The Solar System is the planetary system consisting of the Sun and the objects bound to it by gravity, including eight planets, their natural satellites, dwarf planets, asteroids, comets, and smaller particles. It formed approximately 4.6 billion years ago and lies within the Milky Way galaxy. Its structure extends far beyond the planets, encompassing distant populations of icy bodies and a vast region influenced by the Sun’s outflowing particles. (science.nasa.gov)
The Sun and orbital structure
The Sun contains about 99.8 percent of the Solar System’s mass. It is a star composed predominantly of hydrogen and helium, generating energy through nuclear fusion in its core. Its gravitational attraction dominates planetary motion, while its radiation supplies the principal external energy source for planetary surfaces and atmospheres. (science.nasa.gov)
Distances are commonly expressed in astronomical units (AU), with one AU approximately equal to 150 million kilometres, the characteristic distance between the Sun and Earth. Neptune orbits approximately 30 AU from the Sun. Most planets follow nearly circular paths in approximately the same plane and travel in the same direction, reflecting their origin in a rotating disk. Comets and some smaller bodies can have much more elongated or steeply inclined orbits. (science.nasa.gov)
Planetary motion follows Kepler’s laws to a close approximation: an orbit is an ellipse with the Sun at one focus, an orbiting body sweeps out equal areas in equal times, and the square of its orbital period is proportional to the cube of its semimajor axis. Mutual gravitational interactions introduce departures from simple two-body orbits. More distant planets consequently take longer to complete a revolution around the Sun. (science.nasa.gov)
Planets and their classification
In increasing order of distance from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The four inner planets are terrestrial worlds, composed mainly of rock and metal and possessing solid surfaces. Mercury and Venus have no natural satellites; Earth has the Moon, while Mars has two small moons. Their atmospheres, surface conditions, and geological histories differ substantially. (science.nasa.gov)
Jupiter and Saturn are gas giants whose bulk composition is dominated by hydrogen and helium. Uranus and Neptune are classified as ice giants because heavier compounds, including water, ammonia, and methane, constitute a larger proportion of their interiors. “Ice” here describes a compositional category, not a claim that their deep interiors are frozen. All four giant planets possess rings and multiple moons. (science.nasa.gov)
The International Astronomical Union’s 2006 definition requires a Solar System planet to orbit the Sun, be sufficiently massive for self-gravity to produce a nearly round shape, and have cleared its orbital neighbourhood. A dwarf planet meets the first two conditions but not the third and is not a satellite. This distinction concerns orbital dynamics as well as physical form; dwarf planets are a separate category rather than a subdivision of the eight planets. (iau.org)
Small bodies and outer reservoirs
The main asteroid belt, between Mars and Jupiter, contains rocky remnants of planetary formation. Ceres is its largest member and a dwarf planet. Smaller bodies also occupy many other regions, including orbits near Earth and populations associated with the giant planets. Their varied compositions preserve evidence of conditions in the early Solar System. (science.nasa.gov)
Beyond Neptune lies the Kuiper Belt, a flattened population of predominantly icy bodies. Its main region extends approximately from 30 to 50 AU, although associated populations reach farther. Pluto is a prominent member. More distant objects can follow elongated orbits shaped by gravitational encounters with Neptune and the other giant planets. (science.nasa.gov)
The Oort Cloud is a hypothesized, roughly spherical reservoir of icy bodies thousands to tens of thousands of AU away, possibly extending to about 100,000 AU. It has not been directly observed as a population; its existence is inferred chiefly from long-period comet orbits. Passing stars and the galaxy’s gravitational influence can disturb these distant bodies and send some toward the inner Solar System. (science.nasa.gov)
Formation and evolution
The Solar System originated through the collapse of part of an interstellar cloud of gas and dust. As the material contracted, its rotation produced a protoplanetary disk, with most matter accumulating in the central young Sun. Collisions and gravitational accumulation built progressively larger solid bodies, eventually forming planets and satellites. (science.nasa.gov)
Conditions varied with distance from the Sun. In the hotter inner disk, rock and metal could remain solid, while farther out additional volatile compounds could condense. These differences helped produce rocky inner planets and massive outer planets. Subsequent impacts, gravitational scattering, and changes in planetary orbits modified the original arrangement; surviving small bodies are remnants rather than an unchanged record of the disk. (science.nasa.gov)
Boundaries and exploration
The solar wind, an outward flow of charged particles, inflates the heliosphere within the surrounding interstellar medium. Its outer boundary, the heliopause, marks the transition between solar-wind-dominated and interstellar plasma. This is not the boundary of the Sun’s gravitationally bound system: the proposed Oort Cloud extends much farther. Different physical definitions therefore yield different meanings for the Solar System’s “edge.” (science.nasa.gov)
Spacecraft have investigated planets and smaller bodies through flybys, orbiters, landers, and sample-return missions. Voyager 1 crossed the heliopause in 2012, followed by Voyager 2 in 2018, providing direct measurements of the interstellar environment beyond the heliosphere. These crossings did not carry either spacecraft beyond the distant comet reservoir attributed to the Solar System. (science.nasa.gov)