We Might Need To Redefine 'Planet' Again

You may think you know what a planet is, but celestial bodies often refuse to fully comply with our artificial human labels. We all thought tiny Pluto was a planet, until the 2006 vote in which the International Astronomical Union (IAU) redefinition stripped it of its title. But when is something officially too big to be called a planet?

Artist's rendition of HD 209458 b, an exoplanet 2.5 times Jupiter's mass, orbiting its host star (Image: Lucianomendez/Wikimedia Commons)

At present, there's confusion regarding whether the biggest planets really are planets, or whether they're actually ultra-light stars. The working border set by the IAU is that a star should be massive enough for heavy hydrogen, called deuterium, to undergo nuclear fusion. That's around 13 times the mass of Jupiter. But researcher Kevin Schlaufman from Johns Hopkins points out that this isn't a clear cutoff point - and sometimes objects smaller than that definition just float around on their own, not orbiting any star as a typical planet would. He suggests there's a better way to define maximum planet size, and proposes an update to the definition based on how planets form.

"Objects more massive than [around 10 times the mass of Jupiter] should not be thought of as planets," he writes in the study, which is slated for publication in The Astrophysical Journal.

Schlaufman isn't just tossing up some idea, though. He based his update on the fact that Jupiter-sized planets seem to prefer stars with a lot of metal, such as our sun. This implies that perhaps they formed from clumping in a giant disk of debris orbiting the star. He notes that bigger objects don't care about how much metal is in the disk, and form from collapsing under their own gravity. He sifted through a database of objects orbiting sun-like stars, and noticed that planets less than four times Jupiter's mass preferred metal-rich stars. The planets 10 times heavier than Jupiter (and larger) didn't seem to care about the amount of metal.

"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.

Schlaufman admits that he only uses certain sun-like stars for his analysis, with planets we've detected based on how they eclipse those stars. Our universe obviously has countless more stars and planets than that, but Schlaufman points out that his analysis doesn't strongly depend on the host star's mass or where the planet forms.

What do other people think? "Schlaufman has presented some evidence that planets larger than four Jupiter masses may be forming via a different mechanism to lower-mass planets," David Kipping from Columbia University told Gizmodo in an email. "Whether that constitutes re-classifying planets is a different question, as of course there has been an enormous amount of previous discussion regarding along which axes one draws a line in the sand to distinguish planets from non-planets."

And of course, redefining the word "planet" is an ordeal. Pluto's death came with a lot of hullabaloo and a vote at the IAU's general assembly (the next meeting is in August of this year), although plenty of people still have issues with the present definition, too (just read the Wikipedia page).

Ultimately, the universe doesn't care whether these obese celestial sporks are really spoons, forks or both - that's our problem. But learning more about space is always exciting. Kipping told Gizmodo: "I think the simple question as to whether they form differently is more interesting and meaningful to ask than 'should we call this rose a rose?'"

[ApJ]

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