Proxima Centauri meets all the requirements for Planet status, although it is a star, can an object be a star and a planet at the same time? normally, the answer should be no, but this is the problem you run into when you define words like Planet.. What are the criteria that exclude a star from being classified as a planet?
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5$\begingroup$ I'm pretty sure fusing hydrogen into helium is not an attribute of a planet. See en.wikipedia.org/wiki/Star#Characteristics $\endgroup$– Greg BurghardtAug 4 at 23:11
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3$\begingroup$ What are 'all the criteria' it fulfills to be both? What is the source of that claim which we should 'listen to'? $\endgroup$– planetmakerAug 4 at 23:25
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2 Answers
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Per the IAU definition:
A "planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
Proxima Centauri is not in orbit around the sun, hence it is not a planet.
The IAU has a definition for an exoplanet too:
Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the L4/L5 instability $(M/M_{\rm central} < 2/(25+ \sqrt{621})\approx 1/25 )$ are "planets" (no matter how they formed).
The minimal mass/ size required for an extrasolar object to be considered should be the same as that used in our Solar System.
Proxima, with a mass of 130 Jupiters is well above the upper mass limit for an exoplanet, and hence not an exoplanet. It has stable hydrogen fusion in its core and hence is a star.
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$\begingroup$ The IAU has defined planets for other systems: astronomy.stackexchange.com/questions/43305/… $\endgroup$– snoAug 5 at 19:14
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No, Proxima Centauri is a system, specifically one of the suns in it. Proxima Centauri B, for example, is a planet. It's also actually one of the exoplanets found (in 2016) that is in the "habitable zone" and could possible host life!
