Solar System

Interplanetary environment

Coronal mass ejections play a dominant role in the Solar System's environment and space weather in general.
The outermost layer of the Solar atmosphere is the heliosphere, which permeates much of the Solar planetary system. Along with light, the Sun radiates a continuous stream of charged particles (a plasma) called the solar wind. This stream of particles spreads outwards at speeds from 900,000 kilometres per hour (560,000 mph) to 2,880,000 kilometres per hour (1,790,000 mph),[60] filling the vacuum between the bodies of the Solar System. The result is a thin, dusty atmosphere, called the interplanetary medium, which extends to at least 100 AU (15 billion km; 9.3 billion mi). Beyond the heliosphere, large objects remain gravitationally bound to the sun, but the flow of matter in the interstellar medium homogenizes the distribution of micro-scale objects (see § Farthest regions).[61]
The interplanetary medium is home to at least two disc-like regions of cosmic dust. The first, the zodiacal dust cloud, lies in the inner Solar System and causes the zodiacal light. It may have been formed by collisions within the asteroid belt brought on by gravitational interactions with the planets; a more recent proposed origin is the planet Mars.[62] The second dust cloud extends from about 10 AU (1.5 billion km; 930 million mi) to about 40 AU (6.0 billion km; 3.7 billion mi), and was probably created by collisions within the Kuiper belt.[63][64]
Activity on the Sun's surface, such as solar flares and coronal mass ejections, disturbs the heliosphere, creating space weather and causing geomagnetic storms.[65] Coronal mass ejections and similar events blow a magnetic field and huge quantities of material from the surface of the Sun. The interaction of this magnetic field and material with Earth's magnetic field funnels charged particles into Earth's upper atmosphere, where its interactions create aurorae seen near the magnetic poles.[66] The largest stable structure within the heliosphere is the heliospheric current sheet, a spiral form created by the actions of the Sun's rotating magnetic field on the interplanetary medium.[67][68]

Habitability

Main article: Planetary habitability in the Solar System
Besides solar energy, the primary characteristic of the Solar System enabling the presence of life is the heliosphere and planetary magnetic fields (for those planets that have them). These magnetic fields partially shield the Solar System from high-energy interstellar particles called cosmic rays. The density of cosmic rays in the interstellar medium and the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic-ray penetration in the Solar System varies, though by how much is unknown.[69]
Earth's magnetic field also stops its atmosphere from being stripped away by the solar wind.[70] Venus and Mars do not have magnetic fields, and as a result the solar wind causes their atmospheres to gradually bleed away into space.[71]
The zone of habitability of the Solar System is conventionally located in the inner Solar System, where planetary surface or atmospheric temperatures admit the possibility of liquid water.[72] Habitability might also be possible in subsurface oceans of various outer Solar System moons.[73]

Sun

Main article: Sun
The Sun in true white color
The Sun is the Solar System's star and by far its most massive component. Its large mass (332,900 Earth masses),[74] which comprises 99.86% of all the mass in the Solar System,[75] produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium.[76] This releases an enormous amount of energy, mostly radiated into space as electromagnetic radiation peaking in visible light.[77][78]
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.[79][80]
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.[81] 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".[82]

Inner Solar System

Overview of the Inner Solar System up to the Jovian System
The inner Solar System is the region comprising the terrestrial planets and the asteroid belt.[83] Composed mainly of silicates and metals,[84] the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is less than the distance between the orbits of Jupiter and Saturn. This region is also within the frost line, which is a little less than 5 AU (750 million km; 460 million mi) from the Sun.[24]

Inner planets

Main article: Terrestrial planet
The four terrestrial planets Mercury, Venus, Earth and Mars
The four terrestrial or inner planets have dense, rocky compositions, few or no moons, and no ring systems. They are in hydrostatic equilibrium, forming a rounded shape, and have undergone planetary differentiation, causing chemical elements to accumulate at different radii. They are composed largely of refractory minerals such as silicates—which form their crusts and mantles—and metals such as iron and nickel which form their cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features, such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets that are closer to the Sun than Earth (i.e. Mercury and Venus).[85]

Mercury

Main article: Mercury (planet)
Mercury (0.307–0.588 AU (45.9–88.0 million km; 28.5–54.7 million mi) from the Sun[86]) is the closest planet to the Sun. The smallest planet in the Solar System (0.055 ME), Mercury has no natural satellites. The dominant geological features are impact craters or basins with ejecta blankets, the remains of early volcanic activity including magma flows, and lobed ridges or rupes that were probably produced by a period of contraction early in the planet's history.[87] Mercury's very tenuous atmosphere consists of solar-wind particles trapped by Mercury's magnetic field, as well as atoms blasted off its surface by the solar wind.[88] Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, or that it was prevented from fully accreting by the young Sun's energy.[89][90] There have been searches for "Vulcanoids", asteroids in stable orbits between Mercury and the Sun, but none have been discovered.[91][92]

Venus

Main article: Venus
Venus (0.718–0.728 AU (107.4–108.9 million km; 66.7–67.7 million mi) from the Sun[86]) is close in size to Earth (0.815 ME) and, like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere, and evidence of internal geological activity. It is much drier than Earth, and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C (752 °F), mainly due to the amount of greenhouse gases in the atmosphere.[93] The planet has no magnetic field that would prevent the depletion of its substantial atmosphere, which suggests that its atmosphere is being replenished by volcanic eruptions.[94] A relatively young planetary surface displays extensive evidence of volcanic activity, but is devoid of plate tectonics. It may undergo resurfacing episodes on a time scale of 700 million years.[95]

Earth

Main article: Earth
Earth (0.983–1.017 AU (147.1–152.1 million km; 91.4–94.5 million mi) from the Sun) is the largest and densest of the inner planets, the only one known to have current geological activity, and the only place in the universe where life is known to exist.[96] Its liquid hydrosphere is unique among the terrestrial planets, and it is the only planet where plate tectonics has been observed.[97] Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen.[98][99] The planetary magnetosphere shields the surface from solar and cosmic radiation, limiting atmospheric stripping and maintaining habitability.[100] It has one natural satellite, the Moon.[101]

Mars

Main article: Mars
Mars (1.382–1.666 AU (206.7–249.2 million km; 128.5–154.9 million mi) from the Sun) is smaller than Earth and Venus (0.107 ME). It has an atmosphere of mostly carbon dioxide with a surface pressure of 6.1 millibars (0.088 psi; 0.18 inHg); roughly 0.6% of that of Earth but sufficient to support weather phenomena.[102] Its surface, peppered with volcanoes, such as Olympus Mons, and rift valleys, such as Valles Marineris, shows geological activity that may have persisted until as recently as 2 million years ago.[103] Its red color comes from iron oxide (rust) in its soil,[104] while the polar regions show white ice caps consisting largely of water.[105] Mars has two tiny natural satellites (Deimos and Phobos) thought to be either captured asteroids,[106] or ejected debris from a massive impact early in Mars's history.[107]

Asteroid belt

Main articles: Asteroid belt and Asteroid
Linear map of the inner Solar System, showing many asteroid populations
Asteroids except for the largest, Ceres, are classified as small Solar System bodies[h] and are composed mainly of carbonaceous, refractory rocky and metallic minerals, with some ice.[113][114] They range from a few metres to hundreds of kilometres in size. Asteroids smaller than one meter are usually called meteoroids and micrometeoroids (grain-sized), with the exact division between the two categories being debated over the years.[115] As of 2017, the IAU designates asteroids having a diameter between about 30 micrometres and 1 metre as micrometeoroids, and terms smaller particles "dust".[116]
The asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU (340 and 490 million km; 210 and 310 million mi) from the Sun. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.[117] The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter.[118] Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth.[41] The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident.[119]

Ceres

Main article: Ceres (dwarf planet)
Ceres (2.77 AU (414 million km; 257 million mi) from the Sun) is the largest asteroid, a protoplanet, and a dwarf planet.[h] It has a diameter of slightly under 1,000 km (620 mi) and a mass large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in 1801, but as further observations revealed additional asteroids, it became common to consider it as one of the minor rather than major planets.[120] It was then reclassified again as a dwarf planet in 2006 when the IAU definition of planet was established.[121]: 218 

Pallas and Vesta

Main articles: 2 Pallas and 4 Vesta
Pallas (2.77 AU from the Sun) and Vesta (2.36 AU from the Sun) are the largest asteroids in the asteroid belt, after Ceres. They are the other two protoplanets that survive more or less intact. At about 520 km (320 mi) in diameter, they were large enough to have developed planetary geology in the past, but both have suffered large impacts and been battered out of being round.[122][123][124] Fragments from impacts upon these two bodies survive elsewhere in the asteroid belt, as the Pallas family and Vesta family. Both were considered planets upon their discoveries in 1802 and 1807 respectively, and like Ceres, eventually considered minor planets with the discovery of more asteroids. Some authors today have begun to consider Pallas and Vesta as planets again, along with Ceres, under geophysical definitions of the term.[109]

Asteroid groups

Asteroids in the asteroid belt are divided into asteroid groups and families based on their orbital characteristics. Kirkwood gaps are sharp dips in the distribution of asteroid orbits that correspond to orbital resonances with J