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

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?

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 lots of metal (in astronomy speak, that just means non hydrogen and helium atoms), like 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 non-hydrogen/helium 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.

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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 lots 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).

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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]

Update 3:00PM PST: I clarified the wording, pointing out that in astronomyspeak, “metal” just means any element heavier than helium. Also, a few other astronomers have weighed in on the matter. Konstantin Batygin from CalTech told Gizmodo: “As embarrassing as it sounds, we understand that Jupiter and Saturn grow rapidly (it could have taken Jupiter as little as 10,000 years to graduate from a 30 Earth mass planet to its current 300 Earth masses), but we don’t really understand why and how they stop growing... while this paper does not provide a full resolution to this puzzle, it attacks it from a data-science vantage point, and demonstrates that the maximum mass to which core-accretion can take you probably lies at a ~few Jupiter masses. If true, this means that Jovian-class planets and brown dwarves are hereditarily distinct classes of objects. Pretty sweet.

And Emily Rice from the American Museum of Natural History told me: “Any definition of a planet that includes formation (insert Beyonce reference here) is on the right track. From the abstract... this study combines evidence from a property of the star (heavy element content) that we expect to be correlated with their disks and thus with planet formation in those disks, as well as evidence from the distribution masses of low-mass companions we observe. These are nicely independent arguments pointing toward the same answer, which is promising.”