Planemos and Oph1622


What do you call a planet that’s not a planet?

The IAU recently realized that new discoveries are calling into question exactly how planethood is defined. It seems the old informal definition — a non-fusor in orbit around a hydrogen fusor — doesn’t work for planet-sized bodies that drift freely through space or bodies that don’t orbit stars (hydrogen fusors) at all. What do you call a planet-like body that just wanders through space, or orbits something that isn’t a star?

That latter scenario is exactly the case with newly discovered Oph 162225-240515 (Oph1622 for short). Oph1622 is two bodies — a planetlike giant body as large as seven Jupiters, and a brown dwarf that’s the size of fourteen Jupiters. And the two orbit each other, not a star.

Brown dwarfs are sub-stellar — they’re not stars. But I assume this one is a fusor because its 14 Jupiter-masses would allow it to fuse deuterium and lithium. But it’s not a hydrogen fusor, not in the main sequence. So Oph1622 is a non-star orbiting a non-star.

What do we call bodies like these? Neither is a non-fusing body that orbits a hydrogen fusor. Clearly, there are sub-stellar, planetlike bodies out there that just don’t behave like our familiar local planets. What do we call them?

In articles published today about Oph1622, the BBC and the NYT are calling these bodies planemos, a term recently coined by Gibor Basri, Professor of Astronomy at UC Berkeley, to apply to planetlike bodies that don’t cleave to the traditional (albeit informal) definition of a planet. Planemo is a contraction of planetary mass object.

Read more about planemos and Oph1622 below:

From the BBC — Strange “Twin” New Worlds Found

From the NYT (requires registration *), an article in which the poor Oph1622 brown dwarf is called both “a star” and “a failed star” – A Planet? Maybe It’s a Star. (An obvious first step in improving this article would be to retitle it A Planet? Maybe It’s a Planemo.)

Dr. Basri’s own article, Defining “Planet”.

There’s more on the IAU (the body in charge of naming astronomical objects) and the controversy over what makes a planet a planet here.

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