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A rogue planet, also known as an interstellar planet, nomad planet, free-floating planet or orphan planet, is a planetary-mass object that orbits the galaxy directly. They have either been ejected from the planetary system in which they formed or were never gravitationally bound to any star or brown dwarf.

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 (an example of this is Cha 110913-773444, which may be an ejected rogue planet or may have formed on its own and be a sub-brown dwarf). The closest free-floating planetary mass object to Earth yet discovered, WISE 0855–0714, is around 7 light years away.

In December 2013 a candidate exomoon of a free-floating planet was announced.

Observation

Most methods of detecting exoplanets rely on periodicity of the planet orbiting a host star and thus cannot be used to detect rogue planets. Two methods to detect rogue planets still can be used: 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 emit enough radiation to be detected. On the other hand, without the glare of the host star, the planet itself can be observed more easily once found.

When a planetary-sized object passes in front of a background star, its gravitational field causes a momentary increase in the visible brightness of the background star. This is known as microlensing. The disadvantage of microlensing is that the planet cannot be continuously observed. However, it works better than direct imaging for older and lower-mass planets. 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 our galaxy 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 free-floaters for every star in our galaxy. Other estimations suggest a much larger number, up to 100,000 times more free-floating planets than stars in our Milky Way. In November 2012 astronomers discovered a rogue planet around 100 light-years from Earth.

Retention of heat in interstellar space

In 1998, David J. Stevenson theorized that some planet-sized objects drift in the vast expanses of cold interstellar space and 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 predominantly hydrogen- and helium-containing atmosphere would be easily confined even by an Earth-sized body's gravity.

It is calculated that for an Earth-sized object at a kilobar hydrogen atmospheric pressures in which a convective gas adiabat has formed, geothermal energy from residual core radioisotope decay will be sufficient to heat the surface to temperatures 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 a geodynamo-created protective magnetosphere and possible sea floor volcanism which could provide an energy source for life. 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 one were to pass within 1000 AU of Earth.

A study of simulated planet ejection scenarios has suggested that around five percent 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, it has been discovered that some exoplanets such as the planemo 2M1207b, orbiting the brown dwarf 2M1207, have debris discs. 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.

Known or possible rogue planets

There is no current way of telling whether these are planets that have been ejected from orbiting a star or were originally formed on their own as sub-brown dwarfs.

See also

• Brown dwarf
• Intergalactic star
• Interstellar comet
• Planetar
• Sub-brown dwarf

References

1. Orphan Planets: It's a Hard Knock Life, Space.com, 24 Feb 2005, retrieved 5 Feb 2009.
2. Free-Floating Planets – British Team Restakes Dubious Claim, Space.com, 18 Apr 2001, retrieved 5 Feb 2009.
3. Orphan 'planet' findings challenged by new model, NASA Astrobiology, 18 Apr 2001, retrieved 5 Feb 2009.
4. Working Group on Extrasolar Planets – Definition of a "Planet" POSITION STATEMENT ON THE DEFINITION OF A "PLANET" (IAU)
5. Rogue planet find makes astronomers ponder theory
6. A sub-Earth-mass moon orbiting a gas giant primary or a high-velocity planetary system in the galactic bulge
7. Homeless' Planets May Be Common in Our Galaxy by Jon Cartwright, Science Now ,18 May 2011, Accessed 20 may 2011
8. Planets that have no stars: New class of planets discovered, Physorg.com, May 18, 2011. Accessed May 2011.
9. . {{cite arXiv}}: Explicit use of et al. in: |last= (help); Unknown parameter |version= ignored (help)
10. "Researchers say galaxy may swarm with 'nomad planets'". Stanford University. Retrieved 29 February 2012. {{cite web}}: External link in |publisher= (help)
11. (BBC) (Astron. & Asrophys.) 
12. ^ 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.
13. 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.
14. Dorian S. Abbot (2 June 2011). "The Steppenwolf: A proposal for a habitable planet in interstellar space". arXiv:1102.1108. {{cite arXiv}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)CS1 maint: date and year (link)
15. Debes, John H. (20 October 2007). "The Survival Rate of Ejected Terrestrial Planets with Moons". The Astrophysical Journal Letters. 668 (2): L167 – L170. arXiv:0709.0945. Bibcode:2007ApJ...668L.167D. doi:10.1086/523103. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

• 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"

External links

• Strange New Worlds Could Make Miniature Solar Systems Robert Roy Britt (SPACE.com) 5 June 2006 11:35 am ET
• Working Group on Extrasolar Planets – Definition of a "Planet" POSITION STATEMENT ON THE DEFINITION OF A "PLANET" (IAU) 2003
• The IAU draft definition of "planet" and "plutons" press release (International Astronomical Union) 2006
• Text of A Pail of Air and original radio broadcast

v t e Exoplanets
Planet Definition IAU Planetary science
Main topics	Exoplanet Exoplanet orbital and physical parameters Methods of detecting exoplanets Planetary system Planet-hosting stars
Sizes and types	Terrestrial Carbon planet Coreless planet Desert planet Dwarf planet Hycean planet Ice planet Iron planet (Super-Mercury) Lava planet Ocean world Mega-Earth Sub-Earth Super-Earth Gaseous Eccentric Jupiter Mini-Neptune (Gas dwarf) Helium planet Hot Jupiter Hot Neptune Gas giant Ice giant Super-Jupiter Super-Neptune Super-puff Ultra-hot Jupiter Ultra-hot Neptune Other types Blanet Brown dwarf Chthonian planet Circumbinary planet Circumtriple planet Disrupted planet Double planet Ecumenopolis Eyeball planet Giant planet Mesoplanet Planemo Planet/Brown dwarf boundary Planetesimal Protoplanet Pulsar planet Sub-brown dwarf Sub-Neptune Toroidal planet Ultra-cool dwarf Ultra-short period planet (USP)
Formation and evolution	Accretion Accretion disk Asteroid belt Circumplanetary disk Circumstellar disc Circumstellar envelope Cosmic dust Debris disk Detached object Disrupted planet Excretion disk Exozodiacal dust Extraterrestrial materials Extraterrestrial sample curation Giant-impact hypothesis Gravitational collapse Hills cloud Internal structure Interplanetary dust cloud Interplanetary medium Interplanetary space Interstellar cloud Interstellar dust Interstellar medium Interstellar space Kuiper belt List of interstellar and circumstellar molecules Merging stars Molecular cloud Nebular hypothesis Oort cloud Outer space Planetary migration Planetary system Planetesimal Planet formation Protoplanetary disk Ring system Rubble pile Sample-return mission Scattered disc Star formation
Systems	Exocomet Interstellar Exomoon Tidally detached Rogue planet Orbits Retrograde Trojan Mean-motion resonances Titius–Bode law
Host stars	A B Binary star Brown dwarfs F/Yellow-white dwarfs G/Yellow dwarfs Herbig Ae/Be K/Orange dwarfs M/Red dwarfs Pulsar Red giant Subdwarf B Subgiant T Tauri White dwarfs Yellow giants
Detection	Astrometry Direct imaging list Microlensing list Polarimetry Timing list Radial velocity list Transit method list Transit-timing variation
Habitability	Astrobiology Astrooceanography Circumstellar habitable zone Earth analog Extraterrestrial liquid water Galactic habitable zone Habitability of binary star systems Habitability of F-type main-sequence star systems Habitability of K-type main-sequence star systems Habitability of natural satellites Habitability of neutron star systems Habitability of red dwarf systems Habitability of yellow dwarf systems Habitable zone for complex life List of potentially habitable exoplanets Tholin Superhabitable planet
Catalogues	Nearby Habitable Systems Exoplanet Data Explorer Extrasolar Planets Encyclopaedia NASA Exoplanet Archive NASA Star and Exoplanet Database Open Exoplanet Catalogue
Lists	Exoplanetary systems Host stars Multiplanetary systems Stars with proto-planetary discs Exoplanets Discoveries Extremes Firsts Nearest Largest Heaviest Terrestrial candidates Kepler 1–500 501–1000 1001–1500 1501–2000 K2 Potentially habitable Proper names Discovered exoplanets by year before 2000 2000–2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024
Other	Carl Sagan Institute Exoplanet naming convention Exoplanet phase curves Exoplanetary Circumstellar Environments and Disk Explorer Extragalactic planet Extrasolar planets in fiction Geodynamics of terrestrial exoplanets Neptunian desert Nexus for Exoplanet System Science Planets in globular clusters Small planet radius gap Sudarsky's gas giant classification
Discoveries of exoplanets Search projects

• Use dmy dates from July 2011
• Rogue planets
• Types of planet
• Planemos