Exoplanet

[[File:2M1207b - First image of an exoplanet.jpg|thumb|right|165px|The first direct image of an exoplanet: 2M1207b, orbiting the star 2M1207 (center)]]
{{Astronomy}}
An '''exoplanet''' is a planet orbiting a star other than ours. {{wpl|Rogue planet|Rogue planets,}} planets that drift through deep space instead of orbiting a star, also exist.<ref>Alan Boyle, [http://www.nbcnews.com/science/astronomers-say-theyve-spotted-lonesome-planet-without-sun-8C11366309 Astronomers say they've spotted lonesome planet without a sun]. ''NBC News'', 9 October 2013.</ref>

==History and current state of knowledge==
The first exoplanet discovered was found orbiting a pulsar, {{wpl|PSR B1257+12||,}} during a 1993 study led by Alex Wolszczan of Pennsylvania State University. It wasn't until 1995 that Michel Mayor and Didier Queloz of the University of Geneva confirmed an exoplanet around a main sequence star, 51 Pegasi.

As of August 6, 2019, 4,103 planets in 3,056 planetary systems (with 665 multiple-planet systems) have been found.<ref>[http://exoplanet.eu/catalog.php Interactive Extra-solar Planets Catalog], The Extrasolar Planets Encyclopaedia, accessed 6 August 2019</ref>  Most of these exoplanets are [[gas giant]]s; this is most likely due to the limits of current detection methods.  Some [[rocky planet]]s have been found as well.  Unlike gas giants, rocky planets may, given the right conditions, be able to sustain life.{{efn|This point is highly debatable; there is no real reason why some form of life could not exist elsewhere than rocky planets, so perhaps we should say "life as we know it".}}

Currently, the most notable of these rocky planets is the relatively nearby (20 light-years) planet [[Gliese 581c]]. Of all the known exoplanets, this one comes closest to being capable of sustaining life,{{efn|Again, we are guilty of terracentrism here, imagining that only life like ours on a planet like ours could occur.}} though its existence is disputed.<ref>[http://blogs.discovermagazine.com/badastronomy/2011/01/18/does-gliese-581g-exist/#.Utre_bTnbIU Does Gliese 581g exist?]</ref><ref>[http://www.space.com/10897-alien-planet-gliese-581g-great-debate.html R.I.P. Possibly Habitable Planet Gliese 581g? Not So Fast, Co-Discoverer Says]</ref>

Astronomer Jay Farhi suggests that pollutants in the atmospheres of white dwarf stars may be from either defunct [[asteroid]] belts or shattered rocky planets.<ref>Andrea Thompson, [http://www.space.com/scienceastronomy/many-earth-like-explanets-100412.html Polluted Old Stars Suggest Earth-like Worlds May Be Common]. space.com, 12 April 2010.</ref>

There are estimated 100 billion planets (or more) in the Milky Way alone. There are definitely other Earth-like planets out there somewhere.<ref>[https://www.nasa.gov/multimedia/imagegallery/image_feature_2233.html www.nasa.gov] NASA publishes findings by the PLANET organization</ref><ref>[https://www.space.com/19103-milky-way-100-billion-planets.html 100 Billion Alien Planets Fill Our Milky Way Galaxy: Study]. space.com, 2 January 2013.</ref>

[[File:K2-18 b.jpeg|thumb|left|200px|Artist’s conception of K2-18 b, a giant “water-world” that might have photosynthetic life.]]
On September 2023, the James Webb Space Telescope detected potential traces of dimethyl sulfide in the atmosphere of the sub-Neptune oceanic exoplanet K2-18 b, 120 light years away. If this is true, it could be a sign of ongoing photosynthesis by phytoplankton, as that is the only known method by which the chemical can be produced on Earth. As exciting as this sounds, further investigation is needed, though.<ref>[https://www.livescience.com/space/exoplanets/james-webb-telescope-sees-potential-signs-of-alien-life-in-the-atmosphere-of-a-distant-goldilocks-water-world James Webb telescope sees potential signs of alien life in the atmosphere of a distant goldilocks water world.] ''LiveScience.''</ref>

==Example==
[[File:Epsilon Eridani b.jpg|thumb|left|200px|Artist's conception of planets around ε Eridani]]
{{wpl|Epsilon Eridani}} is a star system 10.5 light years away from Earth. It is an orange dwarf star approximately 850 million years old. There are currently two confirmed planets in the system: A Jupiter-sized planet orbiting outside of ε Eridani's first asteroid belt, and a Saturn-sized planet at about the same distance from the star as [[Pluto]] is from the [[Sun]]. It is expected that more planets will be found in the system within a decade when there are better telescopes.

==Selection bias==
Many scientists cite the number of exoplanets and their rate of discovery as evidence that Earth-like planets, and eventually [[extraterrestrial]] [[life]], will be inevitably found. Critics of this view are quick to point out that all the planets so far found certainly ''don't'' support this conclusion &mdash; as all known exoplanets orbit too close to their star or are gas giants. However, the search for exoplanets is subject to a high degree of [[selection bias]]. All the methods outlined below lead to a huge tendency to find large or closely-orbiting planets. With the transit method, large planets produce a more easily observed signal; with astrometric methods, more massive planets that orbit close to their star will exert a greater force and be more easily seen. As technology improves, smaller planets and more distant planets are being discovered. While it is yet to be confirmed that our own solar system is typical, the selection bias means that exoplanets are not yet strong enough evidence to say that the solar system is unique or part of a special [[creation]].

==Important caveats==
Despite news that often announce the discovery of habitable planets such as {{wpl|Proxima Centauri b}} (yes, around the closest star to our Sun), {{wpl|Kepler-62f||,}} {{wpl|Kepler-186f||,}} and a growing number of others, we just know with our current state-of-the-art technology that ''there is'' a planet (sometimes more) orbiting a given star and depending of the method used for its detection ({{wpl|Doppler_spectroscopy|radial velocity}} measurements and/or looking for {{wpl|Methods_of_detecting_exoplanets#Transit_photometry|transits in front of its home star}}) just its ''minimum'' mass (depending of how inclined is its orbit respect to us, usually unknown. If the inclination is known, it's possible to calculate their actual mass) and/or its radius respectively,{{efn|In systems with several planets transiting it's often possible to determine their individual masses, thus their densities too and make more or less elaborated guesses about their composition, as the case of the famous TRAPPIST-1 system.}} as well in both cases that, according to our climate models, at that distance temperatures allow liquid water to exist and some very rudimentary data on their atmospheres at best. All those cool pictures in websites as NASA's one are just artist's depictions as noted in their captions as we do not know if those planets are actually more or less Earth-like (thus habitable for us), [[Venus]]-like (or not so Venus-like) hells or worse, something in between, or exotic worlds that look like as if they came from a science-fiction work (much less if there is life on them). We are just able to use climate or geological modelling to attempt to deduce what kind of conditions could be expected to be found there,{{efn|That it would not be strange that turned out to be wrong as we've just a couple of worlds close enough to study them in detail (in our Solar System, of course) and our models are based on extrapolations of what happens there}} so it's more than likely we'd not know the actual conditions present on them until we managed to go there assuming that day ''ever'' came.

==External links to sources==
*[http://exoplanets.org/ California & Carnegie Planet Search]
*[http://www.daviddarling.info/  Encyclopedia of Science, ''David Darling'']
*[http://exoplanet.eu/ The Extrasolar Planets Encyclopaedia]

==See also==
*[[Astrobiology]]

==Notes==
{{reflist|group=note}}

==References==
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[[Category:Astronomy]]
[[Category:Planets]]

Categories: Astronomy, Planets