Planet

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The eight planets of the Solar System with size to scale (up to down, left to right): Saturn, Jupiter, Uranus, Neptune (outer planets), Earth, Venus, Mars, and Mercury (inner planets)

A planet is a large, rounded astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself.[1] The Solar System has eight planets by the most restrictive definition of the term: the terrestrial planets Mercury, Venus, Earth, and Mars, and the giant planets Jupiter, Saturn, Uranus, and Neptune. 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 word planet comes from the Greek πλανήται (planḗtai) Template:Gloss. In antiquity, this word referred to the Sun, Moon, and five points of light visible to the naked eye that moved across the background of the stars—namely, Mercury, Venus, Mars, Jupiter, and Saturn. Planets have historically had religious associations: multiple cultures identified celestial bodies with gods, and these connections with mythology and folklore persist in the schemes for naming newly discovered Solar System bodies. Earth itself was recognized as a planet when heliocentrism supplanted geocentrism during the 16th and 17th centuries.

With the development of the telescope, the meaning of planet broadened to include objects only visible with assistance: the moons of the planets beyond Earth; the ice giants Uranus and Neptune; Ceres and other bodies later recognized to be part of the asteroid belt; and Pluto, later found to be the largest member of the collection of icy bodies known as the Kuiper belt. The discovery of other large objects in the Kuiper belt, particularly Eris, spurred debate about how exactly to define a planet. In 2006, the International Astronomical Union (IAU) adopted a definition of a planet in the Solar System, placing the four terrestrial planets and the four giant planets in the planet category; Ceres, Pluto, and Eris are in the category of dwarf planet.[2][3][4] Many planetary scientists have nonetheless continued to apply the term planet more broadly, including dwarf planets as well as rounded satellites like the Moon.[5]

Further advances in astronomy led to the discovery of over 5,900 planets outside the Solar System, termed exoplanets. These often show unusual features that the Solar System planets do not show, such as hot Jupiters—giant planets that orbit close to their parent stars, like 51 Pegasi b—and extremely eccentric orbits, such as HD 20782 b. The discovery of brown dwarfs and planets larger than Jupiter also spurred debate on the definition, regarding where exactly to draw the line between a planet and a star. Multiple exoplanets have been found to orbit in the habitable zones of their stars (where liquid water can potentially exist on a planetary surface), but Earth remains the only planet known to support life.

Formation[edit]

Main article: Nebular hypothesis
Artists' impressions
A protoplanetary disk
Protoplanets colliding during planet formation

It is not known with certainty how planets are formed. The prevailing theory is that they coalesce during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become increasingly dense until they collapse inward under gravity to form protoplanets.[6] After a planet reaches a mass somewhat larger than Mars's mass, it begins to accumulate an extended atmosphere,[7] greatly increasing the capture rate of the planetesimals by means of atmospheric drag.[8][9] Depending on the accretion history of solids and gas, a giant planet, an ice giant, or a terrestrial planet may result.[10][11][12] It is thought that the regular satellites of Jupiter, Saturn, and Uranus formed in a similar way;[13][14] however, Triton was likely captured by Neptune,[15] and Earth's Moon[16] and Pluto's Charon might have formed in collisions.[17]

When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, Poynting–Robertson drag and other effects.[18][19] Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a larger, combined protoplanet or release material for other protoplanets to absorb.[20] Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small bodies.[21][22]

Supernova remnant ejecta producing planet-forming material

The energetic impacts of the smaller planetesimals (as well as radioactive decay) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by density, with higher density materials sinking toward the core.[23] Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comets[24] (smaller planets will lose any atmosphere they gain through various escape mechanisms[25]).

With the discovery and observation of planetary systems around stars other than the Sun, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity—an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 (helium)—appears to determine the likelihood that a star will have planets.[26][27] Hence, a metal-rich population I star is more likely to have a substantial planetary system than a metal-poor, population II star.[28]

Planets in the Solar System[edit]

Main article: Solar System

According to the IAU definition, there are eight planets in the Solar System, which are (in increasing distance from the Sun):[2] Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Jupiter is the largest, at 318 Earth masses, whereas Mercury is the smallest, at 0.055 Earth masses.[29]

The planets of the Solar System can be divided into categories based on their composition. Terrestrials are similar to Earth, with bodies largely composed of rock and metal: Mercury, Venus, Earth, and Mars. Earth is the largest terrestrial planet.[30] Giant planets are significantly more massive than the terrestrials: Jupiter, Saturn, Uranus, and Neptune.[30] They differ from the terrestrial planets in composition. The gas giants, Jupiter and Saturn, are primarily composed of hydrogen and helium and are the most massive planets in the Solar System. Saturn is one third as massive as Jupiter, at 95 Earth masses.[31] The ice giants, Uranus and Neptune, are primarily composed of low-boiling-point materials such as water, methane, and ammonia, with thick atmospheres of hydrogen and helium. They have a significantly lower mass than the gas giants (only 14 and 17 Earth masses).[31]

The Sun's, planets', dwarf planets' and moons' size to scale, labelled. Distance of objects is not to scale. The asteroid belt lies between the orbits of Mars and Jupiter, the Kuiper belt lies beyond Neptune's orbit.

Dwarf planets are gravitationally rounded, but have not cleared their orbits of other bodies. In increasing order of average distance from the Sun, the ones generally agreed among astronomers are Template:Dp, Template:Dp, Template:Dp, Template:Dp, Template:Dp, Template:Dp, Template:Dp, Template:Dp, and Template:Dp.[32][33] Ceres is the largest object in the asteroid belt, located between the orbits of Mars and Jupiter. The other eight all orbit beyond Neptune. Orcus, Pluto, Haumea, Quaoar, and Makemake orbit in the Kuiper belt, which is a second belt of small Solar System bodies beyond the orbit of Neptune. Gonggong and Eris orbit in the scattered disc, which is somewhat further out and, unlike the Kuiper belt, is unstable towards interactions with Neptune. Sedna is the largest known detached object, a population that never comes close enough to the Sun to interact with any of the classical planets; the origins of their orbits are still being debated. All nine are similar to terrestrial planets in having a solid surface, but they are made of ice and rock rather than rock and metal. Moreover, all of them are smaller than Mercury, with Pluto being the largest known dwarf planet and Eris being the most massive.[34][35] There are at least nineteen planetary-mass moons or satellite planets—moons large enough to take on ellipsoidal shapes:[4]

The Moon, Io, and Europa have compositions similar to the terrestrial planets; the others are made of ice and rock like the dwarf planets, with Tethys being made of almost pure ice. Europa is often considered an icy planet, though, because its surface ice layer makes it difficult to study its interior.[4][36] Ganymede and Titan are larger than Mercury by radius, and Callisto almost equals it, but all three are much less massive. Mimas is the smallest object generally agreed to be a geophysical planet, at about six millionths of Earth's mass, though there are many larger bodies that may not be geophysical planets (e.g. Template:Dp).[32]

Reference[edit]

  1. ↑ Lecavelier des Etangs, A.; Lissauer, Jack J. (2022-06-01). "The IAU working definition of an exoplanet". New Astronomy Reviews. 94: 101641. doi:10.1016/j.newar.2022.101641. ISSN 1387-6473.
  2. ↑ 2.0 2.1 "IAU 2006 General Assembly: Result of the IAU Resolution votes". International Astronomical Union. 2006. Archived from the original on 29 April 2014. Retrieved 30 December 2009.
  3. ↑ "Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union". IAU. 2001. Archived from the original on 16 September 2006. Retrieved 23 August 2008.
  4. ↑ 4.0 4.1 4.2 Lakdawalla, Emily (21 April 2020). "What Is A Planet?". The Planetary Society. Archived from the original on 22 January 2022. Retrieved 3 April 2022.
  5. ↑ Grossman, Lisa (24 August 2021). "The definition of planet is still a sore point – especially among Pluto fans". Science News. Archived from the original on 10 July 2022. Retrieved 10 July 2022.
  6. ↑ Wetherill, G. W. (1980). "Formation of the Terrestrial Planets". Annual Review of Astronomy and Astrophysics. 18 (1): 77–113. Bibcode:1980ARA&A..18...77W. doi:10.1146/annurev.aa.18.090180.000453. ISSN 0066-4146.
  7. ↑ D'Angelo, G.; Bodenheimer, P. (2013). "Three-dimensional Radiation-hydrodynamics Calculations of the Envelopes of Young Planets Embedded in Protoplanetary Disks". The Astrophysical Journal. 778 (1): 77 (29 pp.). arXiv:1310.2211. Bibcode:2013ApJ...778...77D. doi:10.1088/0004-637X/778/1/77. S2CID 118522228.
  8. ↑ Inaba, S.; Ikoma, M. (2003). "Enhanced Collisional Growth of a Protoplanet that has an Atmosphere". Astronomy and Astrophysics. 410 (2): 711–723. Bibcode:2003A&A...410..711I. doi:10.1051/0004-6361:20031248.
  9. ↑ D'Angelo, G.; Weidenschilling, S. J.; Lissauer, J. J.; Bodenheimer, P. (2014). "Growth of Jupiter: Enhancement of core accretion by a voluminous low-mass envelope". Icarus. 241: 298–312. arXiv:1405.7305. Bibcode:2014Icar..241..298D. doi:10.1016/j.icarus.2014.06.029. S2CID 118572605.
  10. ↑ Lissauer, J. J.; Hubickyj, O.; D'Angelo, G.; Bodenheimer, P. (2009). "Models of Jupiter's growth incorporating thermal and hydrodynamic constraints". Icarus. 199 (2): 338–350. arXiv:0810.5186. Bibcode:2009Icar..199..338L. doi:10.1016/j.icarus.2008.10.004. S2CID 18964068.
  11. ↑ D'Angelo, G.; Durisen, R. H.; Lissauer, J. J. (2011). "Giant Planet Formation". In Seager, S. (ed.). Exoplanets. University of Arizona Press, Tucson, AZ. pp. 319–346. arXiv:1006.5486. Bibcode:2010exop.book..319D. Archived from the original on 30 June 2015. Retrieved 1 May 2016.
  12. ↑ Chambers, J. (2011). "Terrestrial Planet Formation". In Seager, S. (ed.). Exoplanets. Tucson, AZ: University of Arizona Press. pp. 297–317. Bibcode:2010exop.book..297C. Archived from the original on 30 June 2015. Retrieved 1 May 2016.
  13. ↑ Canup, Robin M.; Ward, William R. (2008). Origin of Europa and the Galilean Satellites. University of Arizona Press. p. 59. arXiv:0812.4995. Bibcode:2009euro.book...59C. ISBN 978-0-8165-2844-8.
  14. ↑ D'Angelo, G.; Podolak, M. (2015). "Capture and Evolution of Planetesimals in Circumjovian Disks". The Astrophysical Journal. 806 (1): 29pp. arXiv:1504.04364. Bibcode:2015ApJ...806..203D. doi:10.1088/0004-637X/806/2/203. S2CID 119216797.
  15. ↑ Agnor, C. B.; Hamilton, D. P. (2006). "Neptune's capture of its moon Triton in a binary–planet gravitational encounter" (PDF). Nature. 441 (7090): 192–4. Bibcode:2006Natur.441..192A. doi:10.1038/nature04792. PMID 16688170. S2CID 4420518. Archived from the original (PDF) on 14 October 2016. Retrieved 1 May 2022.
  16. ↑ Taylor, G. Jeffrey (31 December 1998). "Origin of the Earth and Moon". Planetary Science Research Discoveries. Hawai'i Institute of Geophysics and Planetology. Archived from the original on 10 June 2010. Retrieved 7 April 2010.
  17. ↑ Stern, S.A.; Bagenal, F.; Ennico, K.; Gladstone, G.R.; et al. (16 October 2015). "The Pluto system: Initial results from its exploration by New Horizons". Science. 350 (6258): aad1815. arXiv:1510.07704. Bibcode:2015Sci...350.1815S. doi:10.1126/science.aad1815. PMID 26472913. S2CID 1220226.
  18. ↑ Dutkevitch, Diane (1995). The Evolution of Dust in the Terrestrial Planet Region of Circumstellar Disks Around Young Stars (PhD thesis). University of Massachusetts Amherst. Bibcode:1995PhDT..........D. Archived from the original on 25 November 2007. Retrieved 23 August 2008.
  19. ↑ Matsuyama, I.; Johnstone, D.; Murray, N. (2005). "Halting Planet Migration by Photoevaporation from the Central Source". The Astrophysical Journal. 585 (2): L143–L146. arXiv:astro-ph/0302042. Bibcode:2003ApJ...585L.143M. doi:10.1086/374406. S2CID 16301955.
  20. ↑ Kenyon, Scott J.; Bromley, Benjamin C. (2006). "Terrestrial Planet Formation. I. The Transition from Oligarchic Growth to Chaotic Growth". Astronomical Journal. 131 (3): 1837–1850. arXiv:astro-ph/0503568. Bibcode:2006AJ....131.1837K. doi:10.1086/499807. S2CID 15261426.
  21. ↑ Martin, R. G.; Livio, M. (1 January 2013). "On the formation and evolution of asteroid belts and their potential significance for life". Monthly Notices of the Royal Astronomical Society: Letters. 428 (1): L11–L15. arXiv:1211.0023. doi:10.1093/mnrasl/sls003. ISSN 1745-3925.
  22. ↑ Peale, S. J. (September 1999). "Origin and Evolution of the Natural Satellites". Annual Review of Astronomy and Astrophysics. 37 (1): 533–602. Bibcode:1999ARA&A..37..533P. doi:10.1146/annurev.astro.37.1.533. ISSN 0066-4146. Archived from the original on 13 May 2022. Retrieved 13 May 2022.
  23. ↑ Ida, Shigeru; Nakagawa, Yoshitsugu; Nakazawa, Kiyoshi (1987). "The Earth's core formation due to the Rayleigh-Taylor instability". Icarus. 69 (2): 239–248. Bibcode:1987Icar...69..239I. doi:10.1016/0019-1035(87)90103-5.
  24. ↑ Kasting, James F. (1993). "Earth's early atmosphere". Science. 259 (5097): 920–926. Bibcode:1993Sci...259..920K. doi:10.1126/science.11536547. PMID 11536547. S2CID 21134564.
  25. ↑ Chuang, F. (6 June 2012). "FAQ – Atmosphere". Planetary Science Institute. Archived from the original on 23 March 2022. Retrieved 13 May 2022.
  26. ↑ Fischer, Debra A.; Valenti, Jeff (2005). "The Planet-Metallicity Correlation". The Astrophysical Journal. 622 (2): 1102. Bibcode:2005ApJ...622.1102F. doi:10.1086/428383.
  27. ↑ Wang, Ji; Fischer, Debra A. (2013). "Revealing a Universal Planet-Metallicity Correlation for Planets of Different Sizes Around Solar-Type Stars". The Astronomical Journal. 149 (1): 14. arXiv:1310.7830. Bibcode:2015AJ....149...14W. doi:10.1088/0004-6256/149/1/14. S2CID 118415186.
  28. ↑ Harrison, Edward Robert (2000). Cosmology: The Science of the Universe. Cambridge University Press. p. 114. ISBN 978-0-521-66148-5. Archived from the original on 14 December 2023. Retrieved 13 May 2022.
  29. ↑ "Planetary Physical Parameters". Solar System Dynamics. Jet Propulsion Laboratory. Archived from the original on 4 October 2022. Retrieved 11 July 2022.
  30. ↑ 30.0 30.1 Lewis, John S. (2004). Physics and Chemistry of the Solar System (2nd ed.). Academic Press. p. 59. ISBN 978-0-12-446744-6.
  31. ↑ 31.0 31.1 Marley, Mark (2 April 2019). "Not a Heart of Ice". planetary.org. The Planetary Society. Archived from the original on 12 August 2019. Retrieved 5 May 2022.
  32. ↑ 32.0 32.1 Cite error: Invalid <ref> tag; no text was provided for refs named Grundy2019
  33. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named JWST
  34. ↑ Brown, Michael E.; Schaller, Emily L. (15 June 2007). "The Mass of Dwarf Planet Eris" (PDF). Science. 316 (5831): 1585. Bibcode:2007Sci...316.1585B. doi:10.1126/science.1139415. PMID 17569855. S2CID 21468196. Archived from the original (PDF) on 4 March 2016. Retrieved 27 September 2015.
  35. ↑ "How Big Is Pluto? New Horizons Settles Decades-Long Debate". NASA. 7 August 2017. Archived from the original on 9 November 2019. Retrieved 5 May 2022.
  36. ↑ Lewis, John S. (2004). Physics and Chemistry of the Solar System (2nd ed.). Academic Press. p. 425. ISBN 978-0-12-446744-6.
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