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Kicking Brown Dwarfs Out Of The Family Of Planets

31 Jan 2018, 12:00 UTC
Kicking Brown Dwarfs Out Of The Family Of Planets
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If you want to start an argument among astronomers, ask them if Pluto is a planet.
In 2006 the International Astronomical Union (IAU) defined a planet as an object which 1) orbits the Sun, 2) is massive enough to be in hydrostatic equilibrium (basically that means it’s round), and 3) has cleared the neighborhood around its orbit. Since Pluto doesn’t satisfy the last criteria, they declared that Pluto was no longer a planet.
Naturally this upset a great many people, but it has also been controversial among astronomers, and there have been several proposals to change the definition, some that include Pluto, and some that don’t. But the IAU definition is mainly about how small a planet can be. Pluto isn’t a planet because it is too small to clear its orbit of similar bodies. But on the large side of things there is a similar problem. Where do we draw the line between a really large planet and a really small star?
Artist’s depiction of an ultra-cool dwarf star like TRAPPIST-1 (left) with brown dwarfs of 65 and 30 Jupiter masses (center) and Jupiter (left). Credit: NASA/IPAC/R. Hurt (SSC)
The usual definition is that a star undergoes nuclear ...

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