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A rogue planet (also known as an interstellar planet, nomad planet, free-floating planet, orphan planet, wandering planet, or starless planet) is a planetary-mass object that orbits the galaxy directly. Such objects have either 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 could have billions of rogue planets.
Some planetary-mass objects are thought to have formed in a similar way to stars, and the IAU 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. The closest free-floating planetary-mass object to Earth yet discovered, WISE 0855−0714, is at 7 light years, though it may be a sub-brown dwarf.
Recent observations of a very young free-floating planetary-mass object, OTS 44, with the Herschel Space Observatory and the Very Large Telescope demonstrate that the processes that characterize the canonical star-like mode of formation apply to isolated objects down to a few Jupiter masses. Herschel far-infrared observations show that this young free-floating planetary-mass object is surrounded by a disk with a mass of at least 10 Earth masses and can, therefore, eventually form a mini planetary system. Spectroscopic observations of OTS 44 with the SINFONI spectrograph at the Very Large Telescope reveal that the disk is actively accreting matter, in a similar way to young stars.
Most methods of detecting exoplanets rely on periodic motion of the star caused by the planet in orbit. Since a rogue planet has no star, such a method cannot be used to detect rogue planets. Two methods used to detect rogue planets are gravitational microlensing and direct imaging.
Direct imaging allows astronomers to observe rogue planets continuously. However, only young and massive rogue planets can be observed this way because they are the only ones that emit enough radiation to be detected. On the other hand, without the glare of a host star, the planet itself can be observed more easily once found.
When a planetary-mass object passes in front of a star, the object's gravitational field causes a momentary increase in the visible brightness of the background star. This is known as microlensing. The effect of microlensing cannot be observed continually because the planet is in motion relative to the background star, but it briefly allows the detection of older brown dwarf and lower-mass planets than is possible through direct imaging. Astrophysicist Takahiro Sumi of Osaka University in Japan and colleagues, who form the Microlensing Observations in Astrophysics (MOA) and the Optical Gravitational Lensing Experiment (OGLE) collaborations, carried out a study of microlensing which they published 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 rogue planets for every star in the Milky Way. Other estimations suggest a much larger number, up to 100,000 times more rogue planets than stars in the Milky Way.
In November 2012, astronomers discovered a rogue planet at 100 light-years.
Retention of heat in interstellar space
Interstellar planets generate little heat nor are they heated by a star.
In 1998, David J. Stevenson theorized that some planet-sized objects adrift in the vast expanses of cold interstellar space could possibly sustain a thick atmosphere that would not freeze out. He proposes that atmospheres are preserved by the pressure-induced far-infrared radiation opacity of a thick hydrogen-containing atmosphere.
It is thought that, during planetary-system formation, several small protoplanetary bodies may be ejected from the forming system. With the reduced ultraviolet light that would normally strip the lighter components from an atmosphere, due to its increasing distance from the parent star, the planet's atmosphere, composed predominantly of hydrogen and helium, would be easily confined even by an Earth-sized body's gravity.
It is calculated that, for an Earth-sized object with a kilobar atmospheric pressure of hydrogen, in which a convective gas adiabat has formed, geothermal energy from residual core radioisotope decay will maintain a surface temperature above the melting point of water. Thus, it is proposed that interstellar planetary bodies with extensive liquid-water oceans may exist. It is further suggested that these planets are likely to remain geologically active for long periods, providing there is a geodynamo-created protective magnetosphere, with possible sea floor volcanism providing an energy source for life. Thus humans could theoretically live on a planet without a sun, although food sources would be limited. The author admits these bodies would be difficult to detect due to the intrinsically weak thermal microwave radiation emissions emanating from the lower reaches of the atmosphere, although later research suggests that reflected solar radiation and far-IR thermal emissions may be detectable if such an object were to pass within 1000 AU of Earth.
A study of simulated planet ejection scenarios has suggested that around 5% of Earth-sized planets with Moon-sized natural satellites would retain their satellites after ejection. A large satellite would be a source of significant geological tidal heating.
Proplyds of planetars
Recently[when?], it has been discovered[who?] that some exoplanets such as the planemo 2M1207b, orbiting the brown dwarf 2M1207, have debris disks. If some large interstellar objects are considered stars (sub-brown dwarfs), then the debris could coalesce into planets, meaning the disks are proplyds. If these are considered planets, then the debris would coalesce as satellites. The term planetar exists for those accretion masses that seem to fall between stars and planets.
Rogue planets in fiction
The American science-fiction series Star Trek: Enterprise aired an episode on March 20, 2002 titled "Rogue Planet", where the crew encounters a rogue planet in deep space. Due to geothermal venting along the equator, enough heat and oxygen exist allowing the humans to explore that region of the rogue planet without the need for EVA suits. 
Known or possible rogue planets
The table below lists rogue planets (confirmed or suspected) that have been discovered. It is not yet known whether these planets were ejected from orbiting a star or else formed on their own as sub-brown dwarfs.
|Planet||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 not constrained (could be a foreground brown dwarf)||2000|
|UGPS J072227.51-054031.2||5–40||13||Mass not constrained||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 not constrained (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 called into question||2014|
|2MASS J1119–1137||4–8||7–13||94||Candidate (distance needed)||2016|
|WISEA 1147||5–13||7–13||94||Candidate (distance needed)||2016|
- A Pail of Air (related science fiction story)
- Brown dwarf
- Intergalactic star
- Interstellar comet
- Melancholia (related film)
- Sub-brown dwarf
- Shostak, Seth (2005-02-24). Orphan Planets: It's a Hard Knock Life. Space.com, 24 February 2005. Retrieved on 2009-02-05 from http://www.space.com/searchforlife/seti_orphan_planets_050224.html.
- Lloyd, Robin (2001-04-18). Free-Floating Planets – British Team Restakes Dubious Claim. Space.com, 18 April 2001. Retrieved on 2009-02-05 from http://www.space.com/scienceastronomy/astronomy/free_floaters_010403-1.html. Archived 13 October 2008 at the Wayback Machine.
- Author unknown (2001-04-18). Orphan 'planet' findings challenged by new model. NASA Astrobiology, 18 April 2001. Retrieved on 2009-02-05 from http://astrobiology.arc.nasa.gov/news/expandnews.cfm?id=783[dead link].
- 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). 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 by Jon Cartwright, Science Now ,18 May 2011, Accessed 20 may 2011
- Planets that have no stars: New class of planets discovered, Physorg.com, May 18, 2011. Accessed May 2011.
- [T. Sumi; et al. (2011). "Unbound or Distant Planetary Mass Population Detected by Gravitational Microlensing". arXiv: [astro-ph.EP].
- "Researchers say galaxy may swarm with 'nomad planets'". Stanford University. Retrieved 2012-02-29.
- (BBC) (Astron. & Asrophys.)
- Sean Raymond (9 April 2005). "Life in the dark". Aeon. Retrieved 9 April 2016.
- Stevenson, David J.; Stevens, CF (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". arXiv: .
- 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.|
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