New Proposed Definition of Planets Says They Can Only Get So Big

A dividing line between mega-planets and failed stars.

brown dwarf

The question of "what is a planet?" has always been a thorny one, and even more so since the Pluto demotion fiasco of 2006. Now there's a new proposed definition for planethood, but it focuses not on the small end where objects like Pluto and its counterparts live. It's concerned with the question of how big something can get before it doesn't count as a planet anymore.

In his new paper "Evidence of an Upper Bound on the Masses of Planets and Its Implications for Giant Planet Formation," Kevin Schlaufman sets planetary boundaries at four and 10 times the mass of Jupiter, which itself has a mass equivalent to roughly 317 Earths.

Schlaufman focuses on the differences between gas giants like Jupiter and brown dwarfs. Sometimes referred to as "failed stars," brown dwarfs are relatively new discoveries in the world of astronomy, having been given their name in 1975 by Jill Tarter and only officially detected in the 1995. Brown dwarfs don't have enough mass to properly ignite nuclear fusion in their cores like proper stars. They're too cool to radiate heat. Hell, they aren't really even brown—colors range from deep red to magenta.

What's the difference between a giant planet and a brown dwarf? For Schalufman, the difference comes in their origin stories. Brown dwarf origins are somewhat mysterious, but scientists think that when stars are in their earliest stages, they eject clumps of material that form brown dwarfs. Unlike planets, brown dwarfs don't need a star to keep them in rotation.

Jupiter's origins are radically different than a brown dwarf's. Planet like Jupiter are formed from the bottom-up, Shlaufman says, building a rocky core that is eventually enveloped in a gaseous cloud. Origins like that, Schlaufman says, are only possible when you hit a certain size.

“While we think we know how planets form in a big picture sense, there’s still a lot of detail we need to fill in,” he says in a press release. “An upper boundary on the masses of planets is one of the most prominent details that was missing.”

With an upper limit on how big a planet can be, we'll be able to better prioritize the search through the solar system for interesting bodies, including those that just might hold life.

Source: John Hopkins

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