The Solar System is the gravitationally bound system of the Sun and the objects that directly or indirectly orbit it. The eight planets are the largest of the objects that orbit the Sun directly, with the dwarf planets and other Solar System entities making up the rest. Two of the natural satellites that orbit the Sun indirectly are larger than Mercury, the smallest planet.
The Solar System was formed by the gravitational collapse of a massive interstellar molecular cloud 4.6 billion years ago. The Sun holds the great bulk of the system’s mass, with Jupiter holding the majority of the remaining mass. Mercury, Venus, Earth, and Mars, the four smaller inner planets, are terrestrial planets made mostly of rock and metal. The four outer planets are gigantic planets, with masses far greater than the terrestrial planets. The two largest planets, Jupiter and Saturn, are gas giants, consisting primarily of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, consisting primarily of volatiles, such as water, ammonia, and methane, with relatively high melting points compared to hydrogen and helium. The orbits of all eight planets are roughly round and lie inside a nearly flat disc known as the ecliptic.
Smaller objects can also be found in the Solar System. The asteroid belt, which sits between Mars and Jupiter’s orbits, is primarily made up of rock and metal objects, similar to the terrestrial planets. The Kuiper belt and dispersed disc, which are populations of trans-Neptunian objects largely made of ices, sit beyond Neptune’s orbit, as does a newly discovered population of sednoids. Some objects among these populations are massive enough to have rounded under their own gravity, however how many there will be is a matter of discussion. Dwarf planets are the name given to such objects. Pluto is the only dwarf planet known to exist, while another trans-Neptunian object, Eris, is suspected to be one, and the asteroid Ceres is at least near to being one. Other small-body populations, such as comets, centaurs, and interplanetary dust clouds, freely migrate between these two zones in addition to these two. Natural satellites circle six of the planets, the six largest potential dwarf planets, and many of the smaller bodies, which are commonly referred to as “moons” after the Moon. Planetary rings of dust and other tiny particles encircle each of the outer planets.
The heliosphere is a bubble-like region in the interstellar medium created by the solar wind, a stream of charged particles moving outwards from the Sun. The heliopause, which stretches out to the edge of the dispersed disc, is the point where the solar wind’s pressure equals the opposing pressure of the interstellar medium. The Oort cloud, which is assumed to be the source of long-period comets, could be thousands of times further away than the heliosphere. The Solar System is about 26,000 light-years from the Milky Way’s core in the Orion Arm.
Table of Contents
- 1 Exploration and discovery
- 2 Composition and structure
- 3 Distances and scales
- 4 Evolution and formation
- 5 Sun
- 6 Medium between planets
- 7 Inner Solar System
- 8 Outer Solar System
- 9 Comets
- 10 Trans-Neptunian Region
- 11 The most remote areas
- 12 The context of the galaxy
Exploration and discovery
Humanity did not identify or comprehend the notion of the Solar System for the majority of its history. Most people believed that Earth was immobile at the center of the universe until the Late Middle Ages–Renaissance, and thought it was distinct from the divine or ethereal objects that traveled through the sky. Nicolaus Copernicus was the first to develop a mathematically predictable heliocentric theory, despite the fact that Greek philosopher Aristarchus of Samos had hypothesized on a heliocentric reordering of the cosmos.
Galileo discovered that the Sun had sunspots and that Jupiter had four satellites in orbit around it in the 17th century. Following Galileo’s discoveries, Christiaan Huygens discovered Saturn’s moon Titan as well as the shape of Saturn’s rings. In 1705, Edmond Halley realized that repeated comet observations were documenting the same item, which appeared once every 75–76 years. This was the first proof that the Sun was orbited by something other than planets. The word “Solar System” first emerged in English around this time (1704). Friedrich Bessel successfully measured a stellar parallax, an apparent shift in the position of a star caused by Earth’s motion around the Sun, in 1838, proving heliocentrism for the first time. Since then, advances in observational astronomy and the utilization of uncrewed spacecraft have made it possible to investigate other things orbiting the Sun in more detail.
Composition and structure
The Sun, a G2 main-sequence star that comprises 99.86 percent of the system’s known mass and dominates it gravitationally, is the most important component in the Solar System. The big planets, the Sun’s four largest circling entities, contribute for 99 percent of the remaining mass, with Jupiter and Saturn accounting for more than 90 percent. The remaining Solar System objects (including the four terrestrial planets, dwarf planets, moons, asteroids, and comets) account for less than 0.002% of the total mass of the Solar System.
The ecliptic, or plane of Earth’s orbit, is where most massive objects in orbit around the Sun are found. Comets and Kuiper belt objects are typically at considerable angles to the ecliptic, whereas planets are quite close to it. Planets (and most other objects) orbit the Sun in the same direction that the Sun rotates (counter-clockwise, as seen from above Earth’s north pole) as a result of the Solar System’s origin. Halley’s Comet, for example, is an exception. Most larger moons orbit their planets in this prograde manner (the largest retrograde exception being Triton), and most larger objects rotate in the same direction (with Venus being a notable retrograde exception).
The Sun, four relatively tiny inner planets bordered by a belt of predominantly stony asteroids, and four massive planets encircled by the Kuiper belt of largely frozen objects make up the basic structure of the known parts of the Solar System. This structure is occasionally divided into several sections by astronomers informally. The four terrestrial planets, as well as the asteroid belt, make up the inner Solar System. The four large planets, as well as the asteroids, make up the outer Solar System. Since the discovery of the Kuiper belt, the Solar System’s far reaches have been seen as a distinct zone containing objects beyond Neptune.
The majority of planets in the Solar System have their own secondary systems, which are orbited by planetary objects known as natural satellites or moons (two of which, Titan and Ganymede, are larger than the planet Mercury). Planetary rings are narrow bands of small particles that orbit the four big planets in tandem. The majority of the largest natural satellites rotate synchronously, with one face always towards their parent.
The orbits of objects around the Sun are described by Kepler’s laws of planetary motion. Each object follows Kepler’s laws and moves along an ellipse with the Sun at one point. Because they are more impacted by the Sun’s gravity, objects closer to the Sun (with smaller semi-major axes) travel faster. A body’s distance from the Sun fluctuates over the course of a year in an elliptical orbit. Perihelion refers to a body’s closest approach to the Sun, whereas aphelion refers to its farthest distance from the Sun. Planets have essentially circular orbits, while many comets, asteroids, and Kuiper belt objects have very elliptical orbits. Numerical models can be used to estimate the positions of the bodies in the Solar System.
Despite the fact that the Sun dominates the system in terms of mass, it only accounts for around 2% of the angular momentum. The planets, led by Jupiter, account for the majority of the remaining angular momentum due to a combination of their mass, orbit, and distance from the Sun, with comets potentially contributing a large amount.
The Sun is almost entirely made up of hydrogen and helium, accounting for virtually all of the matter in the Solar System. Jupiter and Saturn, which make up roughly all of the remaining stuff, are mostly made of hydrogen and helium as well. The Solar System has a composition gradient caused by the Sun’s heat and light pressure; things closer to the Sun, which are more impacted by heat and light pressure, are made up of elements with high melting points. The composition of objects further from the Sun is mostly made up of elements with lower melting temperatures. The frost line, which is located about 5 AU from the Sun and beyond which those volatile chemicals may condense, is a barrier in the Solar System beyond which those volatile substances may condense.
Rock, the collective designation for materials with high melting points, such as silicates, iron, or nickel, that stayed solid under practically all conditions in the protoplanetary nebula, makes up the majority of the inner Solar System’s objects. The principal constituents of Jupiter and Saturn are gases, which are materials with extremely low melting temperatures and high vapour pressure, such as hydrogen, helium, and neon, which were always in the gaseous phase in the nebula. Water, methane, ammonia, hydrogen sulfide, and carbon dioxide are examples of ices with melting points of a few hundred kelvins. They can be found as ices, liquids, or gases in diverse locations around the Solar System, but they were either solid or gaseous in the nebula. Icy materials make up the majority of the large planets’ satellites, as well as the majority of Uranus and Neptune (the “ice giants”) and the numerous tiny objects beyond Neptune’s orbit. Volatiles are the combination of gases and ices.
Distances and scales
1 astronomical unit [AU] is the distance between Earth and the Sun (150,000,000 km; 93,000,000 mi). The radius of the Sun, for example, is 0.0047 AU (700,000 km). As a result, the Sun takes approximately 0.00001 percent (105% ) of the volume of a sphere with a radius equal to Earth’s orbit, but Earth’s volume is around one millionth (106) of the Sun’s. The largest planet, Jupiter, is 5.2 astronomical units (780,000,000 km) from the Sun and has a radius of 71,000 km (0.00047 AU), while Neptune is 30 AU (4.5109 km) from the Sun.
The farther a planet or belt is from the Sun, the greater the distance between its orbit and the orbit of the next nearer object to the Sun, with a few exceptions. Venus, for example, is 0.33 AU farther away from the Sun than Mercury, but Jupiter is 4.3 AU away from Saturn, and Neptune is 10.5 AU away from Uranus. A link between these orbital distances has been attempted (for example, the Titius–Bode law), but no such theory has been adopted.
Some Solar System models seek to convey the Solar System’s relative scales in human terms. Some are tiny in scale (and may be mechanical; these are known as orreries), while others span cities or regions. The largest scale model, the Sweden Solar System, uses the 110-meter (361-foot) Ericsson Globe in Stockholm as its substitute Sun, and Jupiter is a 7.5-meter (25-foot) sphere at Stockholm Arlanda Airport, 40 kilometers (25 miles) away, whereas the farthest current object, Sedna, is a 10 cm (4 in) sphere in Lule, 912 kilometers (567 miles) away.
The Sun would be about 3 cm in diameter (roughly two-thirds the diameter of a golf ball) if the Sun–Neptune distance were scaled to 100 meters, the giant planets would all be smaller than 3 mm, and Earth’s diameter, along with that of the other terrestrial planets, would be smaller than a flea (0.3 mm) at this scale.
Evolution and formation
The Solar System was generated by the gravitational collapse of an area within a massive molecular cloud 4.568 billion years ago. This first cloud was probably several light-years across and gave birth to a number of stars. This molecular cloud was primarily hydrogen, with some helium and minor amounts of heavier elements fused by past generations of stars, as is typical of molecular clouds. As the pre-solar nebula, which would later become the Solar System, compressed, conservation of angular momentum led it to rotate faster. The center, where the majority of the mass gathered, grew hotter than the surrounding disc. The contracting nebula began to flatten into a protoplanetary disc with a diameter of around 200 AU and a hot, dense protostar at its center as it rotated faster. Planets evolved as a result of accretion from this disc, in which dust and gas gravitationally attracted one another and coalesced to form progressively larger masses. Hundreds of protoplanets may have formed in the early Solar System, but they combined or were destroyed, leaving planets, dwarf planets, and minor bodies behind.
Only metals and silicates could live in solid form in the warm inner Solar System close to the Sun due to their higher boiling temperatures, and these would eventually create the rocky planets of Mercury, Venus, Earth, and Mars. The terrestrial planets could not develop very large because metallic elements made up such a modest part of the solar nebula. The giant planets (Jupiter, Saturn, Uranus, and Neptune) originated far out, beyond the frost line, the point between Mars’ and Jupiter’s orbits where material is frigid enough to keep volatile icy compounds solid. Because the ices that formed these planets were more abundant than the metals and silicates that formed the terrestrial inner planets, they were able to grow massive enough to capture large atmospheres of hydrogen and helium, the lightest and most abundant elements, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium. Asteroid belts, Kuiper belts, and Oort clouds are examples of leftover debris that never became planets. The Nice model explains how these regions came to be, as well as how the outer planets formed in various positions and migrated to their current orbits via gravitational interactions.
Within 50 million years, the pressure and density of hydrogen in the protostar’s core had increased to the point where thermonuclear fusion could commence. Temperature, response rate, pressure, and density all grew until hydrostatic equilibrium was reached, at which point the thermal pressure equaled gravity’s force. The Sun became a main-sequence star at this epoch. The Sun’s main-sequence phase will endure around 10 billion years from beginning to end, compared to around two billion years for all other periods of its pre-remnant life combined. The heliosphere was formed by the Sun’s solar wind, which blew the remaining gas and dust from the protoplanetary disc into interstellar space, effectively finishing the planetary formation process. The Sun is becoming brighter; it was 70 percent brighter early in its main-sequence life than it is now.
The Solar System will remain mostly unchanged until the hydrogen in the Sun’s core is completely transformed to helium, which will take approximately 5 billion years. The Sun’s main-sequence life will come to an end at this time. The Sun’s core will contract at this time, with hydrogen fusion taking place along a shell around the inert helium, and the energy output will be much higher than it is now. The Sun’s outer layers will grow to nearly 260 times its current diameter, turning it into a red giant. The Sun’s surface will be significantly cooler (2,600 K at its coolest) than it is on the main sequence due to its massively enlarged surface area. Mercury is anticipated to be vaporized by the growing Sun, rendering Earth uninhabitable. The Sun’s core will eventually get hot enough for helium fusion, and it will burn helium for a fraction of the time it burnt hydrogen. The Sun isn’t big enough to start fusing heavier elements, thus nuclear reactions in its core will slow down. Its outer layers will disperse into space, leaving a white dwarf, a dense entity with half the mass of the Sun but only half the size of the Earth. The expelled outer layers will form a planetary nebula, returning to the interstellar medium part of the material that produced the Sun, but now enriched with heavier elements like carbon.
The Sun is the most massive component of the Solar System and its star. Its massive mass (332,900 Earth masses), which accounts for 99.86 percent of all mass in the Solar System, causes temperatures and densities in its core to be high enough to support nuclear fusion of hydrogen into helium, allowing it to be classified as a main-sequence star. This releases a tremendous quantity of energy, which is primarily transmitted into space as electromagnetic radiation with a visible light peak.
The Sun is a main-sequence star of the G2 class. The more brilliant main-sequence stars are, the hotter they are. The temperature of the Sun is in the middle of the range of the hottest and coldest stars. Stars that are brighter and hotter than the Sun are rare, but red dwarfs, which are far dimmer and cooler than the Sun, make up 85 percent of the Milky Way’s stars.
The Sun is a population I star, which means it has a larger quantity of elements heavier than hydrogen and helium (known as “metals” in astronomy) than older population II stars. Heavy elements heavier than hydrogen and helium were produced in the cores of ancient and bursting stars, therefore the cosmos had to be filled with these atoms before the first generation of stars died. Metals are few in the oldest stars, but they are abundant in the stars born later. Because planets form from the accretion of “metals,” this high metallicity is assumed to have been critical to the Sun’s creation of a planetary system.
Medium between planets
The interplanetary medium, which makes up the vast majority of the Solar System, is a near-vacuum. The solar wind is a continuous stream of charged particles (plasma) that the Sun emits in addition to light. This stream of particles extends outwards at a rate of around 1.5 million kilometers per hour, forming a fragile atmosphere that reaches out to at least 100 AU in the interplanetary medium (see Heliosphere). The heliosphere is disturbed by activity on the Sun’s surface, such as solar flares and coronal mass ejections, which causes space weather and geomagnetic storms. The heliospheric current sheet, a spiral formed by the Sun’s spinning magnetic field on the interplanetary medium, is the greatest structure within the heliosphere.
The magnetic field of the Earth prevents the solar wind from destroying its atmosphere. Because Venus and Mars lack magnetic fields, the solar wind is progressively causing their atmospheres to flow away into space. Coronal mass ejections and other related occurrences release a magnetic field and massive amounts of material from the Sun’s surface. The interaction of this magnetic field and material with Earth’s magnetic field funnels charged particles into the upper atmosphere, where they interact to produce aurorae near the magnetic poles.
The heliosphere and planetary magnetic fields (for those planets have them) help to protect the Solar System against cosmic rays, which are high-energy interstellar particles. On very long timeframes, the quantity of cosmic rays in the interstellar medium and the strength of the Sun’s magnetic field change, hence the level of cosmic-ray penetration in the Solar System changes, albeit by how much is unknown.
At least two disc-like areas of cosmic dust can be found in the interplanetary medium. The zodiacal dust cloud, which is located in the inner Solar System and is responsible for the zodiacal light, is the first. It was most likely generated by asteroid belt collisions caused by gravitational interactions with the planets. The second dust cloud, which stretches from 10 to 40 AU, was likely caused by comparable impacts within the Kuiper belt.
Inner Solar System
The region of the Solar System that includes the terrestrial planets and the asteroid belt is known as the inner Solar System. The objects of the inner Solar System, which are mostly silicates and metals, are relatively close to the Sun; their radius is smaller than the distance between Jupiter and Saturn’s orbits. This region is also within the frost line, which is around 700 million kilometers from the Sun and is a little less than 5 AU.
Planets in the inner solar system
The four terrestrial planets, often known as the inner planets, have stony compositions, few or no moons, and no ring systems. They’re mostly made up of refractory minerals like silicates, which make up their crusts and mantles, and metals like iron and nickel, which make up their cores. Three of the four inner planets (Venus, Earth, and Mars) have atmospheres large enough to produce weather, and they all have impact craters and tectonic features like rift valleys and volcanoes. Inner planet is not to be confused with inferior planet, which refers to planets that orbit the Sun closer than Earth (i.e. Mercury and Venus).
Mercury is the nearest planet to the Sun (0.4 AU), as are the other seven planets on average. Mercury (0.055 M) is the smallest planet in the Solar System and has no natural satellites. Its only known geological features, aside from impact craters, are lobed ridges or rupes, which were most likely formed during a period of contraction early in its existence. The solar wind blasts atoms off Mercury’s surface, creating its tenuous atmosphere. Its comparatively massive iron core and thin mantle are still a mystery. Its outer layers may have been taken away by a massive impact, or it may have been stopped from fully accreting by the energy of the young Sun.
Venus (0.7 AU from the Sun) is about the same size as Earth (0.815 M), with a thick silicate mantle surrounding an iron core, a large atmosphere, and evidence of internal geological activity. Its atmosphere is ninety times as dense as Earth’s, and it is significantly drier. There are no natural satellites orbiting Venus. It is the hottest planet, with surface temperatures exceeding 400 degrees Celsius (752 degrees Fahrenheit), owing to high levels of greenhouse gases in the atmosphere. There has been no definite evidence of ongoing geological activity on Venus, yet it lacks a magnetic field that would prevent depletion of its large atmosphere, implying that it is supplied by volcanic eruptions.
Earth (1 AU from the Sun) is the largest and densest of the inner planets, as well as the only one known to have active geological activity and life. It is the only planet where plate tectonics have been detected, and its liquid hydrosphere is unusual among terrestrial planets. The atmosphere of Earth differs dramatically from that of other planets, having been transformed by the advent of life to contain 21% free oxygen. It has only one natural satellite, the Moon, which is the Solar System’s only big satellite of a terrestrial planet.
Mars is smaller than Earth (1.5 AU from the Sun) and Venus (0.107 M). It has a carbon dioxide-dominated atmosphere with a surface pressure of 6.1 millibars (roughly 0.6 percent of that of Earth). Its surface, which is dotted with massive volcanoes like Olympus Mons and rift valleys like Valles Marineris, reveals geological activity that may have lasted as recently as 2 million years ago. Iron oxide (rust) in the soil gives it its red color. Deimos and Phobos, two small natural satellites of Mars, are assumed to be trapped asteroids or ejected debris from a major impact early in the planet’s history.
Belt of asteroids
Asteroids, with the exception of Ceres, are tiny Solar System asteroids made primarily of refractory stony and metallic materials, with some ice. They can range in size from a few meters to hundreds of kilometers. Meteoroids and micrometeoroids (grain-sized asteroids) are terms used to describe asteroids smaller than one meter, depending on different, sometimes arbitrary definitions.
Between 2.3 and 3.3 AU from the Sun, the asteroid belt lies between Mars and Jupiter. It’s supposed to be remains from the birth of the Solar System that failed to merge because to Jupiter’s gravitational influence. There are tens of thousands, if not millions, of objects larger than one kilometer in diameter in the asteroid belt. Despite this, the asteroid belt’s overall mass is unlikely to exceed a thousandth of that of Earth. The asteroid belt is sparsely inhabited, and spacecraft travel through it without incident on a regular basis.
Ceres is the largest asteroid, a protoplanet, and a dwarf planet, with a distance of 2.77 AU. It has a diameter of just about 1000 km and a mass large enough to bring it into a spherical shape by gravity. Ceres was originally classed as a planet when it was found in 1801 but was reclassified as an asteroid in the 1850s as more observations showed more asteroids. When the definition of a planet was created in 2006, it was categorized as a dwarf planet.
Groups of asteroids
Based on their orbital properties, asteroids in the asteroid belt are categorized into asteroid groups and families. Asteroids that orbit larger asteroids are known as asteroid moons. They are not as distinct as planetary moons, and they can be nearly as massive as their counterparts. Main-belt comets, which may have been the source of Earth’s water, are also found in the asteroid belt.
Jupiter trojans are small bodies that are found in Jupiter’s L4 or L5 points (gravitationally stable areas that lead and trail a planet in its orbit); the name trojan is also applied to small bodies that are found in any other planetary or satellite Lagrange point. The Hilda asteroids orbit Jupiter in a 2:3 resonance, which means they orbit the Sun three times for every two Jupiter orbits.
Near-Earth asteroids, many of which cross the orbits of the inner planets, are also found in the inner Solar System. Some of them have the potential to be dangerous.
Outer Solar System
The huge planets and their huge moons exist in the outer region of the Solar System. This is also where centaurs and many short-period comets orbit. Because of their larger distance from the Sun, solid objects in the outer Solar System have a higher proportion of volatiles like water, ammonia, and methane than those in the inner Solar System, as the lower temperatures allow these compounds to remain solid.
The four outer planets, often known as big planets or Jovian planets, account for 99 percent of the mass known to orbit the Sun. Jupiter and Saturn are gas giants because they are more than 400 times the mass of Earth and are primarily made up of the gases hydrogen and helium. Uranus and Neptune are much less substantial than Earth, each weighing less than 20 Earth masses (M) and consisting mostly of ices. For these reasons, some astronomers believe they should be classified as ice giants. Although all four big planets have rings, only Saturn’s ring system can be seen from Earth. The term superior planet refers to planets that are outside of Earth’s orbit, which includes the outer planets as well as Mars.
Jupiter (5.2 AU) has a mass of 318 M, which is 2.5 times that of all the other planets combined. It’s mostly made up of hydrogen and helium. The Great Red Spot and cloud bands are semi-permanent features in Jupiter’s atmosphere due to the planet’s intense internal heat. Jupiter is reported to have 79 satellites. Ganymede, Callisto, Io, and Europa, the four largest, have features in common with terrestrial planets, including as volcanism and interior heating. Ganymede, the Solar System’s biggest satellite, is larger than Mercury.
Saturn (9.5 AU), known for its large ring system, shares some characteristics with Jupiter, including its atmospheric composition and magnetosphere. Saturn, despite having 60% of Jupiter’s volume, is just a third as massive, with a mass of 95 M. Saturn is the Solar System’s sole planet with a density lower than that of water. Saturn’s rings are made up of tiny ice and rock particles. Saturn has 82 verified satellites, the majority of which are made of ice. Titan and Enceladus, two of them, show signs of geological activity. Titan, the Solar System’s second-largest moon, is larger than Mercury and the Solar System’s sole satellite with a significant atmosphere.
Uranus (19.2 AU) is the lightest of the outer planets, weighing 14 M. It is the only planet that orbits the Sun on its side; its axial tilt to the ecliptic is almost ninety degrees. Its core is substantially colder than those of the other major planets, and it emits very little heat into space. Titania, Oberon, Umbriel, Ariel, and Miranda are the largest of Uranus’ 27 known satellites.
Although Neptune (30.1 AU) is somewhat smaller than Uranus, it is more massive (17 M) and so denser. It radiates more internal heat than Jupiter or Saturn, but not nearly as much as Jupiter or Saturn. There are 14 known satellites of Neptune. Triton, the largest, is geologically active, featuring liquid nitrogen geysers. The only major satellite in a retrograde orbit is Triton. Neptune is orbited by a group of small planets known as Neptune trojans, which are in 1:1 resonance with it.
The centaurs are frozen comet-like bodies with semi-major axes that are larger than Jupiter’s (5.5 AU) but smaller than Neptune’s (30 AU). 10199 Chariklo, the largest known centaur, with a diameter of around 250 kilometers. Because it produces a coma as comets do when they approach the Sun, the first centaur found, 2060 Chiron, has also been categorized as a comet (95P).
Comets are tiny entities in the Solar System that are mostly made up of volatile ices. They are typically only a few kilometers across. They have highly eccentric orbits, with perihelions in the inner planets’ orbits and aphelions far beyond Pluto. When a comet enters the inner Solar System, its frozen surface sublimates and ionizes due to its proximity to the Sun, forming a coma: a long tail of gas and dust visible to the human eye.
Comets with orbits of less than two hundred years are known as short-period comets. Comets with long periods of time have orbits that last thousands of years. Short-period comets, like Hale–Bopp, are thought to come from the Kuiper belt, while long-period comets, like Hale–Bopp, are thought to come from the Oort cloud. The Kreutz Sungrazers, for example, originate from the split of a single parent comet. Some comets with hyperbolic orbits may have formed outside of the Solar System, but pinpointing their exact locations is challenging. Asteroids are typically classified as old comets whose volatiles have been mostly driven out by solar heat.
Beyond Neptune’s orbit lies the “trans-Neptunian region,” which includes the doughnut-shaped Kuiper belt, which contains Pluto and several other dwarf planets, as well as an overlapping disc of scattered objects that is tilted toward the plane of the Solar System and extends much further out than the Kuiper belt. The region as a whole remains mostly undiscovered. It appears to be made up of tens of thousands of small worlds made mostly of rock and ice, with the largest having a diameter only a fifth that of Earth and a mass significantly less than that of the Moon. This region, which encompasses both the inner and outer Solar Systems, is frequently referred to as the “third zone of the Solar System.”
The Kuiper belt is a large ring of debris comparable to the asteroid belt, but made up mostly of ice-based objects. Its distance from the Sun is between 30 and 50 AU. Despite the fact that it is thought to include dozens to thousands of dwarf planets, it is mostly made up of minor Solar System bodies. With more information, several of the bigger Kuiper belt objects, such as Quaoar, Varuna, and Orcus, may turn out to be dwarf planets. There are about 100,000 Kuiper belt objects with a diameter more than 50 kilometers, although the Kuiper belt’s overall mass is believed to be a tenth or possibly a hundredth that of Earth. Many Kuiper belt objects have numerous satellites, and the majority of their orbits take them outside the ecliptic plane.
The “classical” belt and the resonances can be generally split in the Kuiper belt. Resonances are orbits that are related to Neptune’s (e.g. twice for every three Neptune orbits, or once for every two). The first resonance occurs within Neptune’s own orbit. The classical belt, which spans 39.4 AU to 47.7 AU, is made up of objects that have no resonance with Neptune. Members of the classical Kuiper belt are known as cubewanos, after the first of their sort to be identified, 15760 Albion (formerly known as 1992 QB1), and are still in low-eccentricity orbits around the planet.
Pluto and Charon
The dwarf planet Pluto (with an average orbit of 39 AU) is the Kuiper belt’s biggest known object. It was once thought to be the ninth planet when it was found in 1930; however, with the adoption of a formal definition of planet in 2006, this was no longer the case. Pluto’s orbit is relatively eccentric, inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU to 49.5 AU from the Sun at perihelion (inside Neptune’s orbit). Pluto and Neptune have a 3:2 resonance, which means Pluto orbits the Sun twice for every three Neptunian orbits. Plutonos are Kuiper belt objects whose orbits have this resonance.
Because the two bodies orbit a barycentre of gravity above their surfaces, Charon, Pluto’s largest moon, is frequently described as part of a binary system with Pluto (i.e. they appear to “orbit each other”). Styx, Nix, Kerberos, and Hydra are four much smaller moons orbiting inside the system beyond Charon.
Haumea and Makemake
Makemake (45.79 AU average) is the largest known object in the classical Kuiper belt, while being smaller than Pluto (that is, a Kuiper belt object not in a confirmed resonance with Neptune). After Pluto, Makemake is the brightest object in the Kuiper belt. In 2008, it was given a naming committee with the anticipation that it will be discovered to be a dwarf planet. Its orbit, at 29°, is far more inclined than Pluto’s.
Haumea (average orbit 43.13 AU) is on a similar orbit to Makemake, with the exception that it is in a temporary 7:12 orbital resonance with Neptune.
It was named with the idea that it would turn out to be a dwarf planet, but later observations have revealed that it may not be one at all.
Short-period comets are thought to come from the dispersed disc, which covers the Kuiper belt but reaches out to around 200 AU. The gravitational effect of Neptune’s early outward migration is assumed to have expelled scattered-disc objects into chaotic orbits. Perihelia is found in most scattered disc objects (SDOs) within the Kuiper belt, whereas aphelia is found well beyond it (some more than 150 AU from the Sun). The orbits of SDOs are similarly significantly inclined to the ecliptic plane, often nearly perpendicular to it. According to some astronomers, the scattered disc is really another part of the Kuiper belt, and scattered disc objects are referred to as “scattered Kuiper belt objects.” Centaurs, like the outward-dispersed occupants of the scattered disc, are classified as inward-scattered Kuiper belt objects by some astronomers.
Eris (with an average orbit of 68 AU) is the largest known dispersed disc object, and because it is 25% more massive than Pluto and has a diameter similar to Pluto, it has sparked controversy regarding what defines a planet. It is the most massive of the dwarf planets that have been discovered. Dysnomia is its only known moon. Its orbit, like Pluto’s, is highly eccentric, with a perihelion of 38.2 AU (approximately Pluto’s distance from the Sun) and an aphelion of 97.6 AU, as well as a sharp tilt to the ecliptic plane.
The most remote areas
Because its outer boundaries are sculpted by two forces, the solar wind and the Sun’s gravity, the point at which the Solar System stops and interstellar space begins is not exactly defined. The heliopause, or outer boundary of the heliosphere, is regarded the start of the interstellar medium because the solar wind’s effect is nearly four times Pluto’s distance from the Sun. The Hill sphere of the Sun, which contains the hypothetical Oort cloud, is thought to stretch up to a thousand times farther.
The heliosphere is a stellar-wind bubble, an area of space dominated by the Sun, in which the Sun discharges its solar wind, a stream of charged particles, at around 400 km/s until it collides with the interstellar medium’s wind.
The collision happens at the termination shock, which is about 80–100 AU upwind of the interstellar medium and about 200 AU downwind of the Sun. The wind here slows drastically, condenses, and gets more turbulent, generating the heliosheath, a large oval structure. This structure is thought to resemble a comet’s tail, extending outward for another 40 AU on the upwind side but tailing many times that distance downwind; evidence from the Cassini and Interstellar Boundary Explorer spacecraft suggests it is pushed into a bubble shape by the interstellar magnetic field’s constraining action.
The heliopause, or outer limit of the heliosphere, is the point where the solar wind eventually stops and interstellar space begins. At 94 and 84 AU from the Sun, respectively, Voyager 1 and 2 are said to have passed the termination shock and entered the heliosheath. In August 2012, Voyager 1 was believed to have passed through the heliopause.
The fluid dynamics of interactions with the interstellar medium, as well as solar magnetic fields prevailing to the south, are likely to influence the shape and form of the heliosphere’s outer edge, which is bluntly shaped with the northern hemisphere extending 9 AU farther than the southern hemisphere. The bow shock is a plasma “wake” left by the Sun as it moves through the Milky Way beyond the heliopause, at roughly 230 AU.
Conditions in local interstellar space are uncertain due to a lack of data. NASA’s Voyager spacecraft is expected to transmit crucial data on radiation levels and solar wind to Earth as they travel through the heliopause. It’s unclear how successfully the heliosphere protects the Solar System from cosmic rays. A NASA-funded team has devised a plan for a “Vision Mission” that would launch a probe into the heliosphere.
90377 Sedna is a big, reddish object with a massive, very elliptical orbit that takes it from 76 AU at perihelion to 940 AU at aphelion and takes 11,400 years to complete. Mike Brown, who discovered the object in 2003, claims that it cannot be a component of the dispersed disc or the Kuiper belt since its perihelion is too far away for Neptune’s migration to have touched it. He and other astronomers believe it is the first of a new class of objects known as “distant detached objects” (DDOs), which could include 2000 CR105, which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3,420 years. Brown dubs this population the “inner Oort cloud” since it appears to have developed in a similar way to the Oort cloud, despite the fact that it is much closer to the Sun. Sedna is most certainly a dwarf planet, while its exact shape is unknown. 2012 VP113, discovered in 2012, is the second unambiguously detached object, with a perihelion around 81 AU further than Sedna’s. Its aphelion, at 400–500 AU, is half that of Sedna.
Cloud of Oort
The Oort cloud is a hypothesized spherical cloud of up to a trillion frozen objects that surrounds the Solar System at around 50,000 AU (about 1 light-year (ly)), and potentially as far as 100,000 AU. It is assumed to be the source of all long-period comets (1.87 ly). It’s supposed to be made up of comets ejected from the inner Solar System as a result of gravitational interactions with the outer planets. Collisions, the gravitational influence of a passing star, or the galactic tide, the tidal force exerted by the Milky Way, can cause Oort cloud objects to travel very slowly and be agitated by uncommon events.
Much of the Solar System remains a mystery. Out to around two light-years, the Sun’s gravitational field is predicted to dominate the gravitational forces of neighboring stars (125,000 AU). Lower estimates for the Oort cloud’s radius, on the other hand, do not put it beyond 50,000 AU. Despite discoveries like Sedna, the region between the Kuiper belt and the Oort cloud, which spans tens of thousands of AU, remains largely unexplored. Studies of the zone between Mercury and the Sun are also ongoing. Uncharted regions of the Solar System may potentially yield objects.
Currently, the farthest known objects, such as Comet West, have aphelia approximately 70,000 AU from the Sun, although this may alter as the Oort cloud is more understood.
The context of the galaxy
The Solar System is situated within the Milky Way, a barred spiral galaxy with a diameter of 100,000 light-years and a population of over 100 billion stars. The Sun is located in the Orion–Cygnus Arm or Local Spur, one of the Milky Way’s outer spiral arms. The Sun is around 26,660 light-years from the Galactic Center, and it travels around the Milky Way at a speed of 247 km/s, completing one revolution every 210 million years. The Solar System’s galactic year is the name given to this revolution. The solar apex, or the direction of the Sun’s passage through interstellar space, is near Hercules, in the direction of the bright star Vega’s current location. The ecliptic plane is at an angle of around 60 degrees to the galactic plane.
The placement of the Solar System in the Milky Way has a role in the evolution of life on Earth. It has a nearly circular orbit, and its orbits near the Sun are almost the same speed as the spiral arms. As a result, the Sun only passes through the arms on rare occasions. Because spiral arms contain a much higher concentration of supernovae, gravitational instabilities, and radiation that may destabilize the Solar System, Earth has experienced long periods of stability. According to the Shiva theory and related hypotheses, the changing position of the Solar System relative to other areas of the Milky Way could explain periodic extinction events on Earth. The Solar System is located far from the galactic center, which is densely populated with stars. Gravitational tugs from neighboring stars near the center might disrupt bodies in the Oort cloud and push many comets into the inner Solar System, resulting in collisions that could be devastating for life on Earth. The galactic center’s powerful radiation may also obstruct the creation of sophisticated life. Some scientists believe that recent supernovae may have harmed life in the last 35,000 years by shooting chunks of expelled star core towards the Sun as radioactive dust grains and larger, comet-like debris, even at the Solar System’s current location.
The Solar System is located within the Local Interstellar Cloud, often known as the Local Fluff. It is assumed to be near the G-Cloud, although it is unknown if the Solar System is immersed in the Local Interstellar Cloud or if it is in the region where the G-Cloud and the Local Interstellar Cloud interact. The Local Interstellar Cloud is a thicker cloud in an otherwise sparse region known as the Local Bubble, a 300-light-year-wide hourglass-shaped hole in the interstellar medium. The bubble is suffused with high-temperature plasma, indicating that it was formed by numerous recent supernovae.
Within ten light-years of the Sun, there are just a few stars. The triple star system Alpha Centauri, which is around 4.4 light-years away, is the closest. Alpha Centauri A and B are a near pair of Sun-like stars, whereas Proxima Centauri, a tiny red dwarf, orbits them at a distance of 0.2 light-year. Proxima Centauri b, the closest verified exoplanet to the Sun, was verified in 2016 to be orbiting Proxima Centauri, the closest confirmed exoplanet to the Sun. The red dwarfs Barnard’s Star (at 5.9 ly), Wolf 359 (7.8 ly), and Lalande 21185 are the stars closest to the Sun (8.3 ly).
Sirius is the nearest star, a bright main-sequence star 8.6 light-years away with a mass nearly twice that of the Sun that is orbited by a white dwarf, Sirius B. The binary Luhman 16 system, which is 6.6 light-years away, is the closest brown dwarf system. The binary red dwarf system Luyten 726-8 (8.7 ly) and the lonely red dwarf Ross 154 are both within ten light-years (9.7 ly). Tau Ceti, at 11.9 light-years from Earth, is the closest isolated Sun-like star to the Solar System. It possesses around 80% of the mass of the Sun but only 60% of its brightness. WISE 08550714, a free-floating planetary-mass object smaller than 10 Jupiter masses and around 7 light-years away, is the closest known free-floating planetary-mass object approaching the Sun.
Comparison with extrasolar systems
In comparison to many other planetary systems, the Solar System is unique in that it lacks planets inside Mercury’s orbit. There are no super-Earths in the known Solar System (Planet Nine could be a super-Earth beyond the known Solar System). It has only small rocky planets and enormous gas giants, which is unusual; elsewhere, planets of intermediate size—both rocky and gas—are abundant, thus there is no “gap” between the sizes of Earth and Neptune (with a radius 3.8 times as large). Furthermore, these super-Earths have orbits that are closer together than Mercury’s. This led to the notion that all planetary systems begin with a large number of close-together planets, and that a series of collisions causes mass to be concentrated into a few larger planets, but that collisions in the Solar System resulted in their destruction and ejection.
Planets’ orbits are almost circular in the Solar System. They have a lower orbital eccentricity than other systems. The exact causes are unknown, despite attempts to explain it partially with a bias in the radial-velocity detection method and partially with extended interactions of a large number of planets.
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