Planetary system
A planetary system consists of a set of non-stellar bodies which are gravitationally bound to and in orbit of a star or star system. Generally speaking, such systems will include planets, and may include other objects such as dwarf planets, asteroids, natural satellites, meteoroids, comets, planetesimals, and circumstellar disks. The Solar System is an example of a planetary system, in which Earth, seven other planets, and other celestial objects are bound to and revolve around the Sun. The term exoplanetary system is sometimes used in reference to planetary systems other than the Solar System. By convention planetary systems are named after their host, or parent, star, as is the case with the Solar System being named after "Sol".
Debris disks are known to be common while 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, and thus, the capacity to support Earth-like life.
Definition
The International Astronomical Union has described a planetary system as the system of planets orbiting one or more stars, brown dwarfs or stellar remnants. The IAU and NASA consider the Solar System a planetary system, including its star the Sun, its planets, and all other bodies orbiting the Sun.Other definitions of planetary system explicitly include all bodies gravitationally bound to one or more stars.
History
Heliocentrism
is a planetary model that places the Sun is at the center of the universe, as opposed to geocentrism.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.
Some also interpret Aryabhatta's writings in Āryabhaṭīya as implicitly heliocentric, although this has also been rebutted.
Discovery of the Solar System
De revolutionibus orbium coelestium by Nicolaus Copernicus, published in 1543, presented the first mathematically predictive heliocentric model of a planetary system. 17th-century successors Galileo Galilei, Johannes Kepler, and Sir Isaac Newton developed an understanding of physics which led to the gradual acceptance of the idea that the Earth moves around the Sun and that the planets are governed by the same physical laws that governed Earth.Speculation on extrasolar planetary systems
In the 16th century the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.In the 18th century, the same possibility was mentioned by Sir 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 centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."
His theories gained popularity 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, particularly science fiction.
Detection of exoplanets
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 then, particularly through advancements in methods of detecting extrasolar planets and dedicated planet-finding programs such as the Kepler mission.Origin and evolution
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 distant orbits, and some planets are ejected completely from the system, becoming rogue planets.
Evolved systems
High-mass stars
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 likely be mostly 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 almost 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 also form planets around black holes.Lower-mass stars
Many low-mass stars are expected to have rocky planets, with their planetary systems primarily consisting of rock- and ice-based bodies. This is because low-mass stars have less material in their planetary disks, making it unlikely that the planetesimals within will reach the critical mass necessary to form gas giants. The planetary systems of low-mass stars also tend to be compact, as such stars tend to have lower temperatures, resulting in the formation of protoplanets closer to the star.As stars evolve and turn into red giants, asymptotic giant branch stars, and eventually planetary nebulae, they engulf the inner planets, evaporating or partially 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, then the mass it loses can transfer to another star, forming new protoplanetary disks and second- and third-generation planets which may differ in composition from the original planets, which may also be affected by the mass transfer.
Planet capture
s in open clusters have similar velocities to the stars and so can be recaptured. They are typically captured into wide orbits between 100 and 105 AU. The capture efficiency decreases with increasing cluster size, and for a given cluster size it increases with the host/primary mass. It is almost independent of the planetary mass. Single and multiple planets could be captured into arbitrary unaligned orbits, non-coplanar with each other or with the stellar host spin, or pre-existing planetary system. Some planet–host metallicity correlation may still exist due to the common origin of the stars from the same cluster. Planets would be unlikely to be captured around neutron stars because these are likely to be ejected from the cluster by a pulsar kick when they form. Planets could even be captured around other planets to form free-floating planet binaries. After the cluster has dispersed some of the captured planets with orbits larger than 106 AU would be slowly disrupted by the galactic tide and likely become free-floating again through encounters with other field stars or giant molecular clouds.System architectures
The Solar System consists of an inner region of small rocky planets and outer region of large giant planets. However, other planetary systems can have quite different architectures. At present, few systems have been found to be analogous to the Solar System with small terrestrial planets in the inner region, as well as a gas giant with a relatively circular orbit, which suggests that this configuration is uncommon. More commonly, systems consisting of multiple Super-Earths have been detected. These super-Earths are usually very close to their star, with orbits smaller than that of Mercury. Other systems have been found to have a hot Jupiter gas giant very close to the star. Theories such as planetary migration or scattering have been proposed to explain the formation of large planets close to their parent stars. Overall, studies suggest that architectures of planetary systems are dependent on the conditions of their initial formation.Classification
Planetary system architectures may be partitioned into four classes based on how the mass of the planets is distributed around the host star:In Similar systems, the masses of all the planets are similar to each other. This architecture class is the most commonly-observed in our galaxy. TRAPPIST-1 is an example of a Similar system. Planets in Similar systems are said to be like 'peas in a pod', and the phrase now refers to a set of specific configuration characteristics.
A 'peas in a pod' system will have planets that are similar or ordered in size, similar and ordered in mass, and tend to display "packing". Packing refers to the tendency of smaller planets to be closer together, and of larger planets to have larger orbital spacing. Lastly, 'peas in a pod' systems tend to display similar spacing between a pair of adjacent planets and the next pair of adjacent planets.
Mixed systems are planetary systems in which the masses of the planets show larger increasing or decreasing variations. Gliese 876 and Kepler-89 are examples of mixed systems.
Anti-Ordered systems have their massive planets close to the host star and the smaller planets further away. There are currently no known examples of this architecture class.
Ordered systems have their planets ordered such that the less massive ones are closer to the star and the more massive planets are further from the star, with the mass of each planet increasing with distance from the star. The Solar System, with small rocky planets in the inner part and giant planets in the outer part, is a type of Ordered system.