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A Failed Star, or Superplanet?

Week of July 27, 1998
I get quite a bit of email from people asking me all sorts of interesting questions. It's not very often, though, that one hits on a topic on which I have actually worked as a scientific researcher. This week I got one asking about brown dwarfs. I have a snack from April 1998 that gives a brief intro to these peculiar objects, but briefly, brown dwarfs (or BDs for short) are objects that are a little less massive than "normal" stars, but bigger than any planets we currently know about. They are a new object, in that we have only just now started finding them. The question I was asked was, is Jupiter a brown dwarf? And if not, how much bigger would Jupiter have to be to be one?

Right now, the thinking in what differentiates a BD from a planet is formation mechanism. A star forms from a collapsing gas cloud. As it forms, the cloud forms a disk around it, and planets form from the disk. A BD forms like a star, from the cloud itself and not from the disk. I think that some people think that there is a minimum mass to form this way, so BDs will always be bigger than planets, or, at least, there is a minimum mass to BDs. No one knows how big planets get; there might be planets bigger than BDs!

Now the difference between a BD and "regular" star is more subtle. The thinking now is that a star eventually settles down into stably fusing hydrogen into helium in the core. A BD never gets that far. The minimum mass for that is about 0.08 or so times the mass of the Sun. What we consider a BD is probably less massive than that magic number. When very young, a BD might fuse deuterium (an isotope, or different flavor, of hydrogen) for a while. That reaction doesn't generate a whole lot of energy, and eventually shuts down anyway. After that, the BD basically just sits and cools. A young one can be hot, and when you observe it with a spectroscope it can mimic a more massive star, but time takes its toll. Eventually, the BD cools (maybe millions of years?) and shows its true colors, so to speak.

So: Jupiter is *not* a BD; it formed like a planet, in the disk around the Sun. It also has about 1/1000 the mass of the Sun, or about 1/80 of the mass it needs to fuse hydrogen.

One more point: is a BD a star? Well, yeah, I guess. Sorta. ;-) It forms like one, and for a short time acts like one. It just never really gets anything going. But then, is a white dwarf a star? A white dwarf is basically the core of a star like the Sun, after the star has aged considerably and blown off all its outer layers. A white dwarf doesn't fuse hydrogen or anything else, and just sits and cools while the Universe ages around it. Sound familiar? Yet we call white dwarfs stars, so I suppose brown dwarfs are as well.

I have always felt that the Universe knows what it's doing, and we are the ones trying to classify everything into neat little boxes. The Universe is under no obligation to make it easy for us.

©2008 Phil Plait. All Rights Reserved.

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