Planets in all the wrong places
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But could a small, dark brown dwarf actually support a viable planetary system? A few months ago, the idea seemed absurd. Then Spitzer found dust disks, very similar to the ones around honest-to-goodness stars, around two brown dwarfs. The first one, OTS 44, is about fifteen times the mass of Jupiter and the other one, Cha 110913-773444, in only four times as massive as our largest planet. It's almost like a planet with its own solar system. Surely such a system could never support life, right? Don't we all depend on our sun for light and warmth? I think it would be unwise to right off these tiny planetary systems just yet. After all, we have the same sort of thing going on with our own giant planets. Both Jupiter and Saturn have moons that really qualify as worlds unto themselves: Io with its active volcanoes, Europa with its likely warm ocean under a protective layer of ice, and especially Titan with its thick atmosphere and liquid methane rain. Many of these moons are kept warm not by sunlight or even infrared light coming from their parent planets, but by gravity. Io and Europa both orbit closely tucked-up next to Jupiter. As they spin around the giant planet, their interiors are heated by friction caused by the colossal tides created by Jupiter's gravity. You don't need sunlight if you have a close enough orbit to keep yourself warmed up like an over-stretched rubber band with Jupiter's tides squeezing and pulling you every go-round. Maybe planets in these brown-dwarf systems are similar.
Spitzer is also finding planetary disks around the most massive stars in the universe, something nobody expected. Far away in the Large Magellanic Cloud, a small galaxy that orbits our Milky Way, astronomers have spied hugely extended planetary disks around two hypergiant stars, R66 and R126. As the term implies, hypergiant stars are really, really big. Each star would comfortably swallow up the Earth, were it in our solar system, and the stars have 30 and 70 times the mass of our sun, respectively. No dust particles could survive anywhere near these monsters; the intense heat and strong stellar particle winds would tear everything apart. But surprisingly, planetary disks have formed around the hypergiants, just far enough away from their fiery hosts to hold together. And compared to our planets, that is very far out - about 60 times more distant than Pluto's orbit around the sun. Could there be viable planets around hypergiants too, orbiting just outside the destructive radiation of their stars? That's still very unlikely. R66's disk shows clear signs of clumping - material is coming together to form silicate crystals and larger dust grains. But such massive, unstable stars blow up in less than a million years. Even if a planet begins to form, chances are it won't get far before its star explodes.
From one end of the spectrum to the other, it's clear that our first observations of other planetary systems weren't what we expected. But that's all the fun, right? We still think planets form out of dusty disks left over from the formation of stars, but now we've got to figure out how planetary disks can exist in such a wide range of systems. Once again, I am amazed by how much our view of the universe has changed in such a short time. As a 1980s kid, I guess it's just about as long as Madonna's been selling records. And hey, she's still going strong.
