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Super-Earths: The next step in planet finding

By Michele / September 15, 2004


When it comes to the subject of extra-solar planets, you've got to move fast.

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Case in point: whenever I give a public talk about all the new planets that have been recently found around other stars, I always check the Planet Quest website to see what the planet count is for that day. The reason is simple: it changes almost every day.

Finding new planets is becoming a popular cottage industry for several groups of astronomers, and it's hard to keep track of every new planet they find. Just for the record, as of September 8, 2004, there are 127 known planets outside of our Solar System.

But recently, three different teams of astronomers announced discoveries which were more significant than the usual, 'ho-hum' detection of a new planet. Not only are we finding smaller and smaller planets, as we get better at it, but it seems that we may have also found an entirely new kind of planet, which some astronomers are calling a "Super-Earth."

Let's start with the basics: so far, almost all of the exoplanets we know about ("exo" means "outside", indicating that these planets are outside our solar system) have been detected by watching their parent stars wobble. Whenever two objects orbit around each other, they actually both orbit around their combined center of mass. It's a little weird to think about it, but it's not entirely correct to say that the Earth orbits the Sun.

Both the Earth and the Sun orbit around their shared center of mass, which is hard to notice because the Sun is so much more massive than the Earth (about 300,000 times more massive, actually). In the case of the Earth-Sun orbital system, the center of mass is physically inside the Sun, although not at the Sun's exact center. The wobble our planet induces in the Sun is so small, that if we were living in another planetary system looking back at our Sun, with our current technology, we wouldn't be able to detect it. But we probably would be able to detect Jupiter. Jupiter is over 300 times more massive than Earth, so the Jupiter-Sun center of mass is a bit farther away from the center of the Sun. That wobble is big enough to notice, even from very far away.

Detecting a planet is even easier if you've got something the mass of Jupiter (or bigger) in a closer orbit around its star. The closer a planet is, the stronger the attraction of gravity between it and the star, and the bigger the star's wobble. That's the reason astronomers weren't too surprised when the first exoplanet we found turned out to be very massive, and very close to its star.

And since then, we've gotten pretty good at finding giant gas planets that are orbiting scorchingly close to their stars. But these worlds are so different from our own system of planets, I often feel we don't really share a true kinship with them. As fascinating as these hot, giant worlds are, we can't easily imagine these planets harboring life or being places we could set foot on someday -there're really just big blobs of super-heated gas.

In the last few years, astronomers have gotten better at detecting and accurately measuring the star-wobbles induced by planets, and soon enough they were able to find Jupiter-mass planets in Jupiter-like orbits, then Saturn-mass ones.

That's intriguing, because as I said before, that's what our own solar system would look like to outsiders with similar technology. They wouldn't be able to detect Earth, just the giant planets. Maybe some of these new systems have smaller planets too, that we just aren't able to make out yet. NASA has several future missions planned that should be able to see the wobbles created by Earth-mass planets, but for the time being, it looks like astronomers will just have to wait.

Recently, three teams of astronomers have announced discoveries of the smallest planets yet. They're still much more massive than the Earth, but for the first time, we may have detected planets that are more similar to the Earth in composition, at least compared to giant gas planets.

Two groups, from the Carnegie Institute of Washington and the University of Texas, have now found planets with approximately twenty times the mass of Earth, about the size of the planet Neptune. In the same week, a European team of planet-finders pushed the limit even farther, claiming the detection of a planet just fourteen times as massive as Earth. All these new planets are much closer to their stars than the planet Mercury is to our Sun, so chances are, these are still pretty hot places we're talking about - not a great place to search for life either. But what's really amazing about these specific planet detections is that some astronomers are beginning to suspect that for the first time, they've found terrestrial planets.

The word "terrestrial" means "Earth-like," but as I said, these planets would be completely scorched, almost certainly lacking liquid water or much of an atmosphere, for that matter. But the reason these planets are more like the Earth is that they may have solid surfaces.