Planemo

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A planemo is a celestial object with mass greater than that of an irregularly shaped asteroid, yet smaller than a nuclear reactive brown dwarf or star. The term covers all bodies within this size range, although most planemos that orbit stars are more regularly referred to with the more specific term, planet (see also dwarf planet). Planemo is a contraction of planetary mass object. The term has yet to achieve common usage in the scientific community: as of October 2006, it appeared in only four papers in the astro-ph archive.

1 Origin of the term
2 Within our solar system
3 Recent discoveries
4 External links
5 See also

[edit] Origin of the term

The description "planemo" was first proposed in 2003 to the International Astronomical Union (IAU) by Gibor Basri, Professor of Astronomy at the University of California, Berkeley, to help clarify the nomenclature of celestial bodies. At the time, the world of astronomy was undergoing a debate (concluded only in 2006) as to what, and what does not, constitute a planet. Under Basri's definition a planemo would be "an object [rounded by self-gravity] that does not achieve core fusion during its lifetime", regardless of its orbit. It is deliberately contrasted with Basri's suggested definition of planet, ("a planemo that orbits a fusor") and was thus intended as a solution to the debate.

[edit] Within our solar system

If applied to our own solar system the list of planemos would include some or all of the following:

Mercury
Venus
Earth
- Moon
Mars
Hygiea
Vesta
Ceres
Pallas
Jupiter
- Io
- Europa
- Ganymede
- Callisto

Saturn
- Mimas
- Enceladus
- Tethys
- Dione
- Rhea
- Titan
- Iapetus
Uranus
- Miranda
- Ariel
- Umbriel
- Titania
- Oberon

The list appears in order of increasing average distance from the Sun. It should be noted that Pluto and Charon would qualify as a double planet system, since they share the same orbit. The numbered planemos could possibly be counted as planets if Basri's definition was used, with the bold ones definitely counting. However many more objects in the distant solar system could be found that would qualify under the definition, with some astronomers predicting hundreds to be discovered.

As "round" is a relative term that would need to be precisely quantified, an eventual list could vary from this. For example, 2003 EL₆₁ is more elliptical than spherical. Basri notes 'roundness' requires "enough mass to allow their self-gravity to overcome any material forces that might produce asymmetric shapes" and that "technically roundness means conformity to the equipotential surface." The IAU's view means an object would qualify if it "has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape."

[edit] Recent discoveries

Cha 110913-77344 was discovered by the Spitzer Space Telescope. It is a planemo 8 times heavier than Jupiter, and an estimated 2 million years old. It is encircled by a disk of dust. It is 500 light-years away from Earth.

In August 2006, a set of 'twin' planemos was discovered roughly 400 light years away, designated Oph 162225-240515, or Oph 1622. Oph 1622 was discovered using the European Southern Observatory New Technology Telescope at La Silla, Chile. This was a memorable discovery due to the nature of the pair, which were the first planemos to be discovered that were not part of a solar system.