Solar System

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Solar System
Solar System true color.jpg
The Sun, planets, moons and dwarf planets [lower-alpha 1]
(true color, size to scale, distances not to scale)
Age4.568 billion years [lower-alpha 2]
Location
Nearest star
Population
Stars Sun
Planets
Known dwarf planets
Known natural satellites 758 [D 3]
Known minor planets 1,358,412 [D 4]
Known comets 4,591 [D 4]
Planetary system
Star spectral type G2V
Frost line ~5 AU [3]
Semi-major axis of outermost planet30.07 AU [D 5] ( Neptune )
Kuiper cliff 50–70 AU [4] [5]
Heliopause detected at 120 AU [6]
Hill sphere 1.1 pc (230,000 au) [7] - 0.865 pc (178,419 au) [8]
Orbit about Galactic Center
Invariable-to-galactic plane inclination~60°, to the ecliptic [lower-alpha 3]
Distance to
Galactic Center
24,000–28,000 ly
[9]
Orbital speed
720,000 km/h (450,000 mi/h) [10]
Orbital period~230 million years [10]

The Solar System [lower-alpha 4] is the gravitationally bound system of the Sun and the objects that orbit it. [11] 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 an ordinary main sequence star that maintains a balanced equilibrium by the fusion of hydrogen into helium at its core, releasing this energy from its outer photosphere.

Contents

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 among astronomers [lower-alpha 5] 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). [lower-alpha 6] 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 from the Sun, 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 to a radius of 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 (269,000 AU) away. Both stars belong to the Milky Way galaxy.

Formation and evolution

Past

Diagram of the early Solar System's protoplanetary disk, out of which Earth and other Solar System bodies formed Soot-line1.jpg
Diagram of the early Solar System's protoplanetary disk, out of which Earth and other Solar System bodies formed

The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud. [lower-alpha 2] This initial cloud was likely several light-years across and probably birthed several stars. [13] 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. [14]

As the pre-solar nebula [14] collapsed, conservation of angular momentum caused it to rotate faster. The center, where most of the mass collected, became increasingly hotter than the surroundings. [13] As the contracting nebula spun faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU (30 billion km; 19 billion mi) [13] and a hot, dense protostar at the center. [15] [16] The planets formed by accretion from this disc, [17] 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. [18] [19]

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. [18]

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. [18] Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud. [18]

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. [20] As helium accumulates at its core the Sun is growing brighter; [21] early in its main-sequence life its brightness was 70% that of what it is today. [22] 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. [23] Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space. [21]

Present and future

The Solar System remains in a relatively stable, slowly evolving state by following isolated, gravitationally bound orbits around the Sun. [24] Although the Solar System has been fairly stable for billions of years, it is technically chaotic, and may eventually be disrupted. There is a small chance that another star will pass through the Solar System in the next few 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. [25]

The current Sun compared to its peak size in the red-giant phase Sun red giant.svg
The current Sun compared to its peak size in the red-giant phase

The Sun's 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. [26] 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. [26]

The expanding Sun is expected to vaporize Mercury as well as Venus, and render Earth uninhabitable (possibly destroying it as well). [27] 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. [26] 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. [28] [29]

General characteristics

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. [30] 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. [31]

Composition

The principal component of the Solar System is the Sun, a G-type main-sequence star star that contains 99.86% of the system's known mass and dominates it gravitationally. [32] The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, asteroids, and comets) together comprise less than 0.002% of the Solar System's total mass. [lower-alpha 7]

The Sun is composed of roughly 98% hydrogen and helium, [36] as are Jupiter and Saturn. [37] [38] A composition gradient exists in the Solar System, created by heat and light pressure from the early 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. [39] The boundary in the Solar System beyond which those volatile substances could coalesce is known as the frost line, and it lies at roughly five times the Earth's distance from the Sun. [3]

Orbits

Animations of the Solar System's inner planets orbiting. Each frame represents 2 days of motion. Solar system orrery inner planets.gif
Animations of the Solar System's inner planets orbiting. Each frame represents 2 days of motion.
Animations of the Solar System's outer planets orbiting. This animation is 100 times faster than the inner planet animation. Solar system orrery outer planets.gif
Animations of the Solar System's outer planets orbiting. This animation is 100 times faster than the inner planet animation.

The planets and other large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic. Smaller icy objects such as comets frequently orbit at significantly greater angles to this plane. [40] [41] Most of the planets in the Solar System have secondary systems of their own, being orbited by natural satellites called moons. All of the largest natural satellites are in synchronous rotation, with one face permanently turned toward their parent. The four giant planets have planetary rings, thin discs of tiny particles that orbit them in unison. [42]

As a result of the formation of the Solar System, planets and most other objects orbit the Sun in the same direction that the Sun is rotating. That is, counter-clockwise, as viewed from above Earth's north pole. [43] There are exceptions, such as Halley's Comet. [44] Most of the larger moons orbit their planets in prograde direction, matching the direction of planetary rotation; Neptune's moon Triton is the largest to orbit in the opposite, retrograde manner. [45] Most larger objects rotate around their own axes in the prograde direction relative to their orbit, though the rotation of Venus is retrograde. [46]

To a good first approximation, Kepler's laws of planetary motion describe the orbits of objects around the Sun. [47] :433–437 These laws stipulate that each object travels along an ellipse with the Sun at one focus, which causes the body's distance from the Sun to vary over the course of its year. A body's closest approach to the Sun is called its perihelion , whereas its most distant point from the Sun is called its aphelion . [48] :9-6 With the exception of Mercury, the orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits. Kepler's laws only account for the influence of the Sun's gravity upon an orbiting body, not the gravitational pulls of different bodies upon each other. On a human time scale, these perturbations can be accounted for using numerical models, [48] :9-6 but the planetary system can change chaotically over billions of years. [49]

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. [50] Although the Sun dominates the system by mass, it accounts for only about 2% of the angular momentum. [51] [52] 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. [51]

Distances and scales

To-scale diagram of distance between planets, with the white bar showing orbital variations. The size of the planets is not to scale. Solar System distance to scale.svg
To-scale diagram of distance between planets, with the white bar showing orbital variations. The size of the planets is not to scale.

The astronomical unit (AU; equal to 150,000,000 km; 93,000,000 mi) is what the distance from the Earth to the Sun would be if the planet's orbit were perfectly circular. [53] For comparison, the radius of the Sun is 0.0047 AU (700,000 km; 400,000 mi). [54] Thus, the Sun occupies 0.00001% (1 part in 107) of the volume of a sphere with a radius the size of Earth's orbit, whereas Earth's volume is roughly one millionth (10−6) that of the Sun. Jupiter, the largest planet, is 5.2 astronomical units (780,000,000 km; 480,000,000 mi) from the Sun and has a radius of 71,000 km (0.00047 AU; 44,000 mi), whereas the most distant planet, Neptune, is 30 AU (4.5×109 km; 2.8×109 mi) from the Sun. [38] [55]

With a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between its orbit and the orbit of the next nearest object to the Sun. For example, Venus is approximately 0.33 AU farther out from the Sun than Mercury, whereas 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, like the Titius–Bode law [56] and Johannes Kepler's model based on the Platonic solids, [57] but ongoing discoveries have invalidated these hypotheses. [58]

Some Solar System models attempt to convey the relative scales involved in the Solar System in human terms. Some are small in scale (and may be mechanical—called orreries)—whereas others extend across cities or regional areas. [59] The largest such scale model, the Sweden Solar System, uses the 110-metre (361 ft) Avicii Arena in Stockholm as its substitute Sun, and, following the scale, Jupiter is a 7.5-metre (25-foot) sphere at Stockholm Arlanda Airport, 40 km (25 mi) away, whereas the farthest current object, Sedna, is a 10 cm (4 in) sphere in Luleå, 912 km (567 mi) away. [60] [61]

If the Sun–Neptune distance is scaled to 100 metres (330 ft), then the Sun would be about 3 cm (1.2 in) in diameter (roughly two-thirds the diameter of a golf ball), the giant planets would be all smaller than about 3 mm (0.12 in), and Earth's diameter along with that of the other terrestrial planets would be smaller than a flea (0.3 mm or 0.012 in) at this scale. [62]

Comparison with extrasolar systems

A diagram depicting the habitable zone boundaries around stars, and how the boundaries are affected by star type. Diagram of different habitable zone regions by Chester Harman.jpg
A diagram depicting the habitable zone boundaries around stars, and how the boundaries are affected by star type.

Compared to many extrasolar systems, the Solar System stands out in lacking planets interior to the orbit of Mercury. [63] [64] It is hypothesized that all planetary systems start with many close-in planets, and that typically a sequence of their collisions causes consolidation of mass into few larger planets, but in case of the Solar System the collisions caused their destruction and ejection. [63] [65] There is a size 'gap' between the largest terrestrial planet (Earth) and the smallest giant planet (Neptune, 3.8 times as large as Earth); elsewhere exoplanets of intermediate size are typical. The known Solar System lacks super-Earths, planets between one and ten times as massive as the Earth. [63] The mass of a potentially habitable exoplanet is estimated to be 0.1–5.0 Earth masses. [66]

The orbits of Solar System planets are nearly circular. Compared to many other systems, they have smaller orbital eccentricity. [63] Although there are attempts to explain it partly with a bias in the radial-velocity detection method and partly with long interactions of a quite high number of planets, the exact causes remain undetermined. [63] [67] Small orbital eccentricity lead to a less drastic range in the planets' surface temperature and more stable condition for life to develop. [68]

Between the orbit of Venus and Mars is the habitable zone, where there is just enough sunlight (and heat) for the planetary surface to have liquid water. [69] For comparison, TRAPPIST-1 has seven planets orbiting to the parent star at a much closer distance (0.012–0.062 AU), [70] but because the parent star is a red dwarf with a lower luminosity than the Sun, [71] its habitable zone is much closer and encompasses three or four planets. [72] It is theorized that the subsurface oceans of outer Solar System moons might be habitable due to tidal heating from the parent planets. [73]

Besides solar energy, a key characteristic of the Solar System enabling the presence of life on Earth is the heliosphere and planetary magnetosphere. 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. [74]

Sun

The Sun in true white color The Sun in white light.jpg
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), [75] which comprises 99.86% of all the mass in the Solar System, [76] produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium. [77] This releases an enormous amount of energy, mostly radiated into space as electromagnetic radiation peaking in visible light. [78] [79]

Because the Sun fuses hydrogen 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 fusor stars in the Milky Way. [80]

The Sun is a population I star, having formed in the spiral arms of the Milky Way galaxy. It has a higher abundance of elements heavier than hydrogen and helium ("metals" in astronomical parlance) than the older population II stars in the galactic bulge and halo. [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]

The region of space dominated by the Solar magnetosphere is the heliosphere, which spans much of the Solar System. Along with light, the Sun radiates a continuous stream of charged particles (a plasma) called the solar wind. This stream spreads outwards at speeds from 900,000 kilometres per hour (560,000 mph) to 2,880,000 kilometres per hour (1,790,000 mph), [83] 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). [84]

Activity on the Sun's surface, such as solar flares and coronal mass ejections, disturbs the heliosphere, creating space weather and causing geomagnetic storms. [85] 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. [86] 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. [87] [88]

Inner Solar System

The inner Solar System is the region comprising the terrestrial planets and the asteroid belt. [89] Composed mainly of silicates and metals, [90] 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 within the frost line, which is a little less than 5 AU (750 million km; 460 million mi) from the Sun. [40]

Inner planets

The four terrestrial planets Mercury, Venus, Earth and Mars Terrestrial planet sizes 3.jpg
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 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. [91]

Asteroids

Overview of the inner Solar System up to Jupiter's orbit Inner solar system objects top view for wiki.png
Overview of the inner Solar System up to Jupiter's orbit

Asteroids except for the largest, Ceres, are classified as small Solar System bodies and are composed mainly of carbonaceous, refractory rocky and metallic minerals, with some ice. [129] [130] They range from a few metres to hundreds of kilometres in size. Many asteroids are divided into asteroid groups and families based on their orbital characteristics. Some asteroids have natural satellites that orbit them, that is, asteroids that orbit larger asteroids. [131]

The asteroid belt occupies a torus-shaped region between 2.3 and 3.3 AU (340 and 490 million km; 210 and 310 million mi) from the Sun, which lies between the orbits of Mars and Jupiter. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter. [132] The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter. [133] Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth. [35] The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident. [134]

The four largest asteroids: Ceres, Vesta, Pallas, Hygiea. Only Ceres and Vesta have been visited by a spacecraft and thus have a detailed picture. The Four Largest Asteroids.jpg
The four largest asteroids: Ceres, Vesta, Pallas, Hygiea. Only Ceres and Vesta have been visited by a spacecraft and thus have a detailed picture.

Below are the descriptions of the three largest bodies in the asteroid belt. They are all considered to be relatively intact protoplanets, a precursor stage before becoming a fully-formed planet (see List of exceptional asteroids): [135] [136] [137]

Below are some exemplar asteroid populations that are not in the asteroid belt and within Jupiter's orbit (see also § Centaurs, trojans and resonant bodies):

Outer Solar System

The outer region of the Solar System is home to the giant planets and their large moons. The centaurs and many short-period comets orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles, such as water, ammonia, and methane than those of the inner Solar System because the lower temperatures allow these compounds to remain solid, without significant rates of sublimation. [18]

Outer planets

The outer planets Jupiter, Saturn, Uranus and Neptune, compared to the inner planets Earth, Venus, Mars, and Mercury at the bottom right Planet collage to scale (captioned).jpg
The outer planets Jupiter, Saturn, Uranus and Neptune, compared to the inner planets Earth, Venus, Mars, and Mercury at the bottom right

The four outer planets, called giant planets or Jovian planets, collectively make up 99% of the mass known to orbit the Sun. [lower-alpha 7] All four giant planets have multiple moons and a ring system, although only Saturn's rings are easily observed from Earth. [91] Jupiter and Saturn are composed mainly of gases with extremely low melting points, such as hydrogen, helium, and neon, [158] hence their designation as gas giants. [159] Uranus and Neptune are ice giants, [160] meaning they are significantly composed of 'ice' in astronomy sense, as in chemical compounds with melting points of up to a few hundred kelvins [158] such as water, methane, ammonia, hydrogen sulfide, and carbon dioxide. [161] Icy substances comprise the majority of the satellites of the giant planets and small objects that lie beyond Neptune's orbit. [161] [162]

Centaurs, trojans and resonant bodies

The centaurs are icy comet-like bodies whose semi-major axes is greater than Jupiter's and less than Neptune's (between 5.5 and 30 AU). These are former Kuiper belt and scattered disc objects (SDOs) 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. [34] While most centaurs are inactive and asteroid-like, some exhibit clear cometary activity, such as the first centaur discovered, 2060 Chiron, which has been classified as a comet (95P) because it develops a coma just as comets do when they approach the Sun. [183] The largest known centaur, 10199 Chariklo, has a diameter of about 250 km (160 mi) and is one of the only few minor planets known to possess a ring system. [184] [185]

Jupiter trojans are located in both of Jupiter's stable Lagrange points: L4, 60° ahead of Jupiter in its orbit, or L5, 60° behind in its orbit. [186] Hilda asteroids are in a 3:2 resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits. [187]

Trans-Neptunian region

Beyond the orbit of Neptune lies the area of the "trans-Neptunian region", with the doughnut-shaped 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. It appears to consist overwhelmingly of many thousands of small worlds—the largest having a diameter only a fifth that of Earth and a mass far smaller than that of the Moon—composed mainly of rock and ice. This region is sometimes described as the "third zone of the Solar System", enclosing the inner and the outer Solar System. [188]

Kuiper belt

Plot of objects around the Kuiper belt and other asteroid populations. J, S, U and N denotes Jupiter, Saturn, Uranus and Neptune. Kuiper belt plot objects of outer solar system.png
Plot of objects around the Kuiper belt and other asteroid populations. J, S, U and N denotes Jupiter, Saturn, Uranus and Neptune.
Orbit classification of Kuiper belt objects. Some clusters that is subjected to orbital resonance are marked. TheKuiperBelt classes-en.svg
Orbit classification of Kuiper belt objects. Some clusters that is subjected to orbital resonance are marked.

The Kuiper belt is a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice. [189] It extends between 30 and 50 AU from the Sun. It is composed mainly of small Solar System bodies, although the largest few are probably large enough to be dwarf planets. [190] There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km (30 mi), but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of Earth. [34] Many Kuiper belt objects have satellites, [191] and most have orbits that are substantially inclined (~10°) to the plane of the ecliptic. [192]

The Kuiper belt can be roughly divided into the "classical" belt and the resonant trans-Neptunian objects. [189] The latter have orbits whose periods are in a simple ratio to that of Neptune: for example, going around the Sun twice for every three times that Neptune does, or once for every two. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 to 47.7 AU. [193] Members of the classical Kuiper belt are sometimes called "cubewanos", after the first of their kind to be discovered, originally designated 1992 QB1; they are still in near primordial, low-eccentricity orbits. [194]

Currently, there are four dwarf planets in the Kuiper belt that have strong consensus among astronomers. [190] [195] Many dwarf planet candidates are being considered, pending further data for verification. [196]

Scattered disc

The orbital eccentricities and inclinations of the scattered disc population compared to the classical and resonant Kuiper belt objects TheKuiperBelt Projections 100AU Classical SDO.svg
The orbital eccentricities and inclinations of the scattered disc population compared to the classical and resonant Kuiper belt objects

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. Scattered-disc objects are believed to have been perturbed into erratic orbits by the gravitational influence of Neptune's early outward migration. Most scattered disc objects have perihelia within the Kuiper belt but aphelia far beyond it (some more than 150 AU from the Sun). SDOs' orbits can be inclined up to 46.8° from the ecliptic plane. [210] Some astronomers consider the scattered disc to be merely another region of the Kuiper belt and describe scattered-disc objects as "scattered Kuiper belt objects". [211] Some astronomers classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc. [212]

Currently, there are two dwarf planets in the scattered disc that have strong consensus among astronomers:

Extreme trans-Neptunian objects

The orbits of Sedna, 2012 VP113, Leleakuhonua, and other very distant objects along with the predicted orbit of the hypothetical Planet Nine Distant object orbits + Planet Nine.png
The orbits of Sedna, 2012 VP113, Leleākūhonua, and other very distant objects along with the predicted orbit of the hypothetical Planet Nine

Some objects in the Solar System have a very large orbit, and therefore are much less affected by the known giant planets than other minor planet populations. These bodies are called extreme trans-Neptunian objects, or ETNOs for short. [216] Generally, ETNOs' semi-major axes are at least 150–250 AU wide. [216] [217] For example, 541132 Leleākūhonua orbits the Sun once every ~32,000 years, with a distance of 65–2000 AU from the Sun. [D 11]

This population is divided into three subgroups by astronomers. The scattered ETNOs have perihelia around 38–45 AU and an exceptionally high eccentricity of more than 0.85. As with the regular scattered disc objects, they were likely formed as result of gravitational scattering by Neptune and still interact with the giant planets. The detached ETNOs, with perihelia approximately between 40–45 and 50–60 AU, are less affected by Neptune than the scattered ETNOs, but are still relatively close to Neptune. The sednoids or inner Oort cloud objects, with perihelia beyond 50–60 AU, are too far from Neptune to be strongly influenced by it. [216]

Currently, there is one ETNO that is classified as a dwarf planet:

Edge of the heliosphere

Diagram of the Sun's magnetosphere and helioshealth Magnetosphere Levels.jpg
Diagram of the Sun's magnetosphere and helioshealth

The Sun's stellar-wind bubble, the heliosphere, a region of space dominated by the Sun, has its boundary at the termination shock, which is roughly 80–100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind. [219] Here the solar wind collides with the interstellar medium [220] and dramatically slows, condenses and becomes more turbulent, forming a great oval structure known as the heliosheath. [219]

The heliosheath has been theorized to look and behave very much like a comet's tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind. [221] Evidence from the Cassini and Interstellar Boundary Explorer spacecraft has suggested that it is forced into a bubble shape by the constraining action of the interstellar magnetic field, [222] [223] but the actual shape remains unknown. [224]

The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics of interactions with the interstellar medium as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU farther than the southern hemisphere. [219] The heliopause is considered the beginning of the interstellar medium. [84] Beyond the heliopause, at around 230 AU, lies the bow shock: a plasma "wake" left by the Sun as it travels through the Milky Way. [225] Large objects outside the heliopause remain gravitationally bound to the sun, but the flow of matter in the interstellar medium homogenizes the distribution of micro-scale objects. [84]

Miscellaneous populations

Comets

Comet Hale-Bopp seen in 1997 Comet Hale-Bopp 1995O1.jpg
Comet Hale–Bopp seen in 1997

Comets are small Solar System bodies, typically only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye. [226]

Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are thought to originate in the Kuiper belt, whereas long-period comets, such as Hale–Bopp, are thought to originate in the Oort cloud. Many comet groups, such as the Kreutz sungrazers, formed from the breakup of a single parent. [227] Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult. [228] Old comets whose volatiles have mostly been driven out by solar warming are often categorised as asteroids. [229]

Meteoroids, meteors and dust

The planets, zodiacal light and meteor shower (top left of image) Meteor shower in the Chilean Desert (annotated and cropped).jpg
The planets, zodiacal light and meteor shower (top left of image)

Solid objects 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. [230] By 2017, the IAU designates any solid object having a diameter between ~30 micrometres and 1 metre as meteoroids, and depreciated the micrometeoroid categorization, instead terms smaller particles simply as 'dust particles'. [231]

Some meteoroids formed via disintegration of comets and asteroids, while a few formed via impact debris ejected from planetary bodies. Most meteoroids are made of silicates and heavier metals like nickel and iron. [232] When passing through the Solar System, comets produce a trail of meteoroids; it is hypothesized that this is caused either by vaporization of the comet's material or by simple breakup of dormant comets. When crossing an atmosphere, these meteoroids will produce bright streaks in the sky due to atmospheric entry, called meteors. If a stream of meteoroids enter the atmosphere on parallel trajectories, meteor will seemingly 'radiate' from a point in the sky, hence the phenomenon's name: meteor shower. [233] [ page needed ]

The inner Solar System is home to the zodiacal dust cloud. It causes the hazy zodiacal light in the dark, unpolluted sky. 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 materials from planet Mars. [234] The outer Solar System also hosts a cosmic dust cloud. It 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. [235] [236]

Boundary area and uncertainties

Much of the Solar System is still unknown. Areas beyond thousands of AU away are still virtually unmapped and learning about this region of space is difficult. Study in this region depends upon inferences from those few objects whose orbits happen to be perturbed such that they fall closer to the Sun, and even then, detecting these objects has often been possible only when they happened to become bright enough to register as comets. [237] Many objects may yet be discovered in the Solar System's uncharted regions. [238]

One of these objects might be the Oort cloud, a theorized spherical cloud of up to a trillion icy objects that is thought to be the source for all long-period comets. [239] [240] No direct observation of the Oort cloud is possible with present imaging technology. [241] It is theorized to surround the Solar System at roughly 50,000 AU (~0.9  ly) from the Sun and possibly to as far as 100,000 AU (~1.8 ly). The Oort cloud is thought to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events, such as collisions, the gravitational effects of a passing star, or the galactic tide, the tidal force exerted by the Milky Way. [239] [240]

As of the 2020s, a few astronomers hypothesized that Planet Nine (a planet beyond Neptune) might exist, based on statistical variance in the orbit of extreme trans-Neptunian objects. [242] Their closest approaches to the Sun are mostly clustered around one sector and their orbits are similarly tilted, suggesting that a large planet might be influencing their orbit over millions of years. [243] [244] [245] However, some astronomers said that this observation might be credited to observational biases or just sheer coincidence. [246]

The Sun's gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light-years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000 AU. [247] Most of the mass is orbiting in the region between 3,000 and 100,000 AU. [248] The furthest known objects, such as Comet West, have aphelia around 70,000 AU from the Sun. [249] The Sun's Hill sphere with respect to the galactic nucleus, the effective range of its gravitational influence, is thought to extend up to a thousand times farther and encompasses the hypothetical Oort cloud. [250] It was calculated by Chebotarev to be 230,000 au. [7]

The Solar System within the interstellar medium, with the different regions and their distances on a steped horizontal distance scale Interstellar medium annotated.jpg
The Solar System within the interstellar medium, with the different regions and their distances on a steped horizontal distance scale

Celestial neighborhood

Diagram of the Local Interstellar Cloud, the G-Cloud and surrounding stars. As of 2022, the precise location of the Solar System in the clouds is an open question in astronomy. The Local Interstellar Cloud and neighboring G-cloud complex.svg
Diagram of the Local Interstellar Cloud, the G-Cloud and surrounding stars. As of 2022, the precise location of the Solar System in the clouds is an open question in astronomy.

Within ten light-years of the Sun there are relatively few stars, the closest being the triple star system Alpha Centauri, which is about 4.4 light-years away and may be in the Local Bubble's G-Cloud. [252] Alpha Centauri A and B are a closely tied pair of Sun-like stars, whereas the closest star to Earth, the small red dwarf Proxima Centauri, orbits the pair at a distance of 0.2 light-year. In 2016, a potentially habitable exoplanet was found to be orbiting Proxima Centauri, called Proxima Centauri b, the closest confirmed exoplanet to the Sun. [253]

The Solar System is surrounded by the Local Interstellar Cloud, although it is not clear if it is embedded in the Local Interstellar Cloud or if it lies just outside the cloud's edge. [254] [255] Multiple other interstellar clouds exist in the region within 300 light-years of the Sun, known as the Local Bubble. [255] The latter feature is an hourglass-shaped cavity or superbubble in the interstellar medium roughly 300 light-years across. The bubble is suffused with high-temperature plasma, suggesting that it may be the product of several recent supernovae. [256]

The Local Bubble is a small superbubble compared to the neighboring wider Radcliffe Wave and Split linear structures (formerly Gould Belt), each of which are some thousands of light-years in length. [257] All these structures are part of the Orion Arm, which contains most of the stars in the Milky Way that are visible to the unaided eye. [258]

The nearest and unaided-visible group of stars beyond the immediate celestial neighborhood is the Ursa Major moving group at roughly 80 light-years, which is within the Local Bubble, like the nearest as well as unaided-visible star cluster the Hyades, which lie at its edge. The closest star-forming regions are the Corona Australis Molecular Cloud, the Rho Ophiuchi cloud complex and the Taurus molecular cloud; the latter lies just beyond the Local Bubble and is part of the Radcliffe wave. [259]

Stellar flybys that pass within 0.8 light-years of the Sun occur roughly once every 100,000 years. The closest well-measured approach was Scholz's Star, which approached to ~50,000 AU of the Sun some ~70 thousands years ago, likely passing through the outer Oort cloud. [260] There is a 1% chance every billion years that a star will pass within 100 AU of the Sun, potentially disrupting the Solar System. [261]

Galactic position

Diagram of the Milky Way, with galactic features and the relative position of the Solar System labelled. Milky Way side view.png
Diagram of the Milky Way, with galactic features and the relative position of the Solar System labelled.

The Solar System is located in the Milky Way, a barred spiral galaxy with a diameter of about 100,000 light-years containing more than 100 billion stars. [262] The Sun is part of one of the Milky Way's outer spiral arms, known as the Orion–Cygnus Arm or Local Spur. [263] [264]

Its speed around the center of the Milky Way is about 220 km/s, so that it completes one revolution every 240 million years. [262] This revolution is known as the Solar System's galactic year. [265] The solar apex, the direction of the Sun's path through interstellar space, is near the constellation Hercules in the direction of the current location of the bright star Vega. [266] The plane of the ecliptic lies at an angle of about 60° to the galactic plane. [lower-alpha 3]

The Sun follows a nearly circular orbit around the Galactic Center (where the supermassive black hole Sagittarius A* resides) at a distance of 26,660 light-years, [268] orbiting at roughly the same speed as that of the spiral arms. [269] If it orbited close to the center, gravitational tugs from nearby stars 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. In this scenario, the intense radiation of the Galactic Center could interfere with the development of complex life. [269]

The Solar System's location in the Milky Way is a factor in the evolutionary history of life on Earth. Spiral arms are home to a far larger concentration of supernovae, gravitational instabilities, and radiation that could disrupt the Solar System, but since Earth stays in the Local Spur and therefore does not pass frequently through spiral arms, this has given Earth long periods of stability for life to evolve. [269] However, according to the controversial Shiva hypothesis, the changing position of the Solar System relative to other parts of the Milky Way could explain periodic extinction events on Earth. [270] [271]

Humanity's perspective

Discovery and exploration

The motion of 'lights' moving across the sky is the basis of the classical definition of planets: wandering stars. Apparent retrograde motion of Mars in 2003.gif
The motion of 'lights' moving across the sky is the basis of the classical definition of planets: wandering stars.

Humanity's knowledge of the Solar System has grown incrementally over the centuries. Up to the Late Middle AgesRenaissance, astronomers from Europe to India believed Earth to be stationary at the center of the universe [272] 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, Nicolaus Copernicus was the first person known to have developed a mathematically predictive heliocentric system. [273] [274]

Heliocentrism did not triumph immediately over geocentrism, but the work of Copernicus had its champions, notably Johannes Kepler. Using a heliocentric model that improved upon Copernicus by allowing orbits to be elliptical, and the precise observational data of Tycho Brahe, Kepler produced the Rudolphine Tables , which enabled accurate computations of the positions of the then-known planets. Pierre Gassendi used them to predict a transit of Mercury in 1631, and Jeremiah Horrocks did the same for a transit of Venus in 1639. This provided a strong vindication of heliocentrism and Kepler's elliptical orbits. [275] [276]

In the 17th century, Galileo publicized the use of the telescope in astronomy; he and Simon Marius independently discovered that Jupiter had four satellites in orbit around it. [277] Christiaan Huygens followed on from these observations by discovering Saturn's moon Titan and the shape of the rings of Saturn. [278] In 1677, Edmond Halley observed a transit of Mercury across the Sun, leading him to realize that observations of the solar parallax of a planet (more ideally using the transit of Venus) could be used to trigonometrically determine the distances between Earth, Venus, and the Sun. [279] Halley's friend Isaac Newton, in his magisterial Principia Mathematica of 1687, demonstrated that celestial bodies are not quintessentially different from Earthly ones: the same laws of motion and of gravity apply on Earth and in the skies. [47] :142

True-scale Solar System diagram made by Emanuel Bowen in 1747. At that time, Uranus, Neptune, nor the asteroid belts have been discovered yet. The Solar System, with the orbits of 5 remarkable comets. LOC 2013593161 (cropped).jpg
True-scale Solar System diagram made by Emanuel Bowen in 1747. At that time, Uranus, Neptune, nor the asteroid belts have been discovered yet.

The term "Solar System" entered the English language by 1704, when John Locke used it to refer to the Sun, planets, and comets. [280] In 1705, Halley realized that repeated sightings of a comet were of the same object, returning regularly once every 75–76 years. This was the first evidence that anything other than the planets repeatedly orbited the Sun, [281] though Seneca had theorized this about comets in the 1st century. [282] Careful observations of the 1769 transit of Venus allowed astronomers to calculate the average Earth–Sun distance as 93,726,900 miles (150,838,800 km), only 0.8% greater than the modern value. [283]

Uranus, having occasionally been observed since antiquity, was recognized to be a planet orbiting beyond Saturn by 1783. [284] In 1838, Friedrich Bessel successfully measured a stellar parallax, an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism. [285] Neptune was identified as a planet some years later, in 1846, thanks to its gravitational pull causing a slight but detectable variation in the orbit of Uranus. [286] Mercury's orbital anomaly observations led to searches for Vulcan, a planet interior of Mercury, but these attempts were quashed with Albert Einstein's theory of general relativity in 1915. [287]

In the 20th century, humans began their space exploration around the Solar System, starting with placing telescopes in space since the 1960s. [288] By 1989, all eight planets have been visited by space probes. [289] Probes have returned samples from comets [290] and asteroids, [291] as well as flown through the Sun's corona [292] and visited two dwarf planets (Pluto and Ceres). [293] [294] Humans have landed on the Moon during the Apollo program in the 1960s and 1970s [295] and will return to the Moon in the 2020s with the Artemis program. [296] Discoveries in the 20th and 21st century has prompted the redefinition of the term planet in 2006, hence the demotion of Pluto to a dwarf planet, [297] and further interest in trans-Neptunian objects. [298]

Spaceflight

An example of a Hohmann transfer orbit between Earth and Mars as used by the InSight probe:

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InSight *
Earth *
Mars Animation of InSight trajectory.gif
An example of a Hohmann transfer orbit between Earth and Mars as used by the InSight probe:
  InSight ·   Earth  ·   Mars

Using in-space propulsion, there are many ways to deliver an object to bodies in the Solar System. Time is a key factor in all transfer maneuvers; depending on the mission parameters, the launch window can range from a few minutes to a few weeks. [299]

A simple and efficient way is to make use of the Hohmann transfer orbit, which involves two maneuvers: the first maneuver make the spacecraft's orbit touches with the target body's orbit, and another maneuver move the spacecraft to the desired orbit around the target body. [300] Variants of the Hohmann transfer maneuver (such as the generalized bi-elliptic transfer and continuous low thrust version) are used in many missions to Mars and other planets. [299]

To save on fuel, some space missions make use of gravity assist maneuvers, such as the two Voyager probes accelerating when flyby planets in the outer Solar System [301] and the Parker Solar Probe decelerating closer towards the Sun after flyby with Venus. [302] When a spacecraft flyby a planet, its velocity can be decomposed to two components: the planet's relative motion to the Sun and the spacecraft's "own" motion. The direction of the spacecraft's motion component is affected, resulting in change in the spacecraft's velocity and consequently change in its orbit. [301]

Even more complicated maneuvers that make use of Lagrange points and aerobraking do exist, and in practice, mission maneuvers are calculated with the use of computer programs and an accurate Solar System model. [303]

See also

Notes

  1. The asteroid belt and Kuiper belt are not added because the individual asteroids are too small to be shown on the diagram.
  2. 1 2 The date is based on the oldest inclusions found to date in meteorites, 4568.2+0.2
    −0.4
    million years, and is thought to be the date of the formation of the first solid material in the collapsing nebula. [12]
  3. 1 2 If is the angle between the north pole of the ecliptic and the north galactic pole then:

    where = 27° 07′ 42.01″ and = 12h 51m 26.282s are the declination and right ascension of the north galactic pole, [267] whereas = 66° 33′ 38.6″ and = 18h 0m 00s are those for the north pole of the ecliptic. (Both pairs of coordinates are for J2000 epoch.) The result of the calculation is 60.19°.
  4. Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar System" and "solar system" structures in their naming guidelines document Archived 25 July 2021 at the Wayback Machine . The name is commonly rendered in lower case ('solar system'), as, for example, in the Oxford English Dictionary and Merriam-Webster's 11th Collegiate Dictionary Archived 27 January 2008 at the Wayback Machine .
  5. The International Astronomical Union's Minor Planet Center has yet to officially list Quaoar, Sedna and Gonggong as dwarf planets as of 2024.
  6. For more classifications of Solar System objects, see List of minor-planet groups and Comet § Classification.
  7. 1 2 The mass of the Solar System excluding the Sun, Jupiter and Saturn can be determined by adding together all the calculated masses for its largest objects and using rough calculations for the masses of the Oort cloud (estimated at roughly 3 Earth masses), [33] the Kuiper belt (estimated at 0.1 Earth mass) [34] and the asteroid belt (estimated to be 0.0005 Earth mass) [35] for a total, rounded upwards, of ~37 Earth masses, or 8.1% of the mass in orbit around the Sun. With the combined masses of Uranus and Neptune (~31 Earth masses) subtracted, the remaining ~6 Earth masses of material comprise 1.3% of the total orbiting mass.

Related Research Articles

<span class="mw-page-title-main">Kuiper belt</span> Area of the Solar System beyond the planets, comprising small bodies

The Kuiper belt is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times as wide and 20–200 times as massive. Like the asteroid belt, it consists mainly of small bodies or remnants from when the Solar System formed. While many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles, such as methane, ammonia, and water. The Kuiper belt is home to most of the objects that astronomers generally accept as dwarf planets: Orcus, Pluto, Haumea, Quaoar, and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, may have originated in the region.

<span class="mw-page-title-main">Oort cloud</span> Distant planetesimals in the Solar System

The Oort cloud, sometimes called the Öpik–Oort cloud, is theorized to be a vast cloud of icy planetesimals surrounding the Sun at distances ranging from 2,000 to 200,000 AU. The concept of such a cloud was proposed in 1950 by the Dutch astronomer Jan Oort, in whose honor the idea was named. Oort proposed that the bodies in this cloud replenish and keep constant the number of long-period comets entering the inner Solar System—where they are eventually consumed and destroyed during close approaches to the Sun.

<span class="mw-page-title-main">Planets beyond Neptune</span> Hypothetical planets further than Neptune

Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century and continued at the start of the 20th with Percival Lowell's quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the giant planets, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.

A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has an orbital semi-major axis of 30.1 astronomical units (au).

<span class="mw-page-title-main">Jupiter trojan</span> Asteroid sharing the orbit of Jupiter

The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroids that share the planet Jupiter's orbit around the Sun. Relative to Jupiter, each trojan librates around one of Jupiter's stable Lagrange points: either L4, existing 60° ahead of the planet in its orbit, or L5, 60° behind. Jupiter trojans are distributed in two elongated, curved regions around these Lagrangian points with an average semi-major axis of about 5.2 AU.

<span class="mw-page-title-main">Asteroid belt</span> Region between the orbits of Mars and Jupiter

The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids or minor planets. The identified objects are of many sizes, but much smaller than planets, and, on average, are about one million kilometers apart. This asteroid belt is also called the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System.

<span class="mw-page-title-main">Centaur (small Solar System body)</span> Type of Solar System object

In planetary astronomy, a centaur is a small Solar System body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets. Centaurs generally have unstable orbits because they cross or have crossed the orbits of the giant planets; almost all their orbits have dynamic lifetimes of only a few million years, but there is one known centaur, 514107 Kaʻepaokaʻawela, which may be in a stable orbit. Centaurs typically exhibit the characteristics of both asteroids and comets. They are named after the mythological centaurs that were a mixture of horse and human. Observational bias toward large objects makes determination of the total centaur population difficult. Estimates for the number of centaurs in the Solar System more than 1 km in diameter range from as low as 44,000 to more than 10,000,000.

<span class="mw-page-title-main">90377 Sedna</span> Dwarf planet

Sedna is a dwarf planet in the outermost reaches of the Solar System, orbiting the Sun beyond the orbit of Neptune. Discovered in 2003, the planetoid's surface is one of the reddest known among Solar System bodies. Spectroscopy has revealed Sedna's surface to be mostly a mixture of the solid ices of water, methane, and nitrogen, along with widespread deposits of reddish-colored tholins, a chemical makeup similar to those of some other trans-Neptunian objects. Within the range of uncertainties, it is tied with the dwarf planet Ceres in the asteroid belt as the largest dwarf planet not known to have a moon. Its diameter is roughly 1,000 km. Owing to its lack of known moons, the Keplerian laws of planetary motion cannot be employed for determining its mass, and the precise figure as yet remains unknown.

<span class="mw-page-title-main">Scattered disc</span> Collection of bodies in the extreme Solar System

The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small Solar System bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc objects (SDOs) have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater than 30 astronomical units (4.5×109 km; 2.8×109 mi). These extreme orbits are thought to be the result of gravitational "scattering" by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune.

<span class="mw-page-title-main">Formation and evolution of the Solar System</span> Modelling its structure and composition

There is evidence that the formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a protoplanetary disk out of which the planets, moons, asteroids, and other small Solar System bodies formed.

<span class="mw-page-title-main">Debris disk</span> Disk of dust and debris in orbit around a star

A debris disk, or debris disc, is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris disks are found around stars with mature planetary systems, including at least one debris disk in orbit around an evolved neutron star. Debris disks can also be produced and maintained as the remnants of collisions between planetesimals, otherwise known as asteroids and comets.

<span class="mw-page-title-main">Minor planet</span> Astronomical object in direct orbit around the Sun that is neither a planet or a comet

According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term minor planet, but that year's meeting reclassified minor planets and comets into dwarf planets and small Solar System bodies (SSSBs). In contrast to the eight official planets of the Solar System, all minor planets fail to clear their orbital neighborhood.

<span class="mw-page-title-main">Detached object</span> Dynamical class of minor planets

Detached objects are a dynamical class of minor planets in the outer reaches of the Solar System and belong to the broader family of trans-Neptunian objects (TNOs). These objects have orbits whose points of closest approach to the Sun (perihelion) are sufficiently distant from the gravitational influence of Neptune that they are only moderately affected by Neptune and the other known planets: This makes them appear to be "detached" from the rest of the Solar System, except for their attraction to the Sun.

<span class="mw-page-title-main">Discovery and exploration of the Solar System</span>

Discovery and exploration of the Solar System is observation, visitation, and increase in knowledge and understanding of Earth's "cosmic neighborhood". This includes the Sun, Earth and the Moon, the major planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune, their satellites, as well as smaller bodies including comets, asteroids, and dust.

<span class="mw-page-title-main">Nice model</span> Scenario for the dynamical evolution of the Solar System

The Nicemodel is a scenario for the dynamical evolution of the Solar System. It is named for the location of the Côte d'Azur Observatory—where it was initially developed in 2005—in Nice, France. It proposes the migration of the giant planets from an initial compact configuration into their present positions, long after the dissipation of the initial protoplanetary disk. In this way, it differs from earlier models of the Solar System's formation. This planetary migration is used in dynamical simulations of the Solar System to explain historical events including the Late Heavy Bombardment of the inner Solar System, the formation of the Oort cloud, and the existence of populations of small Solar System bodies such as the Kuiper belt, the Neptune and Jupiter trojans, and the numerous resonant trans-Neptunian objects dominated by Neptune.

The five-planet Nice model is a numerical model of the early Solar System that is a revised variation of the Nice model. It begins with five giant planets, the four that exist today plus an additional ice giant between Saturn and Uranus in a chain of mean-motion resonances.

The jumping-Jupiter scenario specifies an evolution of giant-planet migration described by the Nice model, in which an ice giant is scattered inward by Saturn and outward by Jupiter, causing their semi-major axes to jump, and thereby quickly separating their orbits. The jumping-Jupiter scenario was proposed by Ramon Brasser, Alessandro Morbidelli, Rodney Gomes, Kleomenis Tsiganis, and Harold Levison after their studies revealed that the smooth divergent migration of Jupiter and Saturn resulted in an inner Solar System significantly different from the current Solar System. During this migration secular resonances swept through the inner Solar System exciting the orbits of the terrestrial planets and the asteroids, leaving the planets' orbits too eccentric, and the asteroid belt with too many high-inclination objects. The jumps in the semi-major axes of Jupiter and Saturn described in the jumping-Jupiter scenario can allow these resonances to quickly cross the inner Solar System without altering orbits excessively, although the terrestrial planets remain sensitive to its passage.

Chimera is a NASA mission concept to orbit and explore 29P/Schwassmann-Wachmann 1 (SW1), an active, outbursting small icy body in the outer Solar System. The concept was developed in response to the 2019 NASA call for potential missions in the Discovery-class, and it would have been the first spacecraft encounter with a Centaur and the first orbital exploration of a small body in the outer Solar System. The Chimera proposal was ranked in the first tier of submissions, but was not selected for further development for the programmatic reason of maintaining scientific balance.

<span class="mw-page-title-main">Solar System belts</span> Solar System belts of asteroids and comets

Solar System belts are asteroid and comet belts that orbit the Sun in the Solar System in interplanetary space. The Solar System has both major and minor asteroid and comet belts in the inner Solar System. The Solar System is unique in that it has multiple belts. The observation of other planetary systems has found these systems to have no asteroid belts or one vast asteroid belt. The stars Fomalhaut, HD 69830 and Epsilon Eridani are examples of systems with one large asteroid belt. The Solar System belts size and placement are mostly a result of the Solar System having four giant planets: Jupiter, Saturn, Uranus and Neptune far from the sun. The giant planets must be in the correct place, not too close or too far from the sun for a system to have Solar System belts.

References

Data sources

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