How well do you know our Solar System?

The Solar System is made up of the Sun and objects that orbit it, including eight planets, four inner terrestrial planets, and four outer gas and ice giants.

There are also many smaller dwarf planets and innumerable small Solar System bodies orbiting the Sun, including natural satellites referred to as moons, planetary rings, and objects in the asteroid belt, the Kuiper belt, and scattered disc.

The Solar System was formed from the gravitational collapse of a giant interstellar molecular cloud 4.6 billion years ago, and hundreds of protoplanets may have existed in the early Solar System.

The planets formed by accretion from a protoplanetary disc, in which dust and gas gravitationally attracted each other, coalescing to form ever larger bodies.

The Sun is the dominant gravitational member of the Solar System, and its planetary system is maintained in a relatively stable, slowly evolving state by following isolated, gravitationally bound orbits around the Sun.

The planets and other large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic, and most of the planets in the Solar System have secondary systems of their own, being orbited by natural satellites called moons.

The overall structure of the charted regions of the Solar System consists of the Sun, four smaller inner planets surrounded by a belt of mostly rocky asteroids, and four giant planets surrounded by the Kuiper belt of mostly icy objects.

The Sun is composed of roughly 98% hydrogen and helium, as are Jupiter and Saturn, while objects farther from the Sun are composed largely of materials with lower melting points, such as ice.

The astronomical unit (AU) is the distance from the Earth to the Sun if the planet's orbit were perfectly circular, and Jupiter, the largest planet, is 5.2 AU from the Sun, while the most distant planet, Neptune, is 30 AU from the Sun.

The Solar System will remain roughly as it is known today until the hydrogen in the core of the Sun has been entirely converted to helium, which will occur roughly 5 billion years from now, marking the end of the Sun's main-sequence life.

The angular momentum of the Solar System is a measure of the total amount of orbital and rotational momentum possessed by all its moving components, with the planets, dominated by Jupiter, accounting for most of the angular momentum.

The Solar System is located 26,000 light-years from the center of the Milky Way galaxy in the Orion Arm, which contains most of the visible stars in the night sky, and the nearest stars are within the so-called Local Bubble, with the closest, Proxima Centauri, at 4.2441 light-years.Overview of the Solar System

• Theories of the Solar System's structure have been proposed and invalidated over time.

• Some models attempt to convey the relative scales of the Solar System on human terms.

• The Sun comprises 99.86% of all the mass in the Solar System and produces energy by nuclear fusion of hydrogen into helium.

• The inner part of the Solar System is composed of the terrestrial planets and the asteroid belt.

• The four inner planets have dense, rocky compositions, few or no moons, and no ring systems.

• The asteroid belt occupies the orbit between Mars and Jupiter and contains objects over one kilometer in diameter.

• Ceres is the largest asteroid, a protoplanet, and a dwarf planet.

• The outer part of the Solar System is composed of the gas giants, their moons, and the Kuiper belt.

• Jupiter is the largest planet and has the most moons in the Solar System.

• Saturn's prominent features are its extensive ring system and numerous moons.

• Uranus and Neptune are ice giants, have ring systems, and have many moons.

• The Kuiper belt is a region beyond Neptune that contains icy bodies, including dwarf planets and other small Solar System bodies.Overview of the Solar System's Outer Regions

• The outer region of the Solar System is home to the giant planets and their large moons, as well as the centaurs and many short-period comets.

• The four outer planets are Jupiter, Saturn, Uranus, and Neptune, collectively making up 99% of the mass known to orbit the Sun. Jupiter and Saturn are gas giants, while Uranus and Neptune are ice giants, and all four have rings.

• Jupiter has 95 known satellites, including the Galilean moons, and is 2.5 times the mass of all the other planets combined.

• Saturn has 83 confirmed satellites, including Titan, which is larger than Mercury and the only satellite in the Solar System to have a substantial atmosphere.

• Uranus has the lowest mass of the outer planets, orbits the Sun on its side, and has extreme seasonal variation due to its axial tilt. It has 27 known satellites and a ring system.

• Neptune is slightly smaller than Uranus but more massive and radiates more internal heat. It has 14 known satellites, including Triton, which is geologically active and has a retrograde orbit.

• The centaurs are icy comet-like bodies that were gravitationally perturbed closer to the Sun by the outer planets, and are expected to become comets or get ejected out of the Solar System. The largest known centaur, 10199 Chariklo, has a diameter of about 250 km and is one of the only few minor planets known to possess a ring system.

• Comets are small Solar System bodies composed largely of volatile ices that have highly eccentric orbits. Short-period comets have orbits lasting less than two hundred years, while long-period comets have orbits lasting thousands of years.

• The trans-Neptunian region includes the Kuiper belt, home of Pluto and several other dwarf planets, and an overlapping disc of scattered objects, which is tilted toward the plane of the Solar System and reaches much further out than the Kuiper belt. The entire region is still largely unexplored and appears to consist overwhelmingly of many thousands of small worlds.

• The Kuiper belt is a great ring of debris consisting mainly of objects composed primarily of ice, estimated to have over 100,000 objects with a diameter greater than 50 km. Pluto is the largest known object in the Kuiper belt, and Charon is its largest moon.

• The scattered disc, which overlaps the Kuiper belt but extends out to near 500 AU, is thought to be the source of short-period comets.

• Eris and Gonggong are the largest known scattered disc objects, with Eris causing a debate about what constitutes a planet. It has one known moon, Dysnomia, while Gonggong has one known moon, Xiangliu.

• The outer boundary of the heliosphere, the heliopause, is the point at which the solar wind finally terminates and is the beginning of interstellar space. Voyager 1 and Voyager 2 have both crossed the heliopause.Overview of the Solar System and its boundaries

• Voyager 1 and Voyager 2 have passed the termination shock and entered the heliosheath, with Voyager 1 crossing the heliopause in August 2012 and Voyager 2 in December 2018.

• The outer edge of the heliosphere is affected by interactions with the interstellar medium and solar magnetic fields.

• Sedna and 2000 CR105 are distant detached objects that may be part of an inner Oort cloud, while the Oort cloud is a hypothetical spherical cloud of up to a trillion icy objects that is thought to be the source for all long-period comets.

• The Solar System is still largely unknown, and the region between the Kuiper belt and the Oort cloud is virtually unmapped.

• The Solar System is located in the Milky Way, a barred spiral galaxy, and is located about 26,660 light-years from the Galactic Center.

• The Solar System's location in the Milky Way has given Earth long periods of stability for life to evolve, but the changing position of the Solar System relative to other parts of the Milky Way could explain periodic extinction events on Earth.

• The Solar System is well outside the star-crowded environs of the Galactic Center, which could perturb bodies in the Oort cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth.

• The Solar System is surrounded by the Local Interstellar Cloud and multiple other interstellar clouds within 300 light-years of the Sun, known as the Local Bubble.

• The Solar System's nearest stars are Alpha Centauri, Proxima Centauri, Barnard's Star, Wolf 359, and Lalande 21185.

• The Solar System's orbits are nearly circular compared to other systems, lacking planets interior to the orbit of Mercury and lacking super-Earths.

• Humanity's knowledge of the Solar System has grown incrementally over the centuries, with the term "Solar System" entering the English language by 1704 and humans beginning their space exploration in the 20th century.