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As far as I know, a brown dwarf is a 'star' whose core never underwent a fusion reaction, so it never became a star.

So I was wondering if, apart from orbiting a star, is there any difference between a massive planet and a brown dwarf?

I heard that they found an exoplanet 5 times the mass of Jupiter, and say a brown dwarf was in a binary pair where the other is a star: What stops it from being classified as a planet?

I guess what I am really asking for is a clearer definition of a brown dwarf.

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Stanley, there really isn't a very clear definition and this is still a keenly argued point.

Definitions include:

Browns dwarfs burn deuterium. In models this happens if they are more massive than about 13 times Jupiter. The weakness of this that we think isolated brown dwarfs could condense from a gas cloud that are less massive than this; and young brown dwarfs won't have got around to fusing deuterium.

Planets must form from the disk around a star. This is ok, but: brown dwarfs may also form from the disk and it is also possible for planets to be tidally stripped from their stars and be found alone in space.

Planets must have a rocky core. This used to be thought definite, but now we think maybe sometimes planets can collapse from a gas instability in the disk in some circumstances, without the need for a rocky/icy core. It is true that brown dwarfs should not have a rocky core. However as an observational definition this is fairly hopeless since we can't even tell yet if Jupiter has a rocky core.

A flavour of the controversy can be gleaned from reading between the lines of the IAU statement on the definition of planets vs brown dwarfs.

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  • $\begingroup$ Astronomic nomenclature is becoming a mess of overlapping and unrelated properties like orbit, size, composition, origin. Instead of attempting to give exact definitions to legacy words, maybe one should talk about the individual characteristics of interest at the moment. Like "A deuterium fusing object", or instead of "comet" say "A halo forming asteroid". No one knows what is meant with words like planet, star or comet anyway. Let the legacy terminology float free from strict definitions. $\endgroup$
    – LocalFluff
    Commented Mar 13, 2015 at 11:38
  • $\begingroup$ @LocalFluff I agree with the sentiment, but it isn't as simple as that. If Jupiter had been 13 times its present size, but formed in the same way, it would have fused its deuterium. Fine you might say. But others want the definition to say something about the origin and formation of the objects. The debate will continue... Personally, I would rather reserve planet, for an object with a rocky core that formed around a star. $\endgroup$
    – ProfRob
    Commented Mar 13, 2015 at 12:11
  • $\begingroup$ In some contexts, an icy atmospheric moon might have more in common with a terrestrial planet, regardless of what it orbits. "Planet" made sense in the ancient times because they were all wandering dots with much in common. Now having Mercury and Jupiter in the same category is hardly a very relevant definition for many purposes. Not that I think it is a real problem, astronomers probably know what they are talking about anyway. (Which means even less reason to make up precise definitions of those words according to the current knowledge about different celestial objects.) $\endgroup$
    – LocalFluff
    Commented Mar 13, 2015 at 13:02

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