Super-Earths: The next step in planet finding

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

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.

Going back to that planetarium talk of a few weeks ago, someone asked me if it would be possible to have solid, rocky planets with more mass than the Earth. For the moment, the Earth is the largest terrestrial planet that we know of. Is there a limit to how large rocky planets can be? Our computer models of how planets form suggest there may indeed be one.

Planets form out of giant, orbiting disks of gas and dust around new stars, basically the stuff left over from the birth of the star itself. The seeds of planets sweep up material as they orbit through the disk, and as they accumulate more mass, their gravity becomes stronger. The increased gravity attracts even more mass, which gives the young planet even more gravity, etc. etc.

All planets, both terrestrial and gas giants, probably start out this way, but at some point, the mass of a planet reaches a trigger point, and the whole process gets a little out of control. After this point, the gravity of a forming planet can pull in a huge envelope of gas in a relatively short amount of time, and you've got yourself another Jupiter or Saturn.

This limit between forming a terrestrial planet versus a gas giant is not well understood, nor is it a specific mass. It probably depends somewhat on the distance from the new planet to its star. Farther out in the disk, more gas is able to cool and condense, leaving more material for planet-forming. Because of this, a planet might reach the "point of no return" at a smaller mass than one closer in, which has hotter, sparser gas to work with. We're just not really sure yet how the whole process works. But our models do seem to show that this mass limit is somewhere around fifteen times the mass of the Earth, suspiciously close to all three of the new planets.

Watching a star wobble only tells you how massive the planet is; it doesn't give you any information about whether the planet is rocky, or gaseous, how physically large it is, or anything like that. So why do astronomers suspect that these planets might be rocky, solid bodies?

For one thing, all three planets are likely to be very hot, as they're all very close to their stars. Why might this be a clue about their composition? Simply put, astronomers are not sure that a 10-20 Earth-mass planet can hold on to a large gaseous atmosphere at those temperatures.

Although it might seem strange, our atmosphere, just like anything else on the surface of the Earth, is held down by gravity. The more massive a planet is, the more atmosphere it can hold down. But temperature is important too. It's much harder for a small-mass planet to hold on to a hot atmosphere, as the hotter gasses get, the faster they move around and bump into each other. At a high enough temperature, gases will literally "boil off" the planet, escaping into space.

Most of the planets we've found racing around the stars in tight, hot, orbits have at least the mass of Jupiter - enough to hold down a large atmosphere even at high temperature. But can a planet with only ten Earth masses hold on to a lot of hot gas? Astronomer aren't sure, and some suspect not.

If that's the case, then we may have just detected the first terrestrial-type worlds outside our own inner solar system. So what might these planets be like?

They would almost certainly be tidally locked to their stars, with one side facing the star (and getting torched), while the other side perpetually gazes out into space (and freezes). The planet Mercury is much like that, being simultaneously one of the hottest places in the Solar System and one of the coldest (interestingly enough, Mercury is not completely tidally locked to the Sun, so the hot side and the cold side switch every couple of years).

But even a harsh environment like that might have a few tricks up its sleeve; on August 3rd, 2004, NASA launched the MESSENGER spacecraft off to take a very close look at Mercury, as astronomers had detected some evidence that there might be water ice (!) on its surface. That's right; up in the polar regions, there are some craters that never see the light of day, but remain permanently in shadow, even as the rest of the planet's surface gets repeatedly scorched, then frozen. Something in those craters is highly reflective, and looks a whole lot like water ice.

Now, all three of these new planets are much close to their stars than Mercury is to our Sun, and their temperatures would be even hotter. But is it possible that there might be a tiny temperate zone between the day and night sides of these planets?

Perhaps. The best guess we have right now is that these planets are unlikely places for life, but not impossible places. And they may be the first places that it would be possible to actually set foot on outside our Solar System.