Free-floating planet

This article is about a type of astronomical object. For other uses, see Rogue planet (disambiguation).

File:Artist's impression of the free-floating planet CFBDSIR J214947.2-040308.9.ogv

A rogue planet — also known as an interstellar planet, nomad planet, free-floating planet or orphan planet — is a planetary-mass object which has either been ejected from its system or was never gravitationally bound to any star, brown dwarf or other such object, and that therefore orbits the galaxy directly.[1][2][3] Astronomers agree that either way, the definition of planet should depend on its current observable state and not its origin.

Larger planetary-mass objects which were not ejected, but have always been free-floating, are thought to have formed in a similar way to stars, and the IAU has proposed that those objects be called sub-brown dwarfs[4] (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).[5] The closest rogue planet to Earth yet discovered, PSO J318.5-22, is around 80 light years away.[6]


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. 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.[7][8][9] Other estimations suggest a much larger number, up to 100,000 times more free-floating planets than stars in our Milky Way.[10]

Retention of heat in interstellar space

In 1998, David J. Stevenson theorized[11] that some planet-sized objects drift in the vast expanses of cold interstellar space and could possibly sustain a thick atmosphere which 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.[12] 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.[11]

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.[11] 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.[11] 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[13] 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.[14]

Proplyds of planetars

Recently, it has been discovered that some extrasolar planets 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.

In popular culture

In the novel When Worlds Collide (serialised 1932) by Edwin Balmer and Phillip Wylie, Earth is first devastated, and then destroyed, by "Bronson Alpha", a gas-giant-sized rogue planet, orbited by "Bronson Beta", an Earth-sized satellite. In the film version (1951) of the novel, Bronson Alpha was reimagined as a dwarf star and renamed "Bellus", while Bronson Beta was designated "Zyra."[15]

Fritz Leiber's short story A Pail of Air tells of Earth after it has been torn away from the Sun by a passing "dark star".

In "Foundation and Empire", by Isaac Asimov, the chapter The War Begins includes mention of General Bel Riose establishing a headquarters on "the rocky barrenness of a wandering sunless planet."

"Rogue Planet" is the title of an early 1950s Dan Dare adventure from the Eagle (comic).

In the novel Wolfbane by Frederik Pohl and C. M. Kornbluth (originally serialised in Galaxy in 1957), a rogue planet, populated by strange machines known as Pyramids, steals the Earth from the Solar System, taking it off into interstellar space.

In Fritz Leiber's novel The Wanderer, Earth encounters two ambulatory rogue planets. In the novel The Witches of Karres (1966) by James H. Schmitz, expanded from a 1949 novelette, the rogue planet Karres can be moved through space by means of psychic powers.

The first known use of "rogue planet" as term for such detached worlds occurred in Poul Anderson's Polesotechnic League novel Satan's World (1969).

In the British science-fiction television series Space: 1999, the pilot episode (1975) has a rogue planet, Meta, coming near Earth. The series itself centers on the Moon being ejected from Earth orbit as well as from the Solar System, thereby becoming a rogue planet.

The rogue planet of Worlorn is the scene of action in George R. R. Martin's novel Dying of the Light.

In the 1980 animated series Thundarr the Barbarian a "runaway planet" passes between the Earth and Moon in the year 1994. The Moon is cracked in half and Earth's civilization is destroyed. The series takes place two thousand years later, when the post-apocalyptic Earth is inhabited by mutants and wizards.

In the first series of Transformers comics published by Marvel Comics, the planet of Cybertron is a rogue planet that was dislodged from its original orbit in the Alpha Centauri system by weapons of mass destruction. In the Red Dwarf books, the Earth becomes a rogue planet when it is torn from its orbit by exploding sewage.

The homeworld of the Founders in Star Trek: Deep Space Nine is a rogue planet in a nebula; it has climatic conditions capable of supporting humanoid life. In the Enterprise episode "Rogue Planet", Enterprise happens upon a rogue planet with an Earth-like atmosphere.

From the 2nd season of Mainframe Entertainment's War Planets cartoon onward, the titular planets were forced to become rogue planets in order to escape being consumed by the Beast Planet, which they achieved with colossal "World Engine" propulsion systems created by a lost civilization.

The planet Zonama Sekot in the Star Wars fictional universe was first introduced in the novel Rogue Planet and later expanded on in the New Jedi Order series.

In the film Melancholia by Lars von Trier, a fictional rogue planet called Melancholia first approaches, then collides with the Earth.

In the online game Cartoon Network Universe: FusionFall, Earth is under attack by Fuse, the ruler of a possibly Saturn-sized rogue planet named Planet Fusion.

The video game Metroid Prime 2: Echoes (2004) takes place on a rogue planet named Aether.

The Starshield novels feature a legendary rogue world. The Mass Effect universe's codex entries identify numerous planets as "captured" rogue planets that have since begun orbiting stars.

In the Dalek Chronicles, comic strip stories featuring the Daleks, cyborg aliens from the long-running British science-fiction series Doctor Who, a rogue planet comes close to destroying the Dalek planet Skaro. However the Dalek Emperor is able to redirect the planet toward Mechanus, planet of the robotic Mechanoids, enemies of the Daleks. The planet is finally destroyed by Dalek weapons due to a robot sent to prevent a war between the Daleks and Mechanoids.

See also


  • 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 ( 5 June 2006 11:35 am ET
  • IAU) 2003
  • International Astronomical Union) 2006
  • original radio broadcast
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.