A planetary system is a set of gravitationally bound non-stellar objects in or out of orbit around a star or star system. Speaking, systems with one or more planets constitute a planetary system, although such systems may consist of bodies such as dwarf planets, natural satellites, comets and circumstellar disks; the Sun together with its planetary system, which includes Earth, is known as the Solar System. The term exoplanetary system is sometimes used in reference to other planetary systems; as of 1 April 2019, there are 4,023 confirmed planets in 3,005 systems, with 656 systems having more than one planet. Debris disks are known to be common, though other objects are more difficult to observe. Of particular interest to astrobiology is the habitable zone of planetary systems where planets could have surface liquid water, thus the capacity to harbor Earth-like life. Heliocentrism was opposed to geocentrism; the notion of a heliocentric Solar System, with the Sun at the center, is first suggested in the Vedic literature of ancient India, which refer to the Sun as the "centre of spheres".
Some interpret Aryabhatta's writings in Āryabhaṭīya as implicitly heliocentric. The idea was first proposed in Western philosophy and Greek astronomy as early as the 3rd century BC by Aristarchus of Samos, but received no support from most other ancient astronomers. De revolutionibus orbium coelestium by Nicolaus Copernicus, published in 1543, was the first mathematically predictive heliocentric model of a planetary system. 17th-century successors Galileo Galilei, Johannes Kepler, Isaac Newton developed an understanding of physics which led to the gradual acceptance of the idea that the Earth moves round the Sun and that the planets are governed by the same physical laws that governed the Earth. In the 16th century the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are accompanied by planets, he was burned at the stake for his ideas by the Roman Inquisition.
In the 18th century the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centers of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."His theories gained traction through the 19th and 20th centuries despite a lack of supporting evidence. Long before their confirmation by astronomers, conjecture on the nature of planetary systems had been a focus of the search for extraterrestrial intelligence and has been a prevalent theme in fiction science fiction; the first confirmed detection of an exoplanet was in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmed detection of exoplanets of a main-sequence star was made in 1995, when a giant planet, 51 Pegasi b, was found in a four-day orbit around the nearby G-type star 51 Pegasi; the frequency of detections has increased since particularly through advancements in methods of detecting extrasolar planets and dedicated planet finding programs such as the Kepler mission.
Planetary systems come from protoplanetary disks that form around stars as part of the process of star formation. During formation of a system much material is gravitationally scattered into far-flung orbits and some planets are ejected from the system becoming rogue planets. Planets orbiting pulsars have been discovered. Pulsars are the remnants of the supernova explosions of high-mass stars, but a planetary system that existed before the supernova would be destroyed. Planets would either evaporate, be pushed off of their orbits by the masses of gas from the exploding star, or the sudden loss of most of the mass of the central star would see them escape the gravitational hold of the star, or in some cases the supernova would kick the pulsar itself out of the system at high velocity so any planets that had survived the explosion would be left behind as free-floating objects. Planets found around pulsars may have formed as a result of pre-existing stellar companions that were entirely evaporated by the supernova blast, leaving behind planet-sized bodies.
Alternatively, planets may form in an accretion disk of fallback matter surrounding a pulsar. Fallback disks of matter that failed to escape orbit during a supernova may form planets around black holes; as stars evolve and turn into red giants, asymptotic giant branch stars, planetary nebulae they engulf the inner planets, evaporating or evaporating them depending on how massive they are. As the star loses mass, planets that are not engulfed move further out from the star. If an evolved star is in a binary or multiple system the mass it loses can transfer to another star, creating new protoplanetary disks and second- and third-generation planets which may differ in composition from the original planets which may be affected by the mass transfer. Planets in evolved binary systems, Hagai B. Perets, 13 Jan 2011 Can Planets survive Stellar Evolution?, Eva Villaver, Mario Livio, Feb 2007 The Orbital Evolution of Gas Giant Planets around Giant Stars, Eva Villaver, Mario Livio, 13 Oct 2009 On the survival of brown dwarfs and planets engulfed by their giant host star, Jean-Claude Passy, Mordecai-Mark Mac Low, Orsola De Marco, 2 Oct 2012 Foretellings of Ragnarök: World-engulfing Asymptotic Giants and the Inheritance of White Dwarfs, Alexander James Mustill, Eva Villaver, 5 Dec 2012 The Solar System consists of an
PDS 70 is a low-mass T Tauri star in the constellation Centaurus. Located 370 light-years from Earth, it has a mass of 0.82 M☉, is 10 million years old. The star has a protoplanetary disk containing a nascent exoplanet, named PDS 70b, imaged, the first confirmed image of a newborn planet; the protoplanetary disk around PDS 70 was first hypothesized in 1992 and confirmed in 2006 along with a jet-like structure. The disk has a radius of 140 au. In 2012 a large gap in the disk was discovered, thought to be caused by planetary formation; the gap was found to have multiple regions: large dust grains were absent out to 80 au, while small dust grains were only absent out to the previously-observed 65 au. There is an asymmetry in the overall shape of the gap. In results published in 2018, a planet in the disk, named PDS 70b, was imaged by the VLT. With a mass estimated to be a few times greater than Jupiter, the planet is thought to have a temperature of around 1000 °C and an atmosphere with clouds. Modelling predicts that the planet has acquired its own accretion disk
Exoplanetology, or exoplanetary science, is an integrated field of astronomical science dedicated to the search for and study of exoplanets. It employs an interdisciplinary approach which includes astrobiology, astronomy, astrogeology and planetary science; the exoplanet naming convention is an extension of the system used for naming multiple-star systems as adopted by the International Astronomical Union. For an exoplanet orbiting a single star, the name is formed by taking the name of its parent star and adding a lowercase letter; the first planet discovered in a system is given the designation "b" and planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbit size. A provisional IAU-sanctioned standard exists to accommodate the naming of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
The official definition of "planet" used by the International Astronomical Union only covers the Solar System and thus does not apply to exoplanets. As of April 2011, the only defining statement issued by the IAU that pertains to exoplanets is a working definition issued in 2001 and modified in 2003; that definition contains the following criteria: Objects with true masses below the limiting mass for thermonuclear fusion of deuterium that orbit stars or stellar remnants are "planets". The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located. Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs"; the IAU's working definition is not always used.
One alternate suggestion is that planets should be distinguished from brown dwarfs on the basis of formation. It is thought that giant planets form through core accretion, which may sometimes produce planets with masses above the deuterium fusion threshold. Brown dwarfs form like stars from the direct gravitational collapse of clouds of gas and this formation mechanism produces objects that are below the 13 MJup limit and can be as low as 1 MJup. Objects in this mass range that orbit their stars with wide separations of hundreds or thousands of AU and have large star/object mass ratios formed as brown dwarfs. Most directly imaged planets as of April 2014 are massive and have wide orbits so represent the low-mass end of brown dwarf formation. One study suggests that objects above 10 MJup formed through gravitational instability and should not be thought of as planets; the 13-Jupiter-mass cutoff does not have precise physical significance. Deuterium fusion can occur in some objects with a mass below that cutoff.
The amount of deuterium fused depends to some extent on the composition of the object. As of 2011 the Extrasolar Planets Encyclopaedia included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around 13 MJup in the observed mass spectrum reinforces the choice to forget this mass limit"; as of 2016 this limit was increased to 60 Jupiter masses based on a study of mass–density relationships. The Exoplanet Data Explorer includes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, observationally problematic due to the sin i ambiguity." The NASA Exoplanet Archive includes objects with less than 30 Jupiter masses. Another criterion for separating planets and brown dwarfs, rather than deuterium fusion, formation process or location, is whether the core pressure is dominated by coulomb pressure or electron degeneracy pressure with the dividing line at around 5 Jupiter masses.
Planets are faint compared with their parent stars. For example, a Sun-like star is about a billion times brighter than the reflected light from any exoplanet orbiting it, it is difficult to detect such a faint light source, furthermore the parent star causes a glare that tends to wash it out. It is necessary to block the light from the parent star in order to reduce the glare while leaving the light from the planet detectable. All exoplanets that have been directly imaged are both large and separated from their parent star. Specially designed direct-imaging instruments such as Gemini Planet Imager, VLT-SPHERE, SCExAO will image dozens of gas giants, but the vast majority of known extrasolar planets have only been detected through indirect methods; the following are the indirect methods that have proven useful: Transit methodIf a planet crosses in front of its parent star's disk the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet, among other factors.
Because the transit method requires that the planet's orbit intersect a line-of-sight between the host star
A dwarf planet is a planetary-mass object, neither a true planet nor a natural satellite. That is, it is in direct orbit of a star, is massive enough for its gravity to compress it into a hydrostatically equilibrious shape, but has not cleared the neighborhood of other material around its orbit; the term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun, brought about by an increase in discoveries of objects farther away from the Sun than Neptune that rivaled Pluto in size, precipitated by the discovery of an more massive object, Eris. The exclusion of dwarf planets from the roster of planets by the IAU has been both praised and criticized; as of July 2008 the International Astronomical Union recognizes five dwarf planets: Ceres in the asteroid belt, Pluto, Haumea and Eris in the outer Solar System. Only two of these bodies and Pluto, have been observed in enough detail to demonstrate that they fit the IAU's definition; the IAU accepted Eris as a dwarf planet.
They subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 are to be named under the assumption that they are dwarf planets. At the time, the only additional bodies to meet this secondary criterion were Makemake. Starting in 1801, astronomers discovered Ceres and other bodies between Mars and Jupiter which were for decades considered to be planets. Between and around 1851, when the number of planets had reached 23, astronomers started using the word asteroid for the smaller bodies and stopped naming or classifying them as planets. With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine planets, along with thousands of smaller bodies. For 50 years Pluto was thought to be larger than Mercury, but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure Pluto's mass and to determine that it was much smaller than initial estimates, it was one-twentieth the mass of Mercury, which made Pluto by far the smallest planet.
Although it was still more than ten times as massive as the largest object in the asteroid belt, Ceres, it had one-fifth the mass of Earth's Moon. Furthermore, having some unusual characteristics, such as large orbital eccentricity and a high orbital inclination, it became evident that it was a different kind of body from any of the other planets. In the 1990s, astronomers began to find objects in the same region of space as Pluto, some farther away. Many of these shared several of Pluto's key orbital characteristics, Pluto started being seen as the largest member of a new class of objects, plutinos; this led. Several terms, including subplanet and planetoid, started to be used for the bodies now known as dwarf planets. By 2005, three trans-Neptunian objects comparable in size to Pluto had been reported, it became clear that either they would have to be classified as planets, or Pluto would have to be reclassified. Astronomers were confident that more objects as large as Pluto would be discovered, the number of planets would start growing if Pluto were to remain a planet.
Eris was discovered in January 2005. As a consequence, the issue became a matter of intense debate during the IAU General Assembly in August 2006; the IAU's initial draft proposal included Charon and Ceres in the list of planets. After many astronomers objected to this proposal, an alternative was drawn up by the Uruguayan astronomers Julio Ángel Fernández and Gonzalo Tancredi: they proposed an intermediate category for objects large enough to be round but which had not cleared their orbits of planetesimals. Dropping Charon from the list, the new proposal removed Pluto and Eris, because they have not cleared their orbits; the IAU's final Resolution 5A preserved this three-category system for the celestial bodies orbiting the Sun. It reads: Although concerns were raised about the classification of planets orbiting other stars, the issue was not resolved; the term dwarf planet has itself been somewhat controversial, as it could imply that these bodies are planets, much as dwarf stars are stars.
This is the conception of the Solar System. The older word planetoid has no such connotation, is used by astronomers for bodies that fit the IAU definition. Brown states that planetoid is "a good word", used for these bodies for years, that the use of the term dwarf planet for a non-planet is "dumb", but that it was motivated by an attempt by the IAU division III plenary session to reinstate Pluto as a planet in a second resolution. Indeed, the draft of Resolution 5A had called these median bodies planetoids, but the plenary session voted unanimously to change the name to dwarf planet; the second resolution, 5B, defined dwarf planets as a subtype of planet, as Stern had intended, distinguished from the other eight that were to be called "classical planets". Under this arrangement, the twelve planets of the rejected proposal were to be preserved in a distinction between eight classical planets and four dwarf planets. Resolution 5B was defeated in the same session; because of the semantic inconsistenc
An ice planet is a theoretical type of exoplanet with an icy surface of volatiles such as water and methane. Ice planets consist of a global cryosphere, they are bigger versions of the small icy worlds of the Solar System, which includes the moons Europa and Triton, the dwarf planets Pluto and Eris, many other small Solar System bodies such as comets. Ice planets appear nearly white with geometric albedos of more than 0.9. An ice planet's surface can be composed of water, ammonia, carbon dioxide, carbon monoxide, other volatiles, depending on its surface temperature. Ice planets would have surface temperatures below 260 K if composed of water, below 180 K if composed of CO2 and ammonia, below 80 K if composed of methane. On the surface, ice planets are hostile to life forms like those living on Earth because they are cold. Many ice worlds have subsurface oceans, warmed by internal heat or tidal forces from another nearby body. Liquid subsurface water would provide habitable conditions for life, including fish and microorganisms.
Subsurface plants as we know it could not exist because there is no sunlight to use for photosynthesis. Microorganisms can produce nutrients using specific chemicals that may provide food and energy for other organisms; some planets, if conditions are right, may have significant atmospheres and surface liquids like Saturn's moon Titan, which could be habitable for exotic forms of life. Although there are many icy objects in the Solar System, there are no known ice planets. If a 9th planet suggested in 2016 is found, it will be an ice planet of many times the earth's mass with a surface temperature under 70K. There are several extrasolar ice planet candidates, including OGLE-2005-BLG-390Lb, OGLE-2013-BLG-0341L b and MOA-2007-BLG-192Lb. Ice giant Ice planets in science fiction Ocean planet Snowball Earth Hoth
A sub-Earth is a planet "substantially less massive" than Earth and Venus. In the Solar System, this category includes Mars. Sub-Earth exoplanets are among the most difficult type to detect because their small sizes and masses produce the weakest signal. Despite the difficulty, one of the first exoplanets found was a sub-Earth around a millisecond pulsar PSR B1257+12; the smallest known is WD 1145+017 b with a size of 0.15 Earth radii, or somewhat smaller than Pluto. However, WD 1145 +017 b is a dwarf planet as it orbits within a cloud of gas; the Kepler space telescope opened the realm of sub-Earths by its discovery of them. On January 10, 2012, Kepler discovered the first three sub-Earths around an ordinary star, Kepler-42; as of June 2014, Kepler has 45 confirmed planets that are smaller than Earth, with 17 of them being smaller than 0.8 Rⴲ. In addition, there are over 310 planet candidates with an estimated radius of <1Rⴲ, with 135 of them being smaller than 0.8 Rⴲ. Sub-Earths lack substantial atmospheres because of their low gravity and weak magnetic fields, allowing stellar radiation to wear away their atmospheres.
Due to their small sizes, unless there are significant tidal forces when orbiting close to the parent star, sub-Earths have short periods of geologic activity
A terrestrial planet, telluric planet, or rocky planet is a planet, composed of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun, i.e. Mercury, Venus and Mars; the terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth, as these planets are, in terms of structure, "Earth-like". These planets are located between the Asteroid Belt. Terrestrial planets have a solid planetary surface, making them different from the larger giant planets, which are composed of some combination of hydrogen and water existing in various physical states. All terrestrial planets in the Solar System have the same basic type of structure, such as a central metallic core iron, with a surrounding silicate mantle; the Moon has a much smaller iron core. Io and Europa are satellites that have internal structures similar to that of terrestrial planets. Terrestrial planets can have canyons, mountains and other surface structures, depending on the presence of water and tectonic activity.
Terrestrial planets have secondary atmospheres, generated through volcanism or comet impacts, in contrast to the giant planets, whose atmospheres are primary, captured directly from the original solar nebula. The Solar System has four terrestrial planets: Mercury, Venus and Mars. Only one terrestrial planet, Earth, is known to have an active hydrosphere. During the formation of the Solar System, there were many more terrestrial planetesimals, but most merged with or were ejected by the four terrestrial planets. Dwarf planets, such as Ceres and Eris, small Solar System bodies are similar to terrestrial planets in the fact that they do have a solid surface, but are, on average, composed of more icy materials; the Earth's Moon has a density of 3.4 g·cm−3 and Jupiter's satellites, Io, 3.528 and Europa, 3.013 g·cm−3. The uncompressed density of a terrestrial planet is the average density its materials would have at zero pressure. A greater uncompressed density indicates greater metal content. Uncompressed density differs from the true average density because compression within planet cores increases their density.
The uncompressed density of terrestrial planets trends towards lower values as the distance from the Sun increases. The rocky minor planet Vesta orbiting outside of Mars is less dense than Mars still at, 3.4 g·cm−3. Calculations to estimate uncompressed density inherently require a model of the planet's structure. Where there have been landers or multiple orbiting spacecraft, these models are constrained by seismological data and moment of inertia data derived from the spacecraft orbits. Where such data is not available, uncertainties are higher, it is unknown. Most of the planets discovered outside the Solar System are giant planets, because they are more detectable, but since 2005, hundreds of terrestrial extrasolar planets have been found, with several being confirmed as terrestrial. Most of these are i.e. planets with masses between Earth's and Neptune's. During the early 1990s, the first extrasolar planets were discovered orbiting the pulsar PSR B1257+12, with masses of 0.02, 4.3, 3.9 times that of Earth's, by pulsar timing.
When 51 Pegasi b, the first planet found around a star still undergoing fusion, was discovered, many astronomers assumed it to be a gigantic terrestrial, because it was assumed no gas giant could exist as close to its star as 51 Pegasi b did. It was found to be a gas giant. In 2005, the first planets around main-sequence stars that may be terrestrial were found: Gliese 876 d, has a mass 7 to 9 times that of Earth and an orbital period of just two Earth days, it orbits the red dwarf 15 light years from Earth. OGLE-2005-BLG-390Lb, about 5.5 times the mass of Earth, orbits a star about 21,000 light years away in the constellation Scorpius. From 2007 to 2010, three potential terrestrial planets were found orbiting within the Gliese 581 planetary system; the smallest, Gliese 581e, is only about 1.9 Earth mass, but orbits close to the star. An ideal terrestrial planet would be 2 Earth masses with a 25-day orbital period around a red dwarf. Two others, Gliese 581c and Gliese 581d, as well as a disputed planet, Gliese 581g, are more-massive super-Earths orbiting in or close to the habitable zone of the star, so they could be habitable, with Earth-like temperatures.
Another terrestrial planet, HD 85512 b, was discovered in 2011. The radius and composition of all these planets are unknown; the first confirmed terrestrial exoplanet, Kepler-10b, was found in 2011 by the Kepler Mission designed to discover Earth-size planets around other stars using the transit method. In the same year, the Kepler Space Observatory Mission team released a list of 1235 extrasolar planet candidates, including six that are "Earth-size" or "super-Earth-size" and in the habitable zone of their star. Since Kepler has discovered hundreds of planets ranging from Moon-sized to super-Earths, with many more candidates in this size range