Solar System
The Solar System[a] consists of the Sun and its planetary system of eight planets, their moons, and other non-stellar objects. It formed4.6 billion years ago from the collapse of a giant molecular cloud. The vast majority of the system's mass is in the Sun, with most of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, called the gas giants, are substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are composed mainly of hydrogen and helium; the two outermost planets, Uranus andNeptune, are composed largely of substances with relatively high melting points (compared with hydrogen and helium), called ices, such as water, ammonia and methane, and are often referred to separately as "ice giants". All planets have almost circular orbits that lie within a nearly flat disc called the ecliptic plane.
The Solar System also contains a number of regions populated by smaller objects.[b] The asteroid belt, which lies between Mars and Jupiter, is similar to the terrestrial planets as it mostly contains objects composed of rock and metal. Beyond Neptune's orbit lie theKuiper belt and scattered disc; linked populations of trans-Neptunian objects composed mostly of ices. Within these populations, several dozen to more than ten thousand objects may be large enough to have been rounded by their own gravity.[5] Such objects are referred to as dwarf planets. Identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto, Eris, Haumea, andMakemake.[b] In addition to these two regions, various other small-body populations including comets, centaurs and interplanetary dustfreely travel between regions. Six of the planets, at least three of the dwarf planets, and many of the smaller bodies are orbited by natural satellites,[c] usually termed "moons" after Earth's Moon. Each of the outer planets is encircled by planetary rings of dust and other small objects.
The solar wind, a flow of plasma from the Sun, creates a bubble in the interstellar medium known as the heliosphere, which extends out to the edge of the scattered disc. The Oort cloud, which is believed to be the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of interstellar wind. The Solar System is located within one of the outer arms of Milky Way galaxy, which contains about 200 billion stars.
Discovery and exploration
For many thousands of years, humanity, with a few notable exceptions, did not recognize the existence of the Solar System. People believed the Earth to be stationary at the centre of theuniverse and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos,[6] Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system.[7] His 17th-century successors, Galileo Galilei, Johannes Kepler andIsaac Newton, developed an understanding of physics that led to the gradual acceptance of the idea that the Earth moves around the Sun and that the planets are governed by the same physical laws that governed the Earth. Additionally, the invention of the telescope led to the discovery of further planets and moons. In more recent times, improvements in the telescope and the use ofunmanned spacecraft have enabled the investigation of geological phenomena such as mountains and craters, and seasonal meteorological phenomena such as clouds, dust storms and ice capson the other planets.
Structure and composition
Solar System showing the plane of the Earth's orbit around the Sun in 3D. Mercury, Venus, Earth, and Mars are shown in both panels; the right panel also shows Jupiter making one full revolution with Saturn and Uranus making less than one full revolution.
The principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99.86 percent of the system's known mass and dominates it gravitationally.[8] The Sun's four largest orbiting bodies, the gas giants, account for 99 percent of the remaining mass, with Jupiter and Saturn together comprising more than 90 percent.[d]
Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic. The planets are very close to the ecliptic while comets and Kuiper belt objects are frequently at significantly greater angles to it.[9][10] All the planets and most other objects orbit the Sun in the same direction that the Sun is rotating (counter-clockwise, as viewed from above the Sun's north pole).[11] There are exceptions, such as Halley's Comet.
The overall structure of the charted regions of the Solar System consists of the Sun, four relatively small inner planets surrounded by a belt of rocky asteroids, and four gas giants surrounded by the Kuiper belt of icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four gas giants.[12] 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.[13]
Most of the planets in the Solar System possess secondary systems of their own, being orbited by planetary objects called natural satellites, or moons (two of which are larger than the planet Mercury), or, in the case of the four gas giants, by planetary rings; thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are insynchronous rotation, with one face permanently turned toward their parent.
Kepler's laws of planetary motion describe the orbits of objects about the Sun. Following Kepler's laws, each object travels along an ellipse with the Sun at one focus. Objects closer to the Sun (with smaller semi-major axes) travel more quickly, as they are more affected by the Sun's gravity. On an elliptical orbit, a body's distance from the Sun varies over the course of its year. A body's closest approach to the Sun is called its perihelion, while its most distant point from the Sun is called its aphelion. The orbits of the planets are nearly circular, but many comets, asteroids and Kuiper belt objects follow highly elliptical orbits. The positions of the bodies in the Solar System can be predicted using numerical models.
Although the Sun dominates the system by mass, it accounts for only about 2% of the angular momentum due to the differential rotation within the gaseous Sun. The planets, dominated by Jupiter, account for most of the rest of the angular momentum due to the combination of their mass, orbit, and distance from the Sun, with a possibly significant contribution from comets.
Due to the vast distances involved, many representations of the Solar System show orbits the same distance apart. In reality, with a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between it and the previous orbit. For example, Venus is approximately 0.33 astronomical units (AU) farther out from the Sun than Mercury, while Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a relationship between these orbital distances (for example, the Titius–Bode law), but no such theory has been accepted.
The Sun, which comprises nearly all the matter in the Solar System, is composed of roughly 98% hydrogen and helium. Jupiter and Saturn, which comprise nearly all the remaining matter, possess atmospheres composed of roughly 99% of those same elements.[18][19] A composition gradient exists in the Solar System, created by heat andlight pressure from the Sun; those objects closer to the Sun, which are more affected by heat and light pressure, are composed of elements with high melting points. Objects farther from the Sun are composed largely of materials with lower melting points. The boundary in the Solar System beyond which those volatile substances could condense is known as the frost line, and it lies at roughly 5 AU from the Sun.
The objects of the inner Solar System are composed mostly of rock,the collective name for compounds with high melting points, such as silicates, iron or nickel, that remained solid under almost all conditions in the protoplanetary nebula. Jupiter and Saturn are composed mainly of gases, the astronomical term for materials with extremely low melting points and high vapor pressure such as molecular hydrogen, helium, and neon, which were always in the gaseous phase in the nebula.[22] Ices, like water, methane, ammonia, hydrogen sulfide and carbon dioxide,[21]have melting points up to a few hundred kelvins, while their phase depends on the ambient pressure and temperature.[22] They can be found as ices, liquids, or gases in various places in the Solar System, while in the nebula they were either in the solid or gaseous phase.[22] Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "ice giants") and the numerous small objects that lie beyond Neptune's orbit.[21][23] Together, gases and ices are referred to as volatiles.[24]
A number of Solar System models on Earth attempt to convey the relative scales involved in the Solar System on human terms. Some models are mechanical — called orreries — while others extend across cities or regional areas.[25] The largest such scale model, the Sweden Solar System, uses the 110-metre Ericsson Globe in Stockholm as its substitute Sun, and, following the scale, Jupiter is a 7.5 metre sphere at Arlanda International Airport, 40 km away, while the farthest current object, Sedna, is a 10-cm sphere in Luleå, 912 km away.
Formation and evolution
The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud.[28] This initial cloud was likely several light-years across and probably birthed several stars.[29] 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. As the region that would become the Solar System, known as the pre-solar nebula,[30] collapsed, conservation of angular momentum caused it to rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc.[29] As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU[29] and a hot, dense protostar at the centre.[31][32] The planets formed by accretionfrom this disc,[33] 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, leaving the planets, dwarf planets, and leftover minor bodies.
Due to their higher boiling points, only metals and silicates could exist in the warm inner Solar System close to the Sun, and these would form the rocky planets of Mercury, Venus, Earth, and Mars. Since metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large. The gas giants (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. Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud. The Nice model is an explanation for the creation of these regions, and how the outer planets could have formed in different positions and migrated to their current orbits through various gravitational interactions.
Within 50 million years, the pressure and density of hydrogen in the centre of the protostar became great enough for it to begin thermonuclear fusion.[34] The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved: the thermal pressure equaled the force of gravity. At this point the Sun became a main-sequence star.[35] Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space, ending the planetary formation process.
The Solar System will remain roughly as we know it today until the hydrogen in the core of the Sun has been entirely converted to helium, which will occur roughly 5.4 billion years from now. This will mark the end of the Sun's main-sequence life. At this time, the core of the Sun will collapse, and the energy output will be much greater than at present. The outer layers of the Sun will expand to roughly up to 260 times its current diameter and the Sun will become a red giant. Because of its vastly increased surface area, the surface of the Sun will be considerably cooler than it is on the main sequence (2600 K at the coolest).[36] The expanding Sun is expected to vaporize Mercury and Venus and render the Earth uninhabitable, as the habitable zone moves out to the orbit of Mars. Eventually, the core will be hot enough for helium fusion to begin in the core; 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 fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will fall away into space, leaving a white dwarf, an extraordinarily dense object, half the original mass of the Sun but only the size of the Earth.[37] The ejected outer layers will form what is known as a planetary nebula, returning some of the material that formed the Sun—but now enriched with heavier elements like carbon—to the interstellar medium.
Sun
The Sun is the Solar System's star, and by far its chief component. Its large mass (332,900 Earth masses)[38] produces temperatures and densities in its corehigh enough to sustain nuclear fusion,[39] which releases enormous amounts of energy, mostly radiated into space as electromagnetic radiation, peaking in the 400–700 nm band of visible light.[40]
The Sun is classified as a type G2 yellow dwarf, but this name is misleading as, compared to the majority of stars in our galaxy, the Sun is rather large and bright.[41] Stars are classified by the Hertzsprung–Russell diagram, a graph that plots the brightness of stars with their surface temperatures. Generally, hotter stars are brighter. Stars following this pattern are said to be on the main sequence, and the Sun lies right in the middle of it. However, stars brighter and hotter than the Sun are rare, while substantially dimmer and cooler stars, known as red dwarfs, are common, making up 85 percent of the stars in the galaxy.[41][42]
Evidence suggests that the Sun's position on the main sequence puts it in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion. The Sun is growing brighter; early in its history it was 70 percent as bright as it is today.[43]
The Sun is a population I star; it was born in the later stages of the universe's evolution, and thus contains more elements heavier than hydrogen and helium ("metals" in astronomical parlance) than older population II stars. 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, while stars born later have more. This high metallicity is thought to have been crucial to the Sun's developing a planetary system, because planets form from accretion of "metals
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