As if Pluto-gate hadn’t caused enough problems, astronomers want to reclassify planets again

When is a planet not a planet? When it forms in the orbit of a metal-less star, according to proposals made in a new study.

How we categorise large planets is the subject of a paper by astronomer Kevin Schlaufman, from John Hopkins University, who calls for a redefinition of planets based on how they form – not simply how large they are.

The International Astronomical Union (IAU) currently divvies up enormous planets from ultra-light stars by claiming the latter should be big enough for the heavy hydrogen isotope, deuterium, to undergo nuclear fusion. If you’re around 13 times the mass of Jupiter, you’re likely looking at a brown dwarf, not a planet. On the other end of the spectrum, the IAU says a planet needs to be big enough to clear smaller objects in its orbit. Pluto was deemed to fail this test, and was reclassified as a dwarf planet in 2006, although this caused a great deal of controversy and led to (as yet unsuccessful) campaigns for it to be knighted as a planet once more.

But Schlaufman, as reported by Gizmodo, wants to find a more precise way to categorise planets by looking at their formation. In a study to be published by The Astrophysical Journal, he argues Jupiter-sized planets tend to form around stars made from lots of metals (in astronomy terms, that just means elements that are heavier than hydrogen and helium), suggesting they formed from analogous debris orbiting the star. Bigger objects, on the other hand, will form regardless of how many ‘metals’ a star is made from, and are created by collapsing under their own gravity.

By looking through a database of objects that orbit stars similar to our own sun, he found that planets around 10-times larger than Jupiter tended to exist around metal-less stars.  “The maximum mass at which celestial bodies no longer preferentially orbit metal-rich solar-type dwarf stars can therefore be used to separate massive planets from brown dwarfs and establish the mass of the largest objects that can be formed through core accretion (planets),” he writes.

Drawing a line around what is and isn’t a planet can be a sore point for Pluto lovers, but is largely an issue of semantics. More interesting is the suggestion that very large planets, several times the mass of Jupiter, may be forming via different mechanisms from smaller planets. There’s still so much we don’t know about the mechanisms of larger planets, and this could be a critical way to understand how they formed and why they subsequently act the way they do.  

As for us earthbound gazers, to borrow from Gertrude Stein, a planet is a planet is a planet.

Image credit: NASA