Friday, August 27, 2004

Discovery of a "Super Earth?"

This story at Space.com talks about the discovery of a new planet around another star and calls the planet a "super-earth." The new planet is about the same mass as Uranus and Neptune or about 14 times the mass of the Earth. This makes it the smallest planet known around a normal star.

How do astronomers find planets outside our solar system? First we need to understand how planets are found around other stars. So far, no planets have been directly observed around other stars. Planets are too faint, especially since they are found very close to a bright star. Seeing a planet directly is like trying to look for lightning bugs around searchlights from several miles away.

So astronomers can only see the effect of the planets on their parent star. All planets have gravity and pull on their parent star. As the planet moves around the star, the star gets pulled in different directions. From Earth, we look for changes in the speed of the star --- the star will move toward us when the planet is between Earth and the star, and the star will move away when the planet is on the other side of its star. But these movements are tiny -- as small as 14 feet per second for this new "Super-Earth." It's impressive that we can measure such small movements from 100 trillion miles away! As massive planets have stronger gravitational pulls, they move the parent star more and are easier to find. That's why most of the planets known so far are larger than Saturn. But improved technology has allowed more accurate measurements, allowing smaller and smaller planets to be found.

As for "Super-Earth": every six days, the planet completes one orbit around the parent star. The small size of the star's wobbles tell us that this is much smaller than Jupiter. It's more like the mass of Uranus or Neptune, about fourteen times the mass of the Earth. This also makes it the smallest planet discovered around a normal star so far.

But "Super-Earth" is also interesting because we're not sure what it is made out of. The Jupiter-sized planets we have discovered around other stars are almost completely made out of gas -- there is no other obvious way to make such a big planet. But it is possible to make a planet as massive as the "Super-Earth" completely out of rock. So the "Super-Earth" may be the first rocky planet ever discovered outside of our solar system. And while Jupiter-size planets cannot have life like we know it, rocky planets could have lakes, oceans, and life.

Unfortunately, it is impossible right now to tell if it is made out of rock or not. And even if "Super-Earth" is made out of rock, it is far too close to its star for there to be any life. Its surface would be hot enough to melt most types of metal and would make a day on Mercury feel chilly.

But now that we can discover these small planets, it is only a matter of time before we find one about the right distance from a star for there to be liquid water. Then things will be really interesting!

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