Saturday, April 16, 2011

Finding Another Earth

 This post will also be found in Tehachapi's The Loop Newspaper

Astronomers have made a great deal of progress in finding extrasolar planets. These are planets that orbit stars other than our Sun. The first (starting back in the 90s) were odd planets, most “hot Jupiters”. Very large planets orbiting close to their star. These included huge planets with years (the time it takes to make an orbit around the star) of mere days.

One of the reasons these planets were discovered first was due to the methods used to detect the planets. The Law of Gravity says that a planet pulls a little bit on its star. Therefore they could use wobbles in the star to see if a planet was there. But this worked best for planets large enough to make big wobbles.
But given the fact that compared to the Sun the Earth is tiny and doesn't give it much of a wobble, this method wasn't going to give us much chance to find another Earth. So other techniques have been developed. 

The Kepler spacecraft (launched in 2009) is currently looking for smaller planets by looking for flickers in the light of a star when a planet passes between it and the Earth. This is known as transiting. Already the Kepler spacecraft has found over a thousand candidates for extrasolar planets. (These have to be confirmed using other methods.) And Kepler is looking in only one direction, for just a small patch of the sky.

And of all of those planets that it appears to have found, several (a few dozen so far) appear to be in the star's habitable zone. A star's habitable zone varies from star to star, but is based on where, given the star's size and temperature, we would expect to find liquid water like on Earth. So for a larger star the habitable zone would be farther out, while for a smaller star the habitable zone would be closer in.

And much of the current search appears to be looking at small stars. Red dwarf stars are very common. They are so much cooler than our own Sun that for a planet in the habitable zone a year could be measured in a couple weeks. So we should be able to see many flickers caused by the planet in a short amount of time. And one candidate is already known from the nearby (around 20 light years) Gliese 581. Though the hypothesized Earth size planet (Gliese 581g) is unconfirmed, there is a confirmed “super-Earth” (a rocky planet several times larger than the Earth) circling that star at the edge of its habitable zone.

Another advantage of looking at these red dwarfs is that the light from these stars is not so over-powering. So we actually have a chance to see the atmosphere of one of these worlds. And if we can see that, we can look for the tell-tale signs of life. We can check the atmosphere for gases associated with life. If we see oxygen, then we could be confident that it was produced by life. (There are several other chemicals that are unlikely to form without life.) 

But even when we find a life bearing world around a red dwarf, it won't be too much like Earth. To be close enough to its star to be in the habitable zone, the planet would end up tidally-locked. That is, only one side of the planet would face the star. So one side would be hot and the other side would be cold since it never sees the star.

Axial tilts could generate weather that would mix the atmosphere, evening things out some, but still, it wouldn't be too much like Earth. So even when we find such a world, in the words of Dorothy, “There's no place like home.”

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