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.

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".

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.