A planemo is a celestial object of planetary mass - one that is larger than an irregularly shaped asteroid, yet smaller than a nuclear reactive star. The term covers all bodies within this size range, although a planemo that orbits a star is more regularly referred to with the more specific term, planet. Planemo is a contraction of planetary mass object.
Origin of the term
The description "planemo" was first proposed to the 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 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.
It can be considered helpful as it creates a designation for so-called "interstellar planets" that are otherwise not covered by suggested definitions for 'planet', and it also creates a category to group large, compositionally-similar moons with their planetary counterparts. Many scientists back the term as it lends itself to a universal definition of planet based on physical characteristics, rather than other definitions which create dividing lines using arbitrary size limits.
Within our solar system
If applied to our own solar system the list of planemos would probably read:
The numbered planemos would count as planets if Basri's definition for the term was used, although potentially several more could yet be discovered.
However, 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". Charon is also a disputed choice as it is controversial whether it directly orbits the Sun or Pluto.
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.
- 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)