Solar System showing plane Solar System showing the planeof the Earths orbit around the of the ecliptic of the Earths orbitSun in 3D view with only around the Sun in 3D viewMercury, Venus, Earth and showing Mercury, Venus, Earth,Mars. Mars and Jupiter making one full revolution. Saturn and Uranus also appear in their own respective orbits around the Sun.
The Solar System is also home to a number of regions populated by smallerobjects. The asteroid belt, which lies between Mars and Jupiter, is similar tothe terrestrial planets as it is composed mainly of rock and metal. BeyondNeptunes orbit lie the Kuiper belt and scattered disc; linked populationsof trans-Neptunian objects composed mostly of ices such as water, ammoniaand methane. Within these populations, five individualobjects, Ceres, Pluto, Haumea, Makemake and Eris, are recognized to belarge enough to have been rounded by their own gravity, and are thustermed dwarf planets.
This asteroid beltlies betweenMars and Jupiter.
MercuryMercury is the closest planet to theSun and the smallest planet in theSolar System. Mercury has no naturalsatellites, and its only knowngeological features besides impactcraters are lobed ridges or rupes,probably produced by a period ofcontraction early in itshistory. Mercurys almost negligibleatmosphere consists of atoms blastedoff its surface by the solar wind. Itsrelatively large iron core and thinmantle have not yet been adequatelyexplained.
VenusVenus is close in size to Earth, and, likeEarth, has a thick silicate mantle aroundan iron core, a substantial atmosphereand evidence of internal geologicalactivity. However, it is much drier thanEarth and its atmosphere is ninety timesas dense. Venus has no natural satellites.It is the hottest planet, with surfacetemperatures over 400 °C, most likely dueto the amount of greenhouse gases in theatmosphere. No definitive evidence ofcurrent geological activity has beendetected on Venus, but it has no magneticfield that would prevent depletion of itssubstantial atmosphere, which suggeststhat its atmosphere is regularlyreplenished by volcanic eruptions.
EarthEarth is the largest and densest ofthe inner planets, the only oneknown to have current geologicalactivity, and is the only place in theSolar System where life is known toexist. Its liquid hydrosphere isunique among the terrestrialplanets, and it is also the only planetwhere plate tectonics has beenobserved. Earths atmosphere isradically different from those of theother planets, having been alteredby the presence of life to contain21% free oxygen. It has one naturalsatellite, the Moon, the only largesatellite of a terrestrial planet in theSolar System.
MarsMars is smaller than Earth and Venus. Itpossesses an atmosphere ofmostly carbon dioxide with a surfacepressure of 6.1 mill bars. Its surface,peppered with vast volcanoes suchas Olympus Mons and rift valleys suchas Valles Marineris, shows geologicalactivity that may have persisted until asrecently as 2 million years ago. Its redcolour comes from iron oxide (rust) in itssoil. Mars has two tiny natural satellites(Deimos and Phobos) thought to becaptured asteroids.
JupiterJupiter, at 318 Earth masses, is 2.5times the mass of all the other planetsput together. It is composed largelyof hydrogen and helium. Jupiters stronginternal heat creates a number of semi-permanent features in its atmosphere,such as cloud bands and the Great RedSpot. Jupiter has 66 known satellites.The fourlargest, Ganymede, Callisto, Io,and Europa, show similarities to theterrestrial planets, such as volcanismand internal heating. Ganymede, thelargest satellite in the Solar System, islarger than Mercury.
SaturnSaturn, distinguished by its extensive ringsystem, has several similarities to Jupiter,such as its atmospheric composition andmagnetosphere. Although Saturn has 60%of Jupiters volume, it is less than a thirdas massive, at 95 Earth masses, making itthe least dense planet in the SolarSystem. The rings of Saturn are made upof small ice and rock particles. Saturnhas 62 confirmed satellites; two ofwhich, Titan and Enceladus, show signs ofgeological activity, though they arelargely made of ice. Titan, the second-largest moon in the Solar System, is largerthan Mercury and the only satellite in theSolar System with a substantialatmosphere.
UranusUranus, at 14 Earth masses, isthe lightest of the outerplanets. Uniquely among theplanets, it orbits the Sun on itsside; its axial tilt is over ninetydegrees to the ecliptic. It has amuch colder core than theother gas giants, and radiatesvery little heat into space.Uranus has 27 knownsatellites, the largest onesbeing Titania, Oberon, Umbriel, Ariel and Miranda.
NeptuneNeptune, though slightly smaller thanUranus, is more massive (equivalent to 17Earths) and therefore more dense. Itradiates more internal heat, but not asmuch as Jupiter or Saturn. Neptunehas 13 known satellites. Thelargest, Triton, is geologically active,with geysers of liquid nitrogen. Triton isthe only large satellite with a retrogradeorbit. Neptune is accompanied in its orbitby a number of minor planets,termed Neptune Trojans, that are in1:1 resonance with it.