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Planets in all the wrong places

At my age, I really should have expected this to happen. All of a sudden I'm seeing lots of little clues that the 1980s are making something of a nostalgic comeback. High school kids I speak to as part of my job have started wearing thin ties and studded belts, and I thoroughly approve of their newly spiked and teased hairstyles. The other day I saw a pair of plastic sandals (remember Jellies?) in a store window and heard Bon Jovi playing on a "classic rock" station. That's right; I'm a golden oldie.

Take, for instance, the fact that when I was in graduate school, a mere 10 years ago, we had no knowledge of planets outside our own solar system. Since we had only one example of a planetary system (our own), we studied its patterns and characteristics and tried to explain them with our best theories of planet formation. It made sense that all the planets close to the sun were small and rocky. After all, the sun puts off so much heat and solar wind pressure that it must have blown all the light material around it farther out into the solar system. That's where you find the giant planets, after all, like Jupiter and Saturn. More volatile substances, like hydrogen, water, or methane, needed the cooler, calmer conditions in the outer solar system to condense. More condensing material meant bigger planets, and - hey! That must be why our outer planets are much bigger than the Earth. It all made sense.

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But then, of course, we had to go and look for other planetary systems. Right from the very first extra-solar planet found, we knew our nice, neat model of planet formation was in trouble. The first planetary system we found outside our own was completely different from our solar system. A planet many times as massive as Jupiter was racing around its parent star in an incredibly close orbit, even closer than our planet Mercury's orbit of the sun. The intense energy from the star would have heated the atmosphere of the planet to well over 1,000 degrees, easily hot enough to cause gases to boil off the planet into space. What was a giant planet doing so close to a star, and how could it possibly manage to survive there?

At first, there seemed to be a comfortable way to explain away this aberration: The planet hadn't formed that close to the star, but fell in later as a consequence of an unstable orbit. Some of the next planets found backed this up; a few gas giants were in highly elongated, almost comet-like orbits around their stars. They must have formed in the outer reaches of their system, but were thrown in, perhaps by a gravitational interaction with another giant planet, toward the scorching embrace of the star. Also, it's important to remember that these first planets were detected indirectly, by watching their stars wobble under the influence of giant planets orbiting around them. The larger the planet, and the closer it was to the star, the bigger the wobble. Of course we had detected a rare and strange planetary system first - that kind was the easiest to see. As our instruments and techniques improved, we were sure to find more systems like our own, with small planets close to their star and massive planets farther out.

But now, after a good several years of planet-hunting and close to 200 extrasolar planets found, it's time we took a hard look at our assumptions about how planets form. Yes, we have found systems with massive planets in orbits similar to our outer planets, and for the time being, our telescopes are still not good enough to detect the wobble caused by relatively puny Earth-like planets tugging on their stars. But we are also finding planetary systems in places we never thought they would exist, and it's looking like our ideas about planet formation will need some revamping.

One of the things that has changed in the last few years is our ability to directly detect not necessarily individual planets, but entire planetary systems. Planets are very small compared to stars, and to make matters worse, they don't give off any visible light of their own. Any light observed from a distant planet would only be the dim reflection of starlight off its surface. For the time being, this is much too faint for our telescopes to see. But what if you had a telescope that could look for planets' warmth instead of their light? Planets absorb light from their stars and warm up in the process. As I said before, in some cases, these planets are boiling hot! NASA's Spitzer Space Telescope sees the universe entirely in infrared light, which we commonly think of as heat radiation. It's sort of a giant night-vision camera in the sky. The reason the military, firefighters, and the police are so fond of these cameras is that they allow them to image the heat coming from an object - whether or not there is any visible light. Now it's possible to catch images of distant vehicles, people hiding in the dark, or even someone passed out in a dark, smoke-filled room with no trouble at all.

Spitzer can do the same thing for objects deep in space. Everything, from planets in our own solar system to the most distant galaxies in the universe gives off infrared radiation that Spitzer can pick up. The trick with planetary systems is that Spitzer does not have the raw imaging power to see individual planets. What it can see are warm disks of dust and gas around stars that are either forming planets now, or are the result of collisions between existing planets. Even our solar system has a thin disk of dust floating around between the planets called the zodiacal dust cloud, which can be observed as a very faint glow around the path of the sun across the sky. Not only can Spitzer see similar disks in other systems, but it can also tell where dust is missing in the disk; that is, where a planet has swept out a comparatively empty ring as it orbits the star.

Spitzer has confirmed that there are planetary systems much like ours around hundreds of stars, but it has also startled astronomers by finding some systems that were completely unpredicted. One of the less dramatic cases is of a star much like our sun called HD69830 (I know; don't get me started about how lame our star-naming protocols are.) This star is one of our close neighbors at only 41 light-years away and like our solar system, it as an asteroid belt in approximately the same region we do. Our asteroid belt is between the planets Mars and Jupiter, and probably formed early in the history of the solar system when material got trapped between the opposing gravitational pulls of Jupiter and the sun, keeping a true planet from coalescing. HD69830's asteroid belt, however, is closer to its star (around where the orbit of Venus would be around our sun), and it contains 25 times as much dusty, chunky material as our belt. It's kind of neat to try to imagine what sunset would be like on a world in that system. As the sky darkened, one of the first things you'd see would be a band of light across the sky, 1,000 times as bright as our own zodiacal dust, well outshining the Milky Way.

Some other recent discoveries have astronomers scratching their heads about what it really means to call something a star versus a planet. Our sun shines because of nuclear explosions going on deep inside its core. The explosions started billions of years ago when the sun accumulated enough mass (and therefore gravity) to drive its internal temperature up to fusion reactor levels. But for a while now, astronomers have known about a class of objects called brown dwarfs, which in some way can be thought of as stillborn stars. These objects never managed to build up enough mass to get the fusion process going. A typical brown dwarf is physically about the size of Jupiter, and usually has between 10 and 50 times Jupiter's mass. Spitzer has no trouble picking these things out; although they don't generate visible light, they're still plenty bright in the infrared due to heat leftover from their formation. You can even think of Jupiter as a mini-brown dwarf. It has almost the exact chemical composition of a star, and it also generates more heat inside its core than it gets from the sun.

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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.

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