The inner Solar System is the region comprising the terrestrial planets and the asteroids.^[89] Composed mainly of silicates and metals,^[90] the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is less than the distance between the orbits of Jupiter and Saturn. This region is within the frost line, which is a little less than 5 AU from the Sun.^[45]

Inner planets

The four terrestrial or inner planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of refractory minerals such as silicates—which form their crusts and mantles—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 and tectonic surface features, such as rift valleys and volcanoes.^[91]

Mercury (0.31–0.59 AU from the Sun)^{[D 6]} is the smallest planet in the Solar System. Its surface is grayish, with an expansive rupes (cliff) system generated from thrust faults and bright ray systems formed by impact event remnants.^[92] The surface has widely varying temperature, with the equatorial regions ranging from −170 °C (−270 °F) at night to 420 °C (790 °F) during sunlight. In the past, Mercury was volcanically active, producing smooth basaltic plains similar to the Moon.^[93] It is likely that Mercury has a silicate crust and a large iron core.^[94]^[95] Mercury has a very tenuous atmosphere, consisting of solar-wind particles and ejected atoms.^[96] Mercury has no natural satellites.^[97]
Venus (0.72–0.73 AU)^{[D 6]} has a reflective, whitish atmosphere that is mainly composed of carbon dioxide. At the surface, the atmospheric pressure is ninety times as dense as on Earth's sea level.^[98] Venus has a surface temperatures over 400 °C (752 °F), mainly due to the amount of greenhouse gases in the atmosphere.^[99] The planet lacks a protective magnetic field to protect against stripping by the solar wind, which suggests that its atmosphere is sustained by volcanic activity.^[100] Its surface displays extensive evidence of volcanic activity with stagnant lid tectonics.^[101] Venus has no natural satellites.^[97]
Earth (0.98–1.02 AU)^{[D 6]} is the only place in the universe where life and surface liquid water are known to exist.^[102] Earth's atmosphere contains 78% nitrogen and 21% oxygen, which is the result of the presence of life.^[103]^[104] The planet has a complex climate and weather system, with conditions differing drastically between climate regions.^[105] The solid surface of Earth is dominated by green vegetation, deserts and white ice sheets.^[106]^[107]^[108] Earth's surface is shaped by plate tectonics that formed the continental masses.^[93] Earth's planetary magnetosphere shields the surface from radiation, limiting atmospheric stripping and maintaining life habitability.^[109]
- The Moon is Earth's only natural satellite.^[110] Its diameter is one-quarter the size of Earth's.^[111] Its surface is covered in very fine regolith and dominated by impact craters.^[112]^[113] Large dark patches on the Moon, maria, are formed from past volcanic activity.^[114] The Moon's atmosphere is extremely thin, consisting of a partial vacuum with particle densities of under 10⁷ per cm⁻³.^[115]
Mars (1.38–1.67 AU)^{[D 6]} has a radius about half of that of Earth.^[116] Most of the planet is red due to iron oxide in Martian soil,^[117] and the polar regions are covered in white ice caps made of water and carbon dioxide.^[118] Mars has an atmosphere composed mostly of carbon dioxide, with surface pressure 0.6% of that of Earth, which is sufficient to support some weather phenomena.^[119] During the Mars year (687 Earth days), there are large surface temperature swings on the surface between −78.5 °C (−109.3 °F) to 5.7 °C (42.3 °F). The surface is peppered with volcanoes and rift valleys, and has a rich collection of minerals.^[120]^[121] Mars has a highly differentiated internal structure, and lost its magnetosphere 4 billion years ago.^[122]^[123] Mars has two tiny moons:^[124]
- Phobos is Mars's inner moon. It is a small, irregularly shaped object with a mean radius of 11 km (7 mi). Its surface is very unreflective and dominated by impact craters.^{[D 7]}^[125] In particular, Phobos's surface has a very large Stickney impact crater that is roughly 4.5 km (2.8 mi) in radius.^[126]
- Deimos is Mars's outer moon. Like Phobos, it is irregularly shaped, with a mean radius of 6 km (4 mi) and its surface reflects little light.^{[D 8]}^{[D 9]} However, the surface of Deimos is noticeably smoother than Phobos because the regolith partially covers the impact craters.^[127]

Asteroids

Asteroids except for the largest, Ceres, are classified as small Solar System bodies and are composed mainly of carbonaceous, refractory rocky and metallic minerals, with some ice.^[128]^[129] They range from a few meters to hundreds of kilometers in size. Many asteroids are divided into asteroid groups and families based on their orbital characteristics. Some asteroids have natural satellites that orbit them, that is, asteroids that orbit larger asteroids.^[130]

Mercury-crossing asteroids are those with perihelia within the orbit of Mercury. At least 362 are known to date, and include the closest objects to the Sun known in the Solar System.^[131] No vulcanoids, asteroids between the orbit of Mercury and the Sun, have been discovered.^[132]^[133] As of 2024, one asteroid has been discovered to orbit completely within Venus's orbit, 594913 ꞌAylóꞌchaxnim.^[134]
Venus-crossing asteroids are those that cross the orbit of Venus. There are 2,809 as of 2015.^[135]
Near-Earth asteroids have orbits that approach relatively close to Earth's orbit,^[136] and some of them are potentially hazardous objects because they might collide with Earth in the future.^[137]^[138]
Mars-crossing asteroids are those with perhihelia above 1.3 AU which cross the orbit of Mars.^[139] As of 2024, NASA lists 26,182 confirmed Mars-crossing asteroids.^[135]

Asteroid belt

The asteroid belt occupies a torus-shaped region between 2.3 and 3.3 AU from the Sun, which lies between the orbits of Mars and Jupiter. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.^[140] The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometer in diameter.^[141] Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth.^[40] The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident.^[142]

Below are the descriptions of the three largest bodies in the asteroid belt. They are all considered to be relatively intact protoplanets, a precursor stage before becoming a fully-formed planet (see List of exceptional asteroids):^[143]^[144]^[145]

Ceres (2.55–2.98 AU) is the only dwarf planet in the asteroid belt.^[146] It is the largest object in the belt, with a diameter of 940 km (580 mi).^[147] Its surface contains a mixture of carbon,^[148] frozen water and hydrated minerals.^[149] There are signs of past cryovolcanic activity, where volatile material such as water are erupted onto the surface, as seen in surface bright spots.^[150] Ceres has a very thin water vapor atmosphere, but practically speaking it is indistinguishable from a vacuum.^[151]
Vesta (2.13–3.41 AU) is the second-largest object in the asteroid belt.^[152] Its fragments survive as the Vesta asteroid family^[153] and numerous HED meteorites found on Earth.^[154] Vesta's surface, dominated by basaltic and metamorphic material, has a denser composition than Ceres's.^[155] Its surface is marked by two giant craters: Rheasilvia and Veneneia.^[156]
Pallas (2.15–2.57 AU) is the third-largest object in the asteroid belt.^[152] It has its own Pallas asteroid family.^[153] Not much is known about Pallas because it has never been visited by a spacecraft,^[157] though its surface is predicted to be composed of silicates.^[158]

Hilda asteroids are in a 3:2 resonance with Jupiter; that is, they go around the Sun three times for every two Jovian orbits.^[159] They lie in three linked clusters between Jupiter and the main asteroid belt.

Trojans are bodies located within another body's gravitationally stable Lagrange points: L₄, 60° ahead in its orbit, or L₅, 60° behind in its orbit.^[160] Every planet except Mercury and Saturn is known to possess at least 1 trojan.^[161]^[162]^[163] The Jupiter trojan population is roughly equal to that of the asteroid belt.^[164] After Jupiter, Neptune possesses the most confirmed trojans, at 28.^[165]