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I just learned about Brown Dwarfs, they are "failed" stars, they narrowly missed the stellar mass mark. I learned that Y Dwarfs have temperature as low as 80 Fahrenheit (The first one found by WISE observatory), why aren't those not called a Jupiter-like planets without a host star? Why do we call them a star and designate a separate category for them (T or Y)?

Ok suppose T and Y dwarfs are okay, let's leave them alone. But why can't Jupiter be a Y Dwarf who is in the binary relationship with the Sun? Sun-Jupiter's Barycenter is just outside the Sun, we could call them in orbit with each other correct?

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    $\begingroup$ I guess it has to do with the mass of the second object (Jupiter in this case). Otherwise Jupiter behaves like a brown dwarf; it even has a moon larger than Mercury (Ganymede). Just the fact Jupiter is not heavy enough is a reason to classify it as a planet. Oh and besides, people would go nuts if Jupiter is not classified as a planet anymore; just look at what happened to Pluto. Anyway, this is all based on my thoughts, no actual sources. $\endgroup$ – Joeytje50 Jun 5 '16 at 19:30
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    $\begingroup$ Jupiter does not behave as if was a brown dwarf. It's spectral lines clearly contain deuterium. Brown dwarfs (as opposed to sub brown dwarfs) are severely depleted in deuterium. $\endgroup$ – David Hammen Jun 6 '16 at 4:03
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But why can't Jupiter be a Y Dwarf who is in the binary relationship with the Sun?

There are two reasons: One is that Jupiter is too small to have ever undergone fusion of any sort. To qualify as a brown dwarf, an object needs to be large enough to have undergone deuterium fusion in its core. This requires a mass of at least 13 Jupiter masses. The other is that Jupiter formed by planetary formation mechanisms rather than by gravitational collapse.

There is debate on what to call objects of less than 13 Jupiter masses that formed by gravitational collapse. Are these sub brown dwarfs or free-floating gas giants, and does it even matter? There is also debate on what to call objects above 13 Jupiter masses that formed by planetary formation mechanisms as opposed to by gravitational collapse. Are these brown dwarfs or hyper planets, and once again, does it even matter?

In any case, Jupiter is not a brown dwarf.

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    $\begingroup$ What's the distinction between "planetary formation mechanisms" and "gravitational collapse"? $\endgroup$ – Russell Borogove Jun 6 '16 at 4:36
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    $\begingroup$ @RussellBorogove: Good question. Why don't you ask it as a question, instead of in a comment? $\endgroup$ – DevSolar Jun 6 '16 at 8:02
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    $\begingroup$ Brown Dwarf don't fuse anything do they? I am sorry, I have not read anybody other than Phil Plait and Bob Berman. So my knowledge isn't that proving I assume. $\endgroup$ – fahadash Jun 6 '16 at 13:29
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    $\begingroup$ @fahadash They did fuse - their mass and composition is just good enough for deuterium fusion. Of course, there's not a lot of deuterium available (compared to simple hydrogen), so they "burn out" rather quickly, but it does mean that there's a very much measurable depletion of deuterium in their spectrum. If it's not fusing and it has a significant fraction (in interstellar-medium terms) of deuterium, it's no brown dwarf. $\endgroup$ – Luaan Jun 6 '16 at 13:50
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    $\begingroup$ To add to Luann's answer, large Brown Dwarf stars also fuse lithium, but there isn't enough of that, either, to make them main sequence stars. $\endgroup$ – AlaskaRon Jun 7 '16 at 3:30

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