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Monday, June 19

A new word for your dictionary ?

Maybe it's time to add a new word to your dictionary of astronomical terms. The works is 'planemo', short for planetary mass object. With all the confusion about what is and isn't a planet, this term is being used to describe something that is big enough to be a 'planet' regardless of it's other classifications, and too small to be a brown dwarf [1] or a star.

A traditional planet, like Earth or Neptune, would be one type of planemo. Others would be; planet sized objects which don't orbit a star, objects which are relatively large but orbit other planets (Jupiter's Galilean moons or Titan for example), or large bodies which share an orbit such as asteroids like Ceres.

We now know planetary systems around stars are quite common. They form from the disk of gas and dust orbiting a star once it has condensed out of a vast cloud of interstellar gas and dust. Recent evidence suggests that planetary systems could form around planemos (interstellar planets) as well as stars or brown dwarfs. This suggests that planetary mass objects could condense from interstellar clouds, independently of stars, and in turn have their own planetary systems orbiting them. If that's that case would those orbiting bodies be called 'planets'?

That's an added dimension to the already complex "what is the definition of a planet ?" debate. It's a topic worthy of a talk / debate at a future CAS meeting. It is anticipated that the International Astronomical Union will provide an update definition of planets at their September 2006 meeting.


[1] A brown dwarf is defined as a object which is massive enough that deuterium burning nuclear reactions start inside when it forms. These reactions are short lived, and the brown dwarf is not massive enough to generate enough heat to start hydrogen burning reaction, and become a fully fledged star. Brown dwarfs are 'failed stars'. Currently the lower mass limit for brown dwarfs is thought to be about 13 Jupiter masses.

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