Super-Earths: The next step in planet finding
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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.
