Planetary mass object

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. This "bizarre class of planet-sized objects has no suns at all, and instead floats untethered through space." 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 2007, it appeared in only four papers in the astro-ph archive.

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 does, 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.

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

Neptune Triton Orcus Pluto Charon Ixion Varuna (55636) 2002 TX300 Haumea Quaoar Makemake (55565) 2002 AW197 Eris Sedna

The list appears in order of increasing average distance from the Sun, with planets and dwarf planets in bold. 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."

Recent discoveries

Cha 110913-773444 was discovered by the Spitzer Space Telescope. It is 8 times more massive 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.

The first Planemos discovered outside our Solar System were those orbiting PSR 1257+12, discovered in 1992 by Aleksander Wolszczan and Dale Frail ; as pulsar planets, they surprised many astronomers who expected to find planets only around main sequence stars.

External links

• Gibor Basri, Michael E. Brown, Planetesimals to Brown Dwarfs: What is a Planet?, 2006 Preprint
• Defining "Planet" by Gibor Basri
• BBC News: Strange 'twin' new worlds found
• ScienceDaily: New Study Suggests 'Planemos' May Spawn Planets And Moons June 6, 2006 (University of Toronto)

See also

• fusor (astronomy)
• mesoplanet
• protoplanet
• planetesimal