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Extrasolar planet

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Artist's impression of the nonexistent Fomalhaut b, an exoplanet directly observed by the Hubble telescope
Planet Fomalhaut b (inset against Fomalhaut's interplanetary dust cloud) imaged by the Hubble Space Telescope's coronagraph (NASA photo)
Discovery image of the Gliese 758 system, taken with Subaru telescope in the near infrared. It is unclear whether the companion should be regarded as a planet or a brown dwarf.
2MASS J044144 is a brown dwarf with a companion about 5-10 times the mass of Jupiter. It is not clear whether this companion object is a sub-brown dwarf or a planet.
Exoplanet discoveries by year

An extrasolar planet (or exoplanet) is a natural planet in a planetary system outside our own solar system. A related concept is an exomoon, a natural satellite orbiting an exoplanet.

In 2013, estimates of the number of terrestrial planets in the Milky Way ranged from at least 17 billion[1] to at least 144 billion.[2] The smaller estimate studied planet candidates gathered by the Kepler space observatory.[3] Among them are 461 Earth-size planets, at least four of which are in the "habitable zone" where liquid water can exist. One of the four, dubbed Kepler-69c, is a mere 1.5 times the size of the Earth and around a star like our own Sun – about as near as the current data allow to finding an "Earth 2.0".[4]

Earlier work suggested that there are at least 100 billion planets of all types in our galaxy, an average of at least one per star. There are also planets that orbit brown dwarfs, and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either type should be called a "planet".[5][6][7]

Analogies with planets in the Solar System apply to few of the extrasolar planets known. Most are quite unlike any of our planets, for example the so-called "hot Jupiters".

History

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Early speculations

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In the sixteenth 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 likewise accompanied by planets. Bruno was burnt at the stake by the Holy Inquisition.[8]

In the eighteenth century, the same possibility was mentioned by Isaac Newton in 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".[9]

Confirmed discoveries

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The first published and confirmed discovery was made in 1988.[10] It was finally confirmed in 2002.

In 1992, radio astronomers announced the discovery of planets around a pulsar.[11] These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation. Otherwise they may be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.

On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[12] This discovery, made at the Observatoire de Haute-Provence, started the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the quick detection of many new exoplanets. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by watching occultations when a star becomes dimmer as an orbiting planet passed in front of it.

In May 2016 NASA announced the discovery of 1,284 exoplanets which brought the total number of exoplanets to over 3,000.[13]

Types

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Extrasolar planets can have many different forms.

  • They can be gas giants or rocky planets
    • They can possibly be dwarf planets, i.e. planets smaller and less dense than regular planets
  • They can orbit several different types of stars
  • They may support life. One recently discovered exoplanet, Gliese 581g is thought to possibly support life, but the existence of this planet is not yet confirmed.


Classes of exoplanets include:


  • A super-terran or super-Earth is a terrestrial planet that is bigger than Earth, but smaller than Neptune
  • A water-world is a hypothetical class of planet between terrestrial and jovian. These worlds would have very very little, if any dry land.
  • Hot Jupiters are gas planets orbiting closer to their stars than Mercury to the sun.
  • Rogue planets orbit no stars at all.


Nearest

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The nearest star with planets is Alpha Centauri. It is 4.3 light years away. Using standard rockets, it would take tens of thousands of years to get there.[14] The nearest star similar to our Sun is Tau Ceti. It has five planets, one of which in the habitable zone, where liquid water may exist.[15][16]

Most Earth-like

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Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8.[17] For comparison, the four solar terrestrial planets are included in this list.

Name ESI SFV HZD COM ATM Planet type Star Habitability Distance (ly) Status Year of
discovery 		Ref
Earth 1.00 0.72 −0.50 −0.31 −0.52 warm terran G mesoplanet 0 Non-exoplanet, inhabited prehistoric
Kepler-438b 0.88 0.88 −0.93 −0.14 −0.73 warm terran M mesoplanet 472.9 confirmed 2015
Kepler-1410b 0.88 0.63 −0.88 −0.16 −0.06 warm superterran K mesoplanet 1213.4 confirmed 2011 [18]
Gliese 667 Cc 0.84 0.64 −0.62 −0.15 +0.21 warm terran M mesoplanet 23.6 confirmed 2011 [19]
Kepler-442b 0.83 0.98 −0.72 −0.15 +0.28 warm superterran K mesoplanet 1291.6 confirmed 2015
Kepler-62e 0.83 0.96 −0.70 −0.15 +0.28 warm superterran K mesoplanet 1199.7 confirmed 2013
Kepler-452b 0.83 0.93 −0.61 -0.15 -0.30 warm superterran G mesoplanet 1402.5 confirmed 2015 [20][21][22]
Gliese 832 c 0.81 0.96 −0.72 −0.15 +0.43 warm superterran M mesoplanet 16.1 confirmed 2014
Kepler-283c 0.79 0.85 −0.58 −0.14 +0.69 warm superterran K mesoplanet 1496.8 confirmed 2011
Kepler-436b 0.79 0.33 −0.87 −0.14 +0.47 warm superterran M mesoplanet 1339.4 confirmed 2015
Kepler-1229b 0.79 0.00 −0.40 −0.15 +0.44 warm superterran M mesoplanet 769.7 confirmed 2016
Tau Ceti e 0.78 0.00 −0.92 −0.15 +0.16 warm superterran G mesoplanet 11.9 unconfirmed 2012
Kepler-296f 0.78 0.15 −0.90 −0.14 +0.53 warm superterran M mesoplanet 1089.6 confirmed 2011
Gliese 180 c 0.77 0.42 −0.53 −0.14 +0.64 warm superterran M mesoplanet 39.5 unconfirmed 2014
Gliese 667 Cf 0.77 0.00 -0.22 −0.16 +0.08 warm terran M psychroplanet 23.6 dubious 2013
Gliese 581 g 0.76 1.00 -0.70 −0.15 +0.28 warm superterran M mesoplanet 20.2 dubious 2010
Gliese 163 c 0.75 0.02 −0.96 −0.14 +0.58 warm superterran M mesoplanet 48.9 confirmed 2012
Gliese 180 b 0.75 0.41 −0.88 −0.14 +0.74 warm superterran M mesoplanet 39.5 unconfirmed 2014
HD 40307 g 0.74 0.04 −0.23 −0.14 +0.77 warm superterran K psychroplanet 41.7 confirmed 2012
Kepler-61b 0.73 0.27 −0.88 −0.13 +1.24 warm superterran M mesoplanet 1062.8 confirmed 2013
Kepler-443b 0.73 0.91 −0.49 −0.13 +1.44 warm superterran K mesoplanet 2564.4 confirmed 2015
Gliese 422 b 0.71 0.17 −0.41 −0.13 +1.11 warm megaterran M mesoplanet 41.3 unconfirmed 2014
Kepler-22b 0.71 0.53 −0.64 −0.12 +1.79 warm superterran G mesoplanet 619.4 confirmed 2011
Kepler-440b 0.70 0.00 +0.01 −0.15 +0.38 warm superterran K psychroplanet 706.5 confirmed 2015
Kepler-298d 0.68 0.00 −0.86 −0.11 +2.11 warm superterran K mesoplanet 1545 confirmed 2012 [23]
Kepler-439b 0.68 0.00 −0.99 −0.13 +1.18 warm superterran G thermoplanet 1914.8 confirmed 2015
Kapteyn b 0.67 0.00 +0.08 −0.15 +0.57 warm superterran M psychroplanet 12.7 unconfirmed 2014
Kepler-62f 0.67 0.05 +0.45 −0.16 +0.19 warm superterran K psychroplanet 1199.7 confirmed 2013
Kepler-186f 0.64 0.00 +0.48 −0.17 −0.26 warm terran M psychroplanet 492 confirmed 2014
Kepler-174d 0.61 0.00 +0.32 −0.13 +1.77 warm superterran K psychroplanet 878.3 confirmed 2011
Gliese 667 Ce 0.60 0.00 +0.51 −0.16 +0.23 warm terran M psychroplanet 23.6 dubious 2013
Gliese 682 c 0.59 0.00 +0.22 −0.14 +1.19 warm superterran M psychroplanet 16.6 unconfirmed 2014
Gliese 581 d 0.53 0.00 +0.78 −0.14 +0.94 warm superterran M hypopsychroplanet 20.2 unconfirmed 2007 [24][25]
Kepler-155c hot superterran K hyperthermoplanet? 965 confirmed 2014
Venus 0.78 0.00 −0.93 −0.28 −0.70 warm terran G hyperthermoplanet close to zero non-exoplanet prehistoric
Mars 0.64 0.00 +0.33 −0.13 −1.12 warm subterran G hypopsychroplanet close to zero non-exoplanet prehistoric
Mercury 0.39 0.00 −1.46 −0.52 −1.37 hot mercurian G non-habitable close to zero non-exoplanet prehistoric
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References

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  1. ↑ Staff (2013). "17 billion Earth-size alien planets inhabit Milky Way". Space.com. Retrieved 8 January 2013.
  2. ↑ Kopparapu, Ravi kumar (2013). "A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around kepler m-dwarfs". Astrophysical Journal - Letters. 767 (1): L8. arXiv:1303.2649v1. Bibcode:2013ApJ...767L...8K. doi:10.1088/2041-8205/767/1/L8. S2CID 119103101.
  3. ↑ Palmer, Jason (2013-01-08). "Earth-sized planets 'number 17bn'". BBC News. Retrieved 2019-01-04.
  4. ↑ Palmer, Jason 2013. Kepler telescope: Earth-sized planets 'number 17bn'. BBC News Science & Environment. [1]
  5. ↑ Claven, Whitney (2013). "Billions and billions of planets". NASA. Archived from the original on 21 April 2020. Retrieved 3 January 2013.
  6. ↑ "100 billion alien planets fill our Milky Way galaxy". Space.com. 2013. Retrieved 3 January 2013. {{cite web}}: Unknown parameter |authors= ignored (help)
  7. ↑ Cassan A; et al. (2012). "One or more bound planets per Milky Way star from microlensing observations". Nature. 481 (7380): 167–169. arXiv:1202.0903. Bibcode:2012Natur.481..167C. doi:10.1038/nature10684. PMID 22237108. S2CID 2614136. Retrieved 2012-01-11.
  8. ↑ "Cosmos" in The New Encyclopædia Britannica (15th edition, Chicago, 1991) 16:787:2a. "For his advocacy of an infinity of suns and earths, he was burned at the stake in 1600."
  9. ↑ Brackenridge, J. Bruce; Newton, Isaac (1999) [1713]. The Principia: A New Translation and Guide. University of California Press. p. 940. ISBN 978-0-520-20217-7.
  10. ↑ Campbell B.; Walker G.A.H.; Yang S. 1988. A search for substellar companions to solar-type stars. Astrophysical Journal 331: 902–921. Bibcode 1988ApJ...331..902C. doi:10.1086/166608
  11. ↑ Wolszczan A. & Frail D.A. 1992. A planetary system around the millisecond pulsar PSR1257+12. Nature 355 (6356): 145–147. doi:10.1038/355145a0
  12. ↑ M. Mayor, D. Queloz (1995). "A Jupiter-mass companion to a solar-type star". Nature. 378 (6555): 355–359. Bibcode:1995Natur.378..355M. doi:10.1038/378355a0. S2CID 4339201.
  13. ↑ "NASA's Kepler Mission Announces Largest Planet Collection Ever Discovered" May 10, 2016 https://www.nasa.gov/press-release/nasas-kepler-mission-announces-largest-collection-of-planets-ever-discovered
  14. ↑ "Travelling to Alpha Centauri". EarthSky.org. Retrieved 19 April 2013.
  15. ↑ Tau Ceti's planets nearest around single, Sun-like star. BBC News Science & Environment. [2]
  16. ↑ Tuomi M. et al [2012]. Signals embedded in the radial velocity noise. Astronomy and Astrophysics (in press). [3]
  17. ↑ "PHL Habitable planet methods". Archived from the original on 2012-09-11. Retrieved 2015-09-05.
  18. ↑ "The New Potential Habitable Exoplanets Candidates of NASA Kepler - Planetary Habitability Laboratory @ UPR Arecibo". phl.upr.edu. Archived from the original on 2021-01-20. Retrieved 2019-01-01.
  19. ↑ "Habitable Exoplanets Catalog". Planetary Habitability Laboratory. 2015. Retrieved 3 June 2015.
  20. ↑ Chou, Felicia; Johnson, Michelle (23 July 2015). "NASA's Kepler Mission Discovers Bigger, Older Cousin to Earth" (Press release). NASA. Retrieved 23 July 2015.
  21. ↑ "News - Astronomers discover Earth's 'bigger, older cousin' - The Weather Network". Archived from the original on 2020-11-24. Retrieved 2015-09-05.
  22. ↑ "NameBright - Coming Soon". Archived from the original on 2017-07-01. Retrieved 2015-09-05.
  23. ↑ "Habitable Exoplanets Catalog: Data of Potentially Habitable Worlds". Planetary Habitability Laboratory. 2015. Archived from the original on 1 December 2017. Retrieved 3 June 2015.
  24. ↑ Reanalysis of data suggests ‘habitable’ planet GJ 581d really could exist – Astronomy Now
  25. ↑ http://www.sciencemag.org/content/347/6226/1080.3.abstract

Other websites

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How can we explore planets beyond our solar system? - lecture by astrophycisist Yamila Miguel (Leiden University) about exoplanets (2021)

Search projects

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Resources

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News

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