Extrasolar planet

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.

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

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

Early speculations

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

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

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 can be free-floating or orbit a brown dwarf
• 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

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

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

Related pages

• Planet
• List of exoplanets
• dwarf planet
• 51 Pegasi b
• 70 Virginis b

References

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.
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.
16. Tuomi M. et al [2012]. Signals embedded in the radial velocity noise. Astronomy and Astrophysics (in press).
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

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

Search projects

• University of California Planet Search Project
• The Geneva Extrasolar Planet Search Programmes
• PlanetQuest distributed computing project
• Extrasolar Planets Archived 2011-05-20 at the Wayback Machine

Resources

• NASA's PlanetQuest Archived 2011-02-25 at the Wayback Machine
  • PlanetQuest 3D Atlas of extrasolar planets within 400 light years of our Solar System Archived 2006-09-29 at the Wayback Machine
  • Planetquest Flash Archived 2006-10-01 at the Wayback Machine
• German Center for Exo-Planet Research Jena/Tautenburg
• Astrophysical Institute & University Observatory Jena (AIU)
• Table of known planetary systems
• Extrasolar Planet XML Database Archived 2006-10-04 at the Wayback Machine
• Andrew Collier Cameron, Extrasolar planets, Physics World (January 2001). (See the online version.)
• searchable dynamic database of extrasolar planets and their parent stars Archived 2018-10-01 at the Wayback Machine
• List of important exoplanets
• Extrasolar Planets - D. Montes, UCM
• Extrasolar Visions Archived 2008-05-09 at the Wayback Machine

News

• 6-8 Earth-Mass Planet Discovered orbiting Gliese 876 Archived 2008-12-20 at the Wayback Machine
• Newfound World Shatters Distance Record from space.com
• Oldest Known World from space.com
• Earth Sized Planets Confirmed from space.com
• Sunshade to Look for Distant Life from news.bbc.co.uk