Planetary-mass object

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Not to be confused with Planet.
A large selection of planetary-mass moons to scale among other planetary-mass objects the planets Mercury, Venus, Earth and Mars, as well as the dwarf planet Pluto.

A planetary-mass object (PMO), planemo,[1] or planetary body is by geophysical definition of celestial objects any celestial object massive enough to achieve hydrostatic equilibrium (to be rounded under its own gravity), but not enough to sustain core fusion like a star.[2][3]

The purpose of this term is to refer to a broader range of celestial objects than the concept of planet, since many objects similar in geophysical terms do not conform to typical expectations for a planet. Planetary-mass objects can be quite distinguished in origin and location. Planetary-mass objects include dwarf planets, planetary-mass moons or free-floating planemos, which may have been ejected from a system (rogue planets) or formed through cloud-collapse rather than accretion (sometimes called sub-brown dwarfs).

Types

Planetary-mass moon

Main article: Satellite planet
Planetary-mass moons larger than Pluto, the largest Solar dwarf planet.

The three largest satellites Ganymede, Callisto, and Titan are of similar size or larger than the planet Mercury; these and four more – Io, Earth's Moon, Europa, and Triton – are larger than the largest dwarf planet, Pluto. Another dozen smaller moons are large enough to be round through their own gravity, tidal forces from their parent planets, or both. In particular, Titan has a thick atmosphere and stable bodies of liquid on its surface, like Earth (though for Titan the liquid is methane rather than water). Proponents of the geophysical definition of planets argue that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet. The term satellite planet is sometimes used for planet-sized satellites.[4]

Dwarf planets

Main article: Dwarf planet

 

The dwarf planet Pluto

A dwarf planet is a planetary-mass object that is neither a true planet nor a natural satellite; it is in direct orbit of a star, and is massive enough for its gravity to compress it into a hydrostatically equilibrious shape (usually a spheroid), but has not cleared the neighborhood of other material around its orbit. Planetary scientist and New Horizons principal investigator Alan Stern, who proposed the term 'dwarf planet', has argued that location should not matter and that only geophysical attributes should be taken into account, and that dwarf planets are thus a subtype of planet. The IAU accepted the term (rather than the more neutral 'planetoid') but decided to classify dwarf planets as a separate category of object.[5]

Planets and exoplanets

These paragraphs are an excerpt from Planet.[edit]
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a young protostar orbited by a protoplanetary disk. Planets grow in this disk by the gradual accumulation of material driven by gravity, a process called accretion. The Solar System has at least eight planets: the terrestrial planets Mercury, Venus, Earth and Mars, and the giant planets Jupiter, Saturn, Uranus and Neptune. These planets each rotate around an axis tilted with respect to its orbital pole. All of them possess an atmosphere, although that of Mercury is tenuous, and some share such features as ice caps, seasons, volcanism, hurricanes, tectonics, and even hydrology. Apart from Venus and Mars, the Solar System planets generate magnetic fields, and all except Venus and Mercury have natural satellites. The giant planets bear planetary rings, the most prominent being those of Saturn.

Former stars

See also: Disrupted planet § Stars

In close binary star systems one of the stars can lose mass to a heavier companion. Accretion-powered pulsars may drive mass loss. The shrinking star can then become a planetary-mass object. An example is a Jupiter-mass object orbiting the pulsar PSR J1719-1438.[6] These shrunken white dwarfs may become a helium planet or carbon planet.

Sub-brown dwarfs

Main article: Sub-brown dwarf
Artist's impression of a super-Jupiter around the brown dwarf 2M1207.[7]

Stars form via the gravitational collapse of gas clouds, but smaller objects can also form via cloud-collapse. Planetary-mass objects formed this way are sometimes called sub-brown dwarfs. Sub-brown dwarfs may be free-floating such as Cha 110913-773444[8] and OTS 44,[9] or orbiting a larger object such as 2MASS J04414489+2301513.

Binary systems of sub-brown dwarfs are theoretically possible; Oph 162225-240515 was initially thought to be a binary system of a brown dwarf of 14 Jupiter masses and a sub-brown dwarf of 7 Jupiter masses, but further observations revised the estimated masses upwards to greater than 13 Jupiter masses, making them brown dwarfs according to the IAU working definitions.[10][11][12]

Captured planets

Rogue planets in stellar 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 volume, 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.[13]

Rogue planets

Main article: Rogue planet
See also: Five-planet Nice model

Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.[14] Such objects are typically called rogue planets.

See also

References

  1. ^ Weintraub, David A. (2014). Is Pluto a Planet?: A Historical Journey through the Solar System. Princeton University Press. p. 226. ISBN 978-1400852970.
  2. ^ Basri, Gibor; Brown, E. M. (May 2006). "Planetesimals to Brown Dwarfs: What is a Planet?". Annual Review of Earth and Planetary Sciences. 34: 193–216. arXiv:astro-ph/0608417. Bibcode:2006AREPS..34..193B. doi:10.1146/annurev.earth.34.031405.125058. S2CID 119338327.
  3. ^ Stern, S. Alan; Levison, Harold F. (2002). Rickman, H. (ed.). "Regarding the criteria for planethood and proposed planetary classification schemes". Highlights of Astronomy. San Francisco, CA: Astronomical Society of the Pacific. 12: 208. Bibcode:2002HiA....12..205S. doi:10.1017/S1539299600013289. ISBN 978-1-58381-086-6.
  4. ^ "Should Large Moons Be Called 'Satellite Planets'?". News.discovery.com. 2010-05-14. Archived from the original on 2010-05-16. Retrieved 2011-11-04.
  5. ^ "Resolution B5 Definition of a Planet in the Solar System" (PDF). IAU 2006 General Assembly. International Astronomical Union. Retrieved January 26, 2008.
  6. ^ Bailes, M.; Bates, S. D.; Bhalerao, V.; Bhat, N. D. R.; et al. (2011). "Transformation of a Star into a Planet in a Millisecond Pulsar Binary". Science. 333 (6050): 1717–20. arXiv:1108.5201. Bibcode:2011Sci...333.1717B. doi:10.1126/science.1208890. PMID 21868629. S2CID 206535504.
  7. ^ "Artist's View of a Super-Jupiter around a Brown Dwarf (2M1207)". Retrieved 22 February 2016.
  8. ^ Luhman, K. L.; Adame, Lucía; D'Alessio, Paola; Calvet, Nuria (2005). "Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk". Astrophysical Journal. 635 (1): L93. arXiv:astro-ph/0511807. Bibcode:2005ApJ...635L..93L. doi:10.1086/498868. S2CID 11685964.
  9. ^ Joergens, V.; Bonnefoy, M.; Liu, Y.; Bayo, A.; et al. (2013). "OTS 44: Disk and accretion at the planetary border". Astronomy & Astrophysics. 558 (7): L7. arXiv:1310.1936. Bibcode:2013A&A...558L...7J. doi:10.1051/0004-6361/201322432. S2CID 118456052.
  10. ^ Close, Laird M.; Zuckerman, B.; Song, Inseok; Barman, Travis; et al. (2007). "The Wide Brown Dwarf Binary Oph 1622–2405 and Discovery of A Wide, Low Mass Binary in Ophiuchus (Oph 1623–2402): A New Class of Young Evaporating Wide Binaries?". Astrophysical Journal. 660 (2): 1492–1506. arXiv:astro-ph/0608574. Bibcode:2007ApJ...660.1492C. doi:10.1086/513417. S2CID 15170262.
  11. ^ Luhman, Kevin L.; Allers, Katelyn N.; Jaffe, Daniel T.; Cushing, Michael C.; Williams, Kurtis A.; Slesnick, Catherine L.; Vacca, William D. (April 2007). "Ophiuchus 1622-2405: Not a Planetary-Mass Binary". The Astrophysical Journal. 659 (2): 1629–1636. arXiv:astro-ph/0701242. Bibcode:2007ApJ...659.1629L. doi:10.1086/512539. S2CID 11153196.
  12. ^ Britt, Robert Roy (2004-09-10). "Likely First Photo of Planet Beyond the Solar System". Space.com. Retrieved 2008-08-23.
  13. ^ On the origin of planets at very wide orbits from the re-capture of free floating planets, Hagai B. Perets, M. B. N. Kouwenhoven, 2012
  14. ^ Lissauer, J. J. (1987). "Timescales for Planetary Accretion and the Structure of the Protoplanetary disk". Icarus. 69 (2): 249–265. Bibcode:1987Icar...69..249L. doi:10.1016/0019-1035(87)90104-7. hdl:2060/19870013947.
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