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posted by martyb on Thursday January 25 2018, @11:33AM   Printer-friendly
from the is-there-a-lower-bound,-too? dept.

A new definition of a planet could help to distinguish gas giants from brown dwarfs:

[Kevin] Schlaufman's definition is based on mass. In a paper published [DOI: 10.3847/1538-4357/aa961c] [DX] January 22, 2018, in the peer-reviewed Astrophysical Journal, Schlaufman has set the upper boundary of planet mass between four and 10 times the mass of the planet Jupiter.

Schlaufman found that objects of at least 10 Jupiter masses tend not to form around metal-rich solar-type dwarf stars:

Planets like Jupiter are formed from the bottom-up by first building-up a rocky core that is subsequently enshrouded in a massive gaseous envelope. It stands to reason that they would be found near stars heavy with elements that make rocks, as those elements provide the seed material for planet formation. Not so with brown dwarfs. Brown dwarfs and stars form from the top-down as clouds of gas collapse under their own weight.

Schlaufman's idea was to find the mass at which point objects stop caring about the composition of the star they orbit. He found that objects more massive than about 10 times the mass of Jupiter do not prefer stars with lots of elements that make rocks and therefore are unlikely to form like planets. For that reason, and while it's possible that new data could change things, he has proposed that objects in excess of 10 Jupiter mass should be considered brown dwarfs, not planets.

Abstract:

Celestial bodies with a mass of M ≈ 10 MJup have been found orbiting nearby stars. It is unknown whether these objects formed like gas-giant planets through core accretion or like stars through gravitational instability. I show that objects with M ≲ 4 MJup orbit metal-rich solar-type dwarf stars, a property associated with core accretion. Objects with M ≳ 10 MJup do not share this property. This transition is coincident with a minimum in the occurrence rate of such objects, suggesting that the maximum mass of a celestial body formed through core accretion like a planet is less than 10 MJup. Consequently, objects with M ≳ 10 MJup orbiting solar-type dwarf stars likely formed through gravitational instability and should not be thought of as planets. Theoretical models of giant planet formation in scaled minimum-mass solar nebula Shakura–Sunyaev disks with standard parameters tuned to produce giant planets predict a maximum mass nearly an order of magnitude larger. To prevent newly formed giant planets from growing larger than 10 MJup, protoplanetary disks must therefore be significantly less viscous or of lower mass than typically assumed during the runaway gas accretion stage of giant planet formation. Either effect would act to slow the Type I/II migration of planetary embryos/giant planets and promote their survival. These inferences are insensitive to the host star mass, planet formation location, or characteristic disk dissipation time.

[Ed. note: I long ago stumbled upon an image showing the relative sizes of the planets against the size of the sun... and now can no longer find it, or anything like the one I remembered. Does anyone have a good link to recommend? --martyb]


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  • (Score: 2, Insightful) by Anonymous Coward on Thursday January 25 2018, @11:45AM (2 children)

    by Anonymous Coward on Thursday January 25 2018, @11:45AM (#627644)

    We see this all the time in medicine/bio. For some reason there are people who think there is some ideal classification system that corresponds to the *real* set of cell types or species or whatever. They are all human constructs that may be more or less useful for different purposes. If you find one that works for you, great; just don't act like you have discovered some facts like this article does.

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  • (Score: 1, Insightful) by Anonymous Coward on Thursday January 25 2018, @12:01PM (1 child)

    by Anonymous Coward on Thursday January 25 2018, @12:01PM (#627646)

    Many things have a continuous spectrum, but people want to put things into boxes (what classification actually is).