The Solar System also contains smaller objects. The asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, and beyond them a newly discovered population of sednoids. Within these populations, some objects are large enough to have rounded under their own gravity, though there is considerable debate as to how many there will prove to be. Such objects are categorized as dwarf planets. The only certain dwarf planet is Pluto, with another trans-Neptunian object, Eris, expected to be, and the asteroid Ceres at least close to being a dwarf planet. In addition to these two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds, freely travel between regions. Six of the planets, the six largest possible dwarf planets, and many of the smaller bodies are orbited by natural satellites, usually termed "moons" after the Moon. Each of the outer planets is encircled by planetary rings of dust and other small objects.
The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium; it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere. The Solar System is located in the Orion Arm, 26,000 light-years from the center of the Milky Way galaxy.
The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy mainly as light and infrared radiation. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometres (864,000 miles), or 109 times that of Earth, and its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System.
Roughly three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.
Saturneclipsing the Sun, as seen by the Cassini orbiter. Individual rings seen in this image include (in order, starting from most distant): E ring, Pallene ring (visible very faintly in an arc just below Saturn), G ring, Janus/Epimetheus ring (faint), F ring (narrow brightest feature), Main rings (A,B,C), and D ring (bluish, nearest Saturn). Interior to the G ring and above the brighter main rings is the pale dot of Earth
The geology of the contact binary object Arrokoth (nicknamed Ultima Thule), the first undisturbed planetesimal visited by a spacecraft, with comet 67P to scale. The eight subunits of the larger lobe, labeled ma to mh, are thought to have been its building blocks. The two lobes came together later, forming a contact binary. Objects such as Arrokoth are believed in turn to have formed protoplanets.
Orrery showing the motions of the inner four planets. The small spheres represent the position of each planet on every Julian day, beginning 6 July 2018 (aphelion) and ending 3 January 2019 (perihelion).
Relative size of the Sun as it is now (inset) compared to its estimated future size as a red giant
Meteor Crater in Arizona. Created 50,000 years ago by an impactor about 50 metres (160 ft) across, it shows that the accretion of the Solar System is not over.
Simulation showing outer planets and Kuiper belt: a) Before Jupiter/Saturn 2:1 resonance b) Scattering of Kuiper belt objects into the Solar System after the orbital shift of Neptune c) After ejection of Kuiper belt bodies by Jupiter
Orbit of Jupiter
Orbit of Saturn
Orbit of Uranus
Orbit of Neptune
Neptune and its moon Triton, taken by Voyager 2. Triton's orbit will eventually take it within Neptune's Roche limit, tearing it apart and possibly forming a new ring system.
Schematic of the hypothetical Oort cloud, with a spherical outer cloud and a disc-shaped inner cloud
Beyond the heliosphere is the interstellar medium, consisting of various clouds of gases. The Solar System currently moves through the Local Interstellar Cloud.
Andreas Cellarius's illustration of the Copernican system, from the Harmonia Macrocosmica (1660)
The Ring nebula, a planetary nebula similar to what the Sun will become
Orrery showing the motions of the outer four planets. The small spheres represent the position of each planet on every 100 Julian days, beginning 21 January 2023 (Jovian perihelion) and ending 2 December 2034 (Jovian perihelion).