A rogue planet (also termed an interstellar planet, nomad planet, free-floating planet, orphan planet, wandering planet, starless planet, or sunless planet) is a planetary-mass object that orbits a galactic center directly. Such objects have been ejected from the planetary system in which they formed or have never been gravitationally bound to any star or brown dwarf. The Milky Way alone may have billions of rogue planets.
Some planetary-mass objects are thought[by whom?] to have formed in a similar way to stars, and the International Astronomical Union has proposed that those objects be called sub-brown dwarfs. A possible example is Cha 110913-773444, which might have been ejected and become a rogue planet, or otherwise formed on its own to become a sub-brown dwarf.
Astronomers have used the Herschel Space Observatory and the Very Large Telescope to observe a very young free-floating planetary-mass object, OTS 44, and demonstrate that the processes characterizing the canonical star-like mode of formation apply to isolated objects down to a few Jupiter masses. Herschel far-infrared observations have shown that OTS 44 is surrounded by a disk of at least 10 Earth masses and thus could eventually form a mini planetary system. Spectroscopic observations of OTS 44 with the SINFONI spectrograph at the Very Large Telescope have revealed that the disk is actively accreting matter, in a similar way to young stars. In December 2013, a candidate exomoon of a rogue planet was announced.
Astrophysicist Takahiro Sumi of Osaka University in Japan and colleagues, who form the Microlensing Observations in Astrophysics and the Optical Gravitational Lensing Experiment collaborations, published their study of microlensing in 2011. They observed 50 million stars in the Milky Way using the 1.8-meter MOA-II telescope at New Zealand's Mount John Observatory and the 1.3-meter University of Warsaw telescope at Chile's Las Campanas Observatory. They found 474 incidents of microlensing, ten of which were brief enough to be planets of around Jupiter's size with no associated star in the immediate vicinity. The researchers estimated from their observations that there are nearly two Jupiter-mass rogue planets for every star in the Milky Way. Other estimates suggest a much larger number, up to 100,000 times more rogue planets than stars in the Milky Way. However, a 2017 study by Przemek Mróz of Warsaw University Observatory and colleagues, with six times larger statistics than the 2011 study, indicates an upper limit on Jupiter-mass free-floating or wide-orbit planets of 0.25 planets per main-sequence star in the Milky Way. 
Retention of heat in interstellar spaceEdit
Interstellar planets generate little heat and are not heated by a star. In 1998, David J. Stevenson theorized that some planet-sized objects adrift in interstellar space might sustain a thick atmosphere that would not freeze out. He proposed that these atmospheres would be preserved by the pressure-induced far-infrared radiation opacity of a thick hydrogen-containing atmosphere.
During planetary-system formation, several small protoplanetary bodies may be ejected from the system. An ejected body would receive less of the stellar-generated ultraviolet light that can strip away the lighter elements of its atmosphere. Even an Earth-sized body would have enough gravity to prevent the escape of the hydrogen and helium in its atmosphere. In an Earth-sized object that has a kilobar atmospheric pressure of hydrogen and a convective gas adiabat, the geothermal energy from residual core radioisotope decay could maintain a surface temperature above the melting point of water, allowing liquid-water oceans to exist. These planets are likely to remain geologically active for long periods. If they have geodynamo-created protective magnetospheres and sea floor volcanism, hydrothermal vents could provide energy for life. Thus, humans could live on such a star-less planet, although food sources would be limited. These bodies would be difficult to detect because of their weak thermal microwave radiation emissions, although reflected solar radiation and far-infrared thermal emissions may be detectable from an object that is less than 1000 astronomical units from Earth. Around five percent of Earth-sized ejected planets with Moon-sized natural satellites would retain their satellites after ejection. A large satellite would be a source of significant geological tidal force heating.
Known or possible rogue planetsEdit
The table below lists rogue planets, confirmed or suspected, that have been discovered. It is yet unknown whether these planets were ejected from orbiting a star or else formed on their own as sub-brown dwarfs.
|Exoplanet||Mass (MJ)||Age (Myr)||Distance (ly)||Status||Discovery|
|OTS 44||~15||0.5–3||160||Likely a low-mass brown dwarf||1998|
|S Ori 52||2–8||1–5||1150||Age and mass uncertain; may be a foreground brown dwarf||2000|
|UGPS J072227.51-054031.2||5–40||13||Mass uncertain||2010|
|MOA-2011-BLG-262||~4||May be a red dwarf||2013|
|2MASS J2208+2921||11–13||21–27||115||Candidate; radial velocity needed||2014|
|WISE 0855−0714||3–10||7.1||Age uncertain; may be a brown dwarf||2014|
|2MASS J12074836–3900043||11–13||7–13||200||Candidate; distance needed||2014|
|SIMP J2154–1055||9–11||30–50||63||Age questioned||2014|
|2MASS J1119–1137||4–8||7–13||94||Candidate; distance needed||2016|
|WISEA 1147||5–13||7–13||94||Candidate; distance needed||2016|
- Shostak, Seth (24 February 2005). Orphan Planets: It's a Hard Knock Life. Space.com, 24 February 2005. Retrieved on 5 February 2009 from http://www.space.com/searchforlife/seti_orphan_planets_050224.html.
- Lloyd, Robin (18 April 2001). Free-Floating Planets – British Team Restakes Dubious Claim. Space.com, 18 April 2001. Retrieved on 5 February 2009 from http://www.space.com/scienceastronomy/astronomy/free_floaters_010403-1.html. Archived 13 October 2008 at the Wayback Machine.
- Author unknown (18 April 2001). Orphan 'planet' findings challenged by new model. NASA Astrobiology, 18 April 2001. Retrieved on 5 February 2009 from "Archived copy". Archived from the original on 22 March 2009. Retrieved 9 February 2009..
- Neil deGrasse Tyson in Cosmos: A Spacetime Odyssey as referred to by National Geographic
- Working Group on Extrasolar Planets – Definition of a "Planet" Position Statement on the Definition of a "Planet" (IAU) Archived 16 September 2006 at the Wayback Machine.
- Rogue planet find makes astronomers ponder theory
- Joergens, V.; Bonnefoy, M.; Liu, Y.; Bayo, A.; Wolf, S.; Chauvin, G.; Rojo, P. (2013). "OTS 44: Disk and accretion at the planetary border". Astronomy & Astrophysics. 558 (7): L7. arXiv: . Bibcode:2013A&A...558L...7J. doi:10.1051/0004-6361/201322432.
- A sub-Earth-mass moon orbiting a gas giant primary or a high-velocity planetary system in the galactic bulge
- Homeless' Planets May Be Common in Our Galaxy Archived 8 October 2012 at the Wayback Machine. by Jon Cartwright, Science Now, 18 May 2011, Accessed 20 May 2011
- Planets that have no stars: New class of planets discovered, Physorg.com, 18 May 2011. Accessed May 2011.
- T. Sumi; et al. (2011). "Unbound or Distant Planetary Mass Population Detected by Gravitational Microlensing". Nature. 473: 349–352. arXiv: [astro-ph.EP]. Bibcode:2011Natur.473..349S. doi:10.1038/nature10092.
- "Researchers say galaxy may swarm with 'nomad planets'". Stanford University. Retrieved 29 February 2012.
- P. Mroz; et al. (2017). "No large population of unbound or wide-orbit Jupiter-mass planets". Nature. 548: 183–186. arXiv: [astro-ph.EP]. Bibcode:2017Natur.548..183M. doi:10.1038/nature23276.
- Luhman, Kevin L.; Esplin, Taran L. (September 2016). "The Spectral Energy Distribution of the Coldest Known Brown Dwarf". The Astronomical Journal. 152 (2). 78. arXiv: . Bibcode:2016AJ....152...78L. doi:10.3847/0004-6256/152/3/78.
- Sean Raymond (9 April 2005). "Life in the dark". Aeon. Retrieved 9 April 2016.
- Stevenson, David J.; Stevens, C. F. (1999). "Life-sustaining planets in interstellar space?". Nature. 400 (6739): 32. Bibcode:1999Natur.400...32S. doi:10.1038/21811. PMID 10403246.
- Lissauer, J. J. (1987). "Timescales for Planetary Accretion and the Structure of the Protoplanetary disk". Icarus. 69 (2): 249–265. Bibcode:1987Icar...69..249L. doi:10.1016/0019-1035(87)90104-7.
- Dorian S. Abbot; Eric R. Switzer (2 June 2011). "The Steppenwolf: A proposal for a habitable planet in interstellar space". The Astrophysical Journal. 735: L27. arXiv: . Bibcode:2011ApJ...735L..27A. doi:10.1088/2041-8205/735/2/L27.
- Debes, John H.; Steinn Sigurðsson (20 October 2007). "The Survival Rate of Ejected Terrestrial Planets with Moons". The Astrophysical Journal Letters. 668 (2): L167–L170. arXiv: . Bibcode:2007ApJ...668L.167D. doi:10.1086/523103.
- Luhman, Kevin L. (10 February 2005). "Spitzer Identification of the Least Massive Known Brown Dwarf with a Circumstellar Disk". Astrophysical Journal Letters. 620 (1): L51–L54. arXiv: . Bibcode:2005ApJ...620L..51L. doi:10.1086/428613.
- Zapatero Osorio, M. R. (6 October 2000). "Discovery of Young, Isolated Planetary Mass Objects in the σ Orionis Star Cluster". Science. 290: 103. Bibcode:2000Sci...290..103Z. doi:10.1126/science.290.5489.103.
- Luhman, Kevin L. (10 December 2005). "Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk". Astrophysical Journal Letters. 635: 93L. arXiv: . Bibcode:2005ApJ...635L..93L. doi:10.1086/498868.
- Marsh, Kenneth A. (1 February 2010). "A Young Planetary-Mass Object in the ρ Oph Cloud Core". Astrophysical Journal Letters. 709: L158. arXiv: . Bibcode:2010ApJ...709L.158M. doi:10.1088/2041-8205/709/2/L158.
- Delorme, Philippe (25 September 2012). "CFBDSIR2149-0403: a 4-7 Jupiter-mass free-floating planet in the young moving group AB Doradus?". Astronomy & Astrophysics. 548A: 26. arXiv: . Bibcode:2012A&A...548A..26D. doi:10.1051/0004-6361/201219984.
- Liu, Michael C. (10 November 2013). "The Extremely Red, Young L Dwarf PSO J318.5338-22.8603: A Free-floating Planetary-mass Analog to Directly Imaged Young Gas-giant Planets". Astrophysical Journal Letters. 777 (1): L20. arXiv: . Bibcode:2013ApJ...777L..20L. doi:10.1088/2041-8205/777/2/L20.
- Gagné, Jonathan (10 March 2014). "BANYAN. II. Very Low Mass and Substellar Candidate Members to Nearby, Young Kinematic Groups with Previously Known Signs of Youth". Astrophysical Journal. 783: 121. arXiv: . Bibcode:2014ApJ...783..121G. doi:10.1088/0004-637X/783/2/121.
- Schneider, Adam C. (9 January 2014). "Discovery of the Young L Dwarf WISE J174102.78-464225.5". Astronomical Journal. 147: 34. arXiv: . Bibcode:2014AJ....147...34S. doi:10.1088/0004-6256/147/2/34.
- Luhman, Kevin L. (10 May 2014). "Discovery of a ~250 K Brown Dwarf at 2 pc from the Sun". Astrophysical Journal Letters. 786: L18. arXiv: . Bibcode:2014ApJ...786L..18L. doi:10.1088/2041-8205/786/2/L18.
- Gagné, Jonathan (10 April 2014). "The Coolest Isolated Brown Dwarf Candidate Member of TWA". Astrophysical Journal Letters. 785 (1): L14. arXiv: . Bibcode:2014ApJ...785L..14G. doi:10.1088/2041-8205/785/1/L14.
- Liu, Michael C. (9 December 2016). "The Hawaii Infrared Parallax Program. II. Young Ultracool Field Dwarfs". Astrophysical Journal. 833: 96. arXiv: . Bibcode:2016ApJ...833...96L. doi:10.3847/1538-4357/833/1/96.
- Gagné, Jonathan (1 September 2014). "SIMP J2154-1055: A New Low-gravity L4β Brown Dwarf Candidate Member of the Argus Association". Astrophysical Journal Letters. 792: L17. arXiv: . Bibcode:2014ApJ...792L..17G. doi:10.1088/2041-8205/792/1/L17.
- Gagné, Jonathan (20 July 2015). "SDSS J111010.01+011613.1: A New Planetary-mass T Dwarf Member of the AB Doradus Moving Group". Astrophysical Journal Letters. 808: L20. arXiv: . Bibcode:2015ApJ...808L..20G. doi:10.1088/2041-8205/808/1/L20.
- Kellogg, Kendra (11 April 2016). "The Nearest Isolated Member of the TW Hydrae Association is a Giant Planet Analog". Astrophysical Journal Letters. 821 (1): L15. arXiv: . Bibcode:2016ApJ...821L..15K. doi:10.3847/2041-8205/821/1/L15.
- Schneider, Adam C. (21 April 2016). "WISEA J114724.10-204021.3: A Free-floating Planetary Mass Member of the TW Hya Association". Astrophysical Journal Letters. 822 (1): L1. arXiv: . Bibcode:2016ApJ...822L...1S. doi:10.3847/2041-8205/822/1/L1.
- Stevenson, D. (1999). "Life-sustaining planets in interstellar space?". Nature. 400 (6739): 32. Bibcode:1999Natur.400...32S. doi:10.1038/21811. PMID 10403246.
- Article by Stevenson similar to the Nature article but containing more information, titled: "Possibility of Life Sustaining Planets in Interstellar Space"
|Look up Interstellar planet in Wiktionary, the free dictionary.|
|Wikimedia Commons has media related to Exoplanets.|
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