Extrasolar moon

Extrasolar moon

An extrasolar moon, or exomoon, is a term that refers to a smaller, natural (non-man-made) satellite that orbits an extrasolar planet or other extrasolar body larger than itself.

Definition of satellites around brown dwarfs

While traditional usage implies that moons orbit a planet, the discovery of planet-sized satellites around brown dwarfs blurs the distinction between planets and moons, due to the low mass of such failed stars. To resolve this confusion, the International Astronomical Union declared, "Objects with true masses below the limiting mass for thermonuclear fusion of deuterium," (currently calculated to be 13 Jupiter masses for objects of solar metallicity) "that orbit stars or stellar remnants, are planets" (no matter how they formed).

Theory and search

There are no known extrasolar moons, but their existence is theorized around many exoplanets. Future space telescopes such as Kepler, Darwin and New Worlds Mission will be able to search for moons orbiting extrasolar planets. Methods of doing this include imaging extrasolar planets directly and using the spectrograph to look for a wobble in their movement (just as wobbling in parent stars caused by planets); looking for the transit of the parent planet by its moon; or possibly imaging the moon directly.


Since no extrasolar moons have yet been discovered, their properties remain unknown. They are likely to vary, as do the properties of moons in our solar system. Some might be capable of supporting life.


A system of nomenclature for extrasolar moons has not yet been decided by the International Astronomical Union, since no such moons have yet been discovered. Such a system would probablyFact|date=August 2008 use Roman numerals after the planet designations, with the numeral increasing according to order of discovery or of the moon's distance to its parent planets. For example, the moons discovered around 51 Pegasi b would be designated as "51 Pegasi b I", "51 Pegasi b II", and so on.Fact|date=August 2008

See also

* Extrasolar planet
* Natural satellite

External links

* [http://www.space.com/scienceastronomy/shadow_moons_021008-1.html Shadow Moons: The Unknown Sub-Worlds that Might Harbor Life]
* [http://www.space.com/scienceastronomy/planet_photo_040910.html Likely First Photo of Planet Beyond the Solar System]
* [http://www.dtm.ciw.edu/boss/definition.html Working Group on Extrasolar Planets - Defintion of a "Planet"] Position statement on the definition of a planet. (IAU)

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