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A rogue planet (also termed an interstellar planet, nomad planet, free-floating planet, orphan planet, wandering planet, starless planet, sunless planet, or Planemo) 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 may have billions of rogue planets.
Contents
- Observation
- Retention of heat in interstellar space
- Proplyds of planetars
- Rogue planets in fiction
- Known or possible rogue planets
- References
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 of at least 10 Earth masses, and thus eventually, can 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. In December 2013, a candidate exomoon of a rogue planet was announced.
Observation
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, with no glare of a host star, an isolated planet can be observed more easily, once found. When a planetary-mass object passes in front of a star, the object's gravitational field causes momentary changes in the visible brightness of the background star. This is termed microlensing. Such effects 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 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.
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 might 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 theoretically, humans might 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 force 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.
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 with no need for EVA suits.
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.