Solar System
The solar system is the system consisting of the Sun and all the celestial bodies that orbit it under the influence of its gravity. It includes eight planets, their natural satellites, five recognized dwarf planets, hundreds of thousands of asteroids, billions of comets, and a multitude of small bodies. Its formation, approximately 4.6 billion years ago, is the best-documented history of a planetary system in the universe.
The Planets
The eight planets of the solar system are divided into two groups. The terrestrial planets—Mercury, Venus, Earth, Mars—are rocky, relatively small, and close to the Sun. The giant planets—Jupiter, Saturn, Uranus, Neptune—are massive, gaseous or icy, and distant. The boundary between the two groups corresponds to the ice line, beyond which volatile compounds such as water could condense in the protoplanetary disk.
Other inhabitants
The solar system is home to much more than just planets. The main asteroid belt, between Mars and Jupiter, contains hundreds of thousands of objects, including the dwarf planet Ceres. The Kuiper Belt, beyond Neptune, is home to Pluto and many other dwarf planets. Even further out, the Oort Cloud, a hypothetical spherical structure, is believed to contain the nuclei of long-period comets. More than 200 moons orbit the planets and dwarf planets.
The Origin of the Solar System
The solar system formed through the gravitational collapse of an interstellar molecular cloud enriched by previous supernovae. The core of the cloud condensed to form the Sun, while the remaining material formed a protoplanetary disk in which the planets coalesced through accretion. This history is confirmed by isotopic analysis of primitive meteorites, all of which date back 4.567 billion years.
The heliosphere
The solar system is surrounded by a heliosphere— a plasma bubble created by the solar wind that repels the interstellar medium. The outer boundary, the heliopause, lies about 120 AU from the Sun. The Voyager 1 and 2 probes crossed this boundary in 2012 and 2018, respectively, becoming the first human-made objects to enter interstellar space.
Did you know?
More than 99.8% of the Solar System’s mass is concentrated in the Sun. All the planets, moons, asteroids, and comets combined account for only 0.2% of the total mass— and Jupiter alone accounts for more than two-thirds of that fraction.
Frequently Asked Questions
Is the Solar System unique?
No. More than 5,000 exoplanets have been discovered to date, and estimates suggest thatthere could be billions of planetary systems in the Milky Way alone. But no system identical to ours has yet been found—each system discovered has its own unique architecture.
Where does the solar system end?
The boundary depends on the definition. The heliopause at 120 AU marks the limit of the solar wind’s influence. The Oort Cloud, which represents the gravitational boundary of the system, extends toabout 100,000 AU—or nearly 1.5 light-years—halfway to the nearest star.
Is Pluto part of the solar system?
Of course! Pluto orbits the Sun and is a full-fledged member of the solar system. It was simply reclassified in 2006 as a dwarf planet ratherthan as a major planet, due to the discovery of many other similar objects in the Kuiper Belt.