Solar System

The Solar System^[c] is the gravitationally bound system of the Sun and the objects that orbit it.^[4] It was formed 4.6 billion years ago when a dense region of a molecular cloud collapsed, forming the Sun and a protoplanetary disc. The Sun is a main-sequence star, where hydrogen in its core is fused into helium. Most of fusion energy is released into space as electromagnetic radiation (light) and neutrinos.

The largest objects that orbit the Sun are the eight planets. In order from the Sun, they are four terrestrial planets (Mercury, Venus, Earth and Mars); two gas giants (Jupiter and Saturn); and two ice giants (Uranus and Neptune). All terrestrial planets have solid surfaces. Inversely, all giant planets do not have a definite surface, as they are mainly composed of gases and liquids. Over 99.86% of the Solar System's mass is in the Sun and nearly 90% of the remaining mass is in Jupiter and Saturn.

There is a strong consensus between astronomers^[d] that the Solar System has at least eight dwarf planets: Ceres, Pluto, Haumea, Quaoar, Makemake, Gonggong, Eris, and Sedna. There are a vast number of small Solar System bodies, such as asteroids, comets, centaurs, meteoroids, and interplanetary dust clouds. Some of these bodies are in the asteroid belt (between Mars's and Jupiter's orbit) and the Kuiper belt (just outside Neptune's orbit).^[e] Six planets, six dwarf planets, and other bodies have orbiting natural satellites, which are commonly called 'moons'.

The Solar System is constantly flooded by the Sun's charged particles, the solar wind, forming the heliosphere. Around 75–90 astronomical units, the solar wind is halted, resulting in the heliopause. This is the boundary of the Solar System to interstellar space. The outermost region of the Solar System is the theorized Oort cloud, the source for long-period comets, extending 2,000–200,000 astronomical units (0.032–3.2 light-years). The closest star to the Solar System, Proxima Centauri, is 4.25 light-years away. Both stars belong to the Local Group and the Milky Way galaxy.

As of April 2024, the Solar System has 8 planets, 5 International Astronomical Union–confirmed dwarf planets,^[2] 753 natural satellites,^[1] and more than 1.3 million detected minor planets.^[2] The number of dwarf planets, natural satellites and minor planets are expected to increase over time with new detections.

Astronomers sometimes divide the Solar System structure into separate regions. The inner Solar System includes the Mercury, Venus, Earth, Mars and bodies in the asteroid belt. The outer Solar System includes the Jupiter, Saturn, Uranus, Neptune and bodies in the Kuiper belt.^[5] Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune.^[6]

The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud.^[f] This initial cloud was likely several light-years across and probably birthed several stars.^[8] As is typical of molecular clouds, this one consisted mostly of hydrogen, with some helium, and small amounts of heavier elements fused by previous generations of stars.^[9]

As the pre-solar nebula^[9] collapsed, conservation of angular momentum caused it to rotate faster. The center, where most of the mass collected, became increasingly hotter than the surrounding disc.^[8] As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU (30 billion km; 19 billion mi)^[8] and a hot, dense protostar at the center.^[10][11] The planets formed by accretion from this disc,^[12] in which dust and gas gravitationally attracted each other, coalescing to form ever larger bodies. Hundreds of protoplanets may have existed in the early Solar System, but they either merged or were destroyed or ejected, leaving the planets, dwarf planets, and leftover minor bodies.^[13][14]

Due to their higher boiling points, only metals and silicates could exist in solid form in the warm inner Solar System close to the Sun (within the frost line). They would eventually form the rocky planets of Mercury, Venus, Earth, and Mars. Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.^[13]

The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid. The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium, the lightest and most abundant elements.^[13] Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud.^[13]

Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion.^[15] As helium accumulates at its core the Sun is growing brighter;^[16] early in its main-sequence life its brightness was 70% that of what it is today.^[17] The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved: the thermal pressure counterbalancing the force of gravity. At this point, the Sun became a main-sequence star.^[18]

The main-sequence phase, from beginning to end, will last about 10 billion years for the Sun compared to around two billion years for all other subsequent phases of the Sun's pre-remnant life combined.^[19] Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space.^[16]

The Solar System is in a relatively stable, slowly evolving state by following isolated, gravitationally bound orbits around the Sun.^[20] Although the Solar System has been fairly stable for billions of years, it is technically chaotic, and may eventually be disrupted. There is also a small chance that another star will pass through the Solar System in the next billion years. Although this could destabilize the system and eventually lead millions of years later to expulsion of planets, collisions of planets, or planets hitting the Sun, it would most likely leave the Solar System much as it is today.^[21]

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. This will mark the end of the Sun's main-sequence life. At that time, the core of the Sun will contract with hydrogen fusion occurring along a shell surrounding the inert helium, and the energy output will be greater than at present. The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a red giant. Because of its increased surface area, the surface of the Sun will be cooler (2,600 K (2,330 °C; 4,220 °F) at its coolest) than it is on the main sequence.^[19]

The expanding Sun is expected to vaporize Mercury as well as Venus, and render Earth uninhabitable (possibly destroying it as well).^[22] Eventually, the core will be hot enough for helium fusion; the Sun will burn helium for a fraction of the time it burned hydrogen in the core. The Sun is not massive enough to commence the fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will be ejected into space, leaving behind a dense white dwarf, half the original mass of the Sun but only the size of Earth.^[19] The ejected outer layers may form a planetary nebula, returning some of the material that formed the Sun—but now enriched with heavier elements like carbon—to the interstellar medium.^[23][24]

The Sun 

The Sun is the Solar System's star and by far its most massive component. Its large mass (332,900 Earth masses),^[25] which comprises 99.86% of all the mass in the Solar System,^[26] produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium.^[27] This releases an enormous amount of energy, mostly radiated into space as electromagnetic radiation peaking in visible light.^[28][29]

Because the Sun fuses hydrogen into helium at its core, it is a main-sequence star. More specifically, it is a G2-type main-sequence star, where the type designation refers to its effective temperature. Hotter main-sequence stars are more luminous but shorter lived. The Sun's temperature is intermediate between that of the hottest stars and that of the coolest stars. Stars brighter and hotter than the Sun are rare, whereas substantially dimmer and cooler stars, known as red dwarfs, make up about 75% of the stars in the Milky Way.^[30]

The Sun is a population I star; it has a higher abundance of elements heavier than hydrogen and helium ("metals" in astronomical parlance) than the older population II stars.^[31] Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, whereas stars born later have more. This higher metallicity is thought to have been crucial to the Sun's development of a planetary system because the planets form from the accretion of "metals".^[32]