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.

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.

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