such as those orbiting the stars Mu Arae, 55 Cancri and GJ 436.^[61] These planets have been nicknamed "Neptunes" because they roughly approximate that planet's mass (17 Earths).^[62] Another new category are the so-called "super-Earths", possibly terrestrial planets far larger than Earth but smaller than Neptune or Uranus. To date, five possible super-Earths have been found: Gliese 876 d, which is roughly six times Earth's mass,^[63] OGLE-2005-BLG-390Lb and MOA-2007-BLG-192Lb, frigid icy worlds discovered through gravitational microlensing,^[64][65] and two planets orbiting the nearby red dwarf Gliese 581. Gliese 581 d is roughly 7.7 times Earth's mass,^[66] while Gliese 581 c is five times Earth's mass and the first terrestrial planet found within a star's habitable zone.^[67]

It is far from clear if the newly discovered large planets would resemble the gas giants in the Solar System or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. In particular, some of the newly-discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in the Solar System, which makes it questionable whether they are the same type of planet at all. There may also exist a class of hot Jupiters, called Chthonian planets, that orbit so close to their star that their atmospheres have been blown away completely by stellar radiation. While many hot Jupiters have been found in the process of losing their atmospheres, as of 2008, no genuine Chthonian planets have been discovered.^[68]

More detailed observation of extrasolar planets will require a new generation of instruments, including space telescopes. Currently the COROT spacecraft is searching for stellar luminosity variations due to transiting planets. Several projects have also been proposed to create an array of space telescopes to search for extrasolar planets with masses comparable to the Earth. These include the proposed NASA's Kepler Mission, Terrestrial Planet Finder, and Space Interferometry Mission programs, the ESA's Darwin, and the CNES' PEGASE.^[69] The New Worlds Mission is an occulting device that may work in conjunction with the James Webb Space Telescope. However, funding for some of these projects remains uncertain. The first spectra of extrasolar planets were reported in February 2007 (HD 209458 b and HD 189733 b).^[70][71] The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy.

planet

A planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.[a][1][2]

The term planet is ancient, with ties to history, science, myth, and religion. The planets were originally seen by many early cultures as divine, or as emissaries of the gods. Even today, many people believe in astrology, which holds that the movement of the planets affects people's lives, although such a causation is rejected by the scientific community. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. Even now there is no uncontested definition of what a planet is. In 2006, the IAU officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticized, and remains disputed by some scientists.

The planets were thought by Ptolemy to orbit the Earth in deferent and epicycle motions. Though the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. By careful analysis of the observation data, Johannes Kepler found the planets' orbits to be not circular, but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some share such features as ice-caps and seasons. Since the dawn of the Space Age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology. Since 1992, through the discovery of hundreds of extrasolar planets (planets around other stars), scientists are beginning to understand that planets throughout the Milky Way Galaxy share characteristics in common with our own.

Planets are generally divided into two main types: large, low-density gas giants, and smaller, rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System. In order from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four gas giants, Jupiter, Saturn, Uranus, and Neptune. Many of these planets are orbited by one or more moons, which can be larger than small planets. As of January 13th 2009, there are 335 known extrasolar planets, ranging from the size of gas giants to that of terrestrial planets.[3] This brings the total number of identified planets to at least 343. The Solar System also contains at least five dwarf planets: Ceres, Pluto (formerly considered to be the Solar System's ninth planet), Makemake, Haumea and Eris. No extrasolar dwarf planets have yet been detected.