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 never been gravitationally bound to any star or brown dwarf. There may be billions of rogue planets in the Milky Way.
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 may have been ejected and become a 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 at 7 light years.
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, it has been discovered 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.
Known or possible rogue planets
The table below lists rogue planets (confirmed or suspected) that have been discovered. 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.
|Planet||Mass (MJ)||Distance (ly)||Status||Discovery|
|WISE 0855−0714||3–10||7.1||May be a brown dwarf||2014|
|S Ori 52||2–8 (or brown dwarf)||Mass not constrained|
|UGPS J072227.51-054031.2||5–40||13||Mass not constrained||2010|
|Cha 110913-773444||5–15||163||Mass not constrained||2004|
|MOA-2011-BLG-262||~4||May be a red dwarf||2013|
- Orphan Planets: It's a Hard Knock Life, Space.com, 24 Feb 2005, retrieved 5 Feb 2009.
- Free-Floating Planets – British Team Restakes Dubious Claim, Space.com, 18 Apr 2001, retrieved 5 Feb 2009. Archived 13 October 2008 at the Wayback Machine.
- Orphan 'planet' findings challenged by new model, NASA Astrobiology, 18 Apr 2001, retrieved 5 Feb 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). 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 29 February 2012.
- (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.
- 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.|
- Definition of a "Planet" (Resolution B5 - IAU)
- Strange New Worlds Could Make Miniature Solar Systems Robert Roy Britt (SPACE.com) 5 June 2006 11:35 am ET
- The IAU draft definition of "planet" and "plutons" press release (International Astronomical Union) 2006