Brown Dwarfs (and Giant Planets)
Published on Sep 12, 2006 at 5:04 pm.
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Filed under extrasolar planets, planets, stars.
Some years ago, when I was in graduate school, I was asked to write a column for the university’s astronomy club’s monthly newsletter. It was the “Astroquestion of the Month” column. Anyway, the last question that I posed before leaving was, “What is a brown dwarf?” At the time, no one had yet positively identified a brown dwarf, so it was a purely theoretical discussion.  Now, we have identified brown dwarfs. So, now I pose the question again, what is a brown dwarf?
This is a topic that comes up in stellar astronomy, so we should begin with stars. A star is a large ball of gas, mostly hydrogen and helium, that is undergoing nuclear fusion at its core. That is a simplistic definition, and not entirely right (or complete), but it gets the basic idea across. Jupiter is also a big ball of mostly hydrogen and helium, but we consider it a planet. The difference is what goes on in its core.  When a star forms, it forms from a cloud of gas and dust (mostly hydrogen and helium) that starts to collapse under its own gravitational field. Near the middle of such a cloud, the gasses swirl together to form a big ball of hot gas that we call a protostar. This protostar then accretes more material. As the mass of the protostar increases, gravity causes it to compress. Compressing the interior of the protostar also heats it. Eventually it gets dense enough and hot enough to begin to fuse hydrogen into helium. This fusion process releases energy, which keeps the star from further compressing through a hydrostatic equilibrium. In fact, the energy production is well regulated, and the star remains mostly stable until it begins to run low on hydrogen in its core. A star like the Sun can keep this up for about 10 billion years before it runs into problems. But, Jupiter does not sustain fusion in its core. Why not? Well, quite simply, Jupiter did not form with enough mass to get the job done. Astronomers used to refer to Jupiter as a “failed star” because it has a composition similar to a star, but is far too small to be stellar in nature.
Well, this is all fine and dandy. But, not all stars form with the same mass. Some are much more massive than the Sun, and some are much less massive. I’ll save for another entry discussion of the biggest that a star can be (about the most massive star observed is 150 times the mass of the Sun, though theory seems to permit slightly larger stars). But, what is the smallest star? If a star is too small (below about 0.08 the mass of the Sun), then it can’t sustain fusion. Jupiter is far too small. But what properties would something just a little too small have? Would such a body be more planetlike or more starlike?
Theorists thinking about such things decided that a body with just a tiny bit too little mass would have more starlike properties than planetlike properties. What are the differences? Well, Jupiter is differentiated, that is the heavy stuff has settled to its center. It is believed to have an iron and rock core several times the size of the Earth in its core.   However, convection in a star keeps things stirred up, and the heat keeps a solid body from forming. So, presumably these sub-stellar bodies would have convection, a non-solid core, and would form just like stars. Planets form in a disk around a star. Stars form from their own disk of material collapsing into a protostar. Some of these failed stars would be companions to bigger stars, and some were thought to possibly form as isolated bodies with their own planetary systems. But, it is pretty awkward to keep calling these things “failed stars” or “sub-stellar bodies” or any such thing. So, the astronomer Jill Tarter is credited with coining the term brown dwarf to refer to one of these things. Eventually, we began to find brown dwarfs, with the first confirmed brown dwarf being Gliese 229B (picture above).
According to our theories of stellar structure and nuclear fusion, you need a body of about 0.08 solar masses to initiate hydrogen fusion. Actually, this number is not hard and fast, as a slightly higher mass of 0.09 solar masses may be needed for stars that are composed of practically only hydrogen and helium, and a slightly lower value of 0.07 solar masses might get the job done with stars that have about 1% other elements. The composition that stars have when they form depends upon where and when they form. Stars in the distant past had fewer other things besides hydrogen and helium in them (a tale for another blog entry).
At any rate, a mass of 0.08 solar masses is about 84 times the mass of Jupiter. So, calling Jupiter a “failed star” as has been done in many textbooks is like saying that a brick is a “failed airplane.” But, there are objects that are 3 times Jupiter’s mass, 5 times Jupiter’s mass, 10 times Jupiter’s mass, 50 times Jupiter’s mass. Where does something quit being a planet and begin to be a brown dwarf?
Unfortunately, we don’t actually have a definition for the difference. Yep. That’s right. More planet controversy. For years there’s been talk about how big something had to be to be a planet. But, there is also talk about what is the biggest that something can be and still be a planet.
As it turns out, size isn’t the determining factor. Something with a bit more mass than Jupiter may be slightly bigger, but only slightly so. Something with a lot more mass is about the size of Jupiter. The more mass one of these things has, the more gravity, and since gas is easy to compress, the more compressed it is. Pretty much every one of these things from about 1 Jupiter mass up to about 100 Jupiter masses is about the same size! The difference is in interior structure — density, temperature, convection, and the like.
We know that stars generate energy through nuclear fusion, so brown dwarfs don’t generate any energy, right? Well, actually, they do a little. As they are compressed by gravity, they heat up. The hot gas in their core provides stabalization against further collapse (just like in a star). But, the gas radiates heat away and cools off. In a star, that heat is replentished by the nuclear fusion. In a brown dwarf, the gas simply cools and compresses a bit more under gravity. But, that compression heats it up. So, a brown dwarf will slowly and inexorably compress its core providing heat in a process called Kelvin-Helmholtz contraction. This is a dead end process, though, and it can’t keep it up forever. But, this isn’t a way to tell a brown dwarf from a planet, since Jupiter is doing the same thing! Actually, brown dwarfs, with more gravity, might get the process finished quicker, but for a while, it is the same process.
But, is there any other difference? Well, perhaps. It turns out that some elements are easier to fuse than others. Lithium, a rather rare element in the cosmos, fuses more easily, and at a lower temperature, than hydrogen. In fact, you almost never find any in stars, because if a star is hot enough to fuse hydrogen, it is certainly hot enough to fuse lithium, and there is so little lithium around that it gets used up very quickly. It is believed that any object more massive than about 65 Jupiter masses might be hot enough to begin fusing lithium, but not hot enough to fuse hydrogen. 65 Jupiter masses is clearly going to be a brown dwarf. But, even if a brown dwarf were able to fuse lithium, it isn’t a star because the lithium fusion would be too little and too slow to provide enough energy to achieve the sort of equilibrium that a star has.
So, what about below 65 Jupiter masses? Well, even easier to fuse than lithium is deuterium, and there is more of it. So, high mass brown dwarfs fuse both deuterium and lithium. In fact, it seems that you possibly only need about 13 or so Jupiter masses to initiate a little deuterium fusion. So, would that make a good cutoff size for a brown dwarf? Many astronomers feel that initiating any fusion should be among the criteria for determining whether an object is a brown dwarf as opposed to a big planet.
This has been a nice rule of thumb for a while. But, no one really fretted over Pluto, either, until we started finding things close to Pluto sized, and then the definition of planet became and issue.  
But, we also have an issue at the other end of the planet definition. A team of Penn State astronomers has announced the discovery of a body about 12 Jupiter masses. It is currently being called CHXR 73 B. This is how we normally desginate stars and brown dwarfs, but is it a giant planet or a baby brown dwarf? It isn’t really clear. It is far enough away from its parent star, CHXR 73, that it likely formed from its own accretion disk, rather than forming in the accretion disk of a star like a planet would. That sounds starlike, so perhaps that makes it more like a brown dwarf than a planet, and it might (or might not) be fusing deuterium. Its mass is not absolutely determined, but it appears to be just barely below the threshold mass to fuse deuterium, but who knows. So, we have yet another question as to the definition of a planet.  We might have to revisit this topic soon.
-Astroprof
Image Credit:Â NASA, HST
