Rogue planet

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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 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.

Video Rogue planet

Observation

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 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. In November 2012, astronomers discovered a rogue planet at 100 light-years.

Maps Rogue planet

Retention of heat in interstellar space

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.

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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.

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Known or possible rogue planets

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.

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See also

• Intergalactic star
• Interstellar comet

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References

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Bibliography

• 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"

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External links

• 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

Source of the article : Wikipedia