In general, a solar system is an astronomical feature consisting of a central star together with the planets and other celestial bodies that are held in orbit around it by gravitational attraction. The term often refers to the particular solar system to which our earth belongs. This is dominated by our sun, as the central star, and contains a large number of different objects, including planets, dwarf planets, SSSBs (small solar system bodies), etc. In any solar system, some objects are under secondary gravitational influence, i.e. they are satellites (such as our moon) and are influenced by their parent body, although they also accompany their parent in its orbit around the sun.

The solar system itself is part of a larger system, the Galaxy, which is itself rotating. Our solar system rotates around the centre of the Galaxy once every 2.2×10⁸ years.

Origin of the solar system

The current theory of the origin of the solar system is called the nebular hypothesis. It holds that the system originated in a large cloud of gas and dust (a nebula), which collapsed under its own gravitational attraction to form the sun and its associated celestial objects. It is possible that this collapse was initiated by nearby supernova explosions. As the nebular cloud collapsed, it formed a flat spinning disc with the sun at its centre (which is why the planets mostly occupy the same plane in space). The planets continued to grow by accretion and by about 4.6 billion years ago the earth had formed.

Regions of the solar system

It is convenient to partition the solar system into three main parts:

• • the inner solar system, containing the planets Mercury, Venus, earth, and Mars, together with the main asteroid belt.

• • the outer solar system (above 5 au), containing the planets Jupiter, Saturn, Uranus, and Neptune, together with their planets and other objects.

• • the trans-Neptunian region (above 30 au), essentially consisting of the region lying outside the orbit of Neptune.

Note that this is not an official classification and some writers speak of the ‘inner solar system’ as the region containing the main planets and the ‘outer solar system’ as the largely unmapped trans-Neptunian region. The trans-Neptunian region may be partitioned further into three other regions:

• • the Kuiper belt ranging from 3–50 au, discovered in 1992 and named after the Dutch–American astronomer Gerard Kuiper (1905–73), who had speculated about trans-Neptunian bodies in the early 1950s. The belt is home to thousands of objects, known as Kuiper belt objects or KBOs. This region includes a number of significant astronomical objects; for example, Pluto, which was originally regarded as a planet, is a Kuiper belt object (and is now classified as a dwarf planet).

• • the scattered disk lying outside the Kuiper belt and probably the region in which most periodic comets originate. Some authors regard the scattered disk as an outer part of the Kuiper belt.

• • the Oort cloud, a hypothetical region way outside the Kuiper belt and scattered disk, about 1 light year from the sun, thought to be the origin of long-period comets. Originally proposed in 1932 by the Estonian astronomer Ernst Öpik (1893–1985), it is named after the Dutch astronomer Jan Hendrik Oort (1900–92), who revived the idea in 1950.

The planets

There are eight bodies now recognised as planets, and they are divided into two groups. The inner or terrestrial planets—Mercury, Venus, earth, and Mars—are comparatively small. They are composed of rock and metal, with the metal part forming a dense central core. Mercury and Venus have no natural satellites. The earth has one satellite (the moon) and Mars has two (Phobos and Deimos). These inner-planet satellites may all be captured asteroids.

The outer or giant planets—Jupiter, Saturn, Uranus, and Neptune—have a rock and metal core surrounded by layers of solid, liquid, and gaseous hydrogen and helium. They are much further from the sun and therefore much cooler and consequently have been able to retain gases of low density. All have large numbers of satellites: Jupiter has at least 63; Saturn 60; Uranus 27; and Neptune 13. The outer planets also have ring systems composed of smaller bodies, rocks, dust, and ice particles.

The four largest satellites of Jupiter—Ganymede, Callisto, Io, and Europa are known as the Galilean satellites after their discovery by Galileo in 1610. Ganymede is the largest natural satellite in the solar system followed by Titan (Saturn), Callisto (Jupiter), Io (Jupiter), the Moon (earth), Europa (Jupiter), Triton (Neptune), Titania (Uranus), Rhea (Saturn), and Oberon (Uranus).

Dwarf planets

These are bodies that are in direct orbit around the sun and have reached hydrostatic equilibrium, but have not cleared their neighbourhood. There are five objects presently classified as dwarf planets:

• • Pluto—a Kuiper-belt object (KBO), considered to be a planet until 2006. It is named after the Greek god of the underworld.

• • Makemake—another KBO, discovered in 2005 and about 75% of the size of Pluto. Makemake is the name of the creator of humanity in the mythology of Easter Island.

• • Ceres—the largest member of the asteroid belt, named after the Roman goddess of planets.

• • Eris—the largest dwarf planet, about 27% larger than Pluto, found in the scattered disk. It is named after the Greek goddess for strife.

• • Haumea—a KBO about one third the mass of Pluto. It is named after the Hawaiian goddess of fertility.

There are many other objects that are candidates for dwarf planet status. They include a number of satellites of the outer planets and some named trans-Neptunian objects (Orcus, Ixion, Huya, Varuna, Quaoar, and Sedna).

Other objects

The space between the orbits of Mars and Jupiter is occupied by thousands of asteroids, or minor planets. Most of these are small, often irregularly shaped chunks of rock, with perhaps only 150 of them more than 100 km across. Even smaller are hundreds of thousands of meteoroids, some no larger than grains of dust. Those that enter the earth’s atmosphere and burn up as trails of light (shooting stars) are termed meteors. The largest ones that reach the ground are called meteoroids.

The other important members of the solar system are comets. They consist of a nucleus of dust and ice a kilometre or two across, surrounded by a gaseous coma and with a tail that appears as the comet nears the sun. A dusty tail behind the comet is the source of many meteoroids.

https://ssd.jpl.nasa.gov/?bodies Data for the solar system from NASA/JPL