Crochet, knitting, astronomy & life in general.

Tuesday, January 18, 2011

Astronomy Tuesday: Elusive Habitable Planets

I know I said that I'd have lots of time to write about astronomy over the holidays, but really, I got so caught up in watching Farscape, reading webcomics and practising extreme relaxation, that I totally forgot. And then the semester started again and I got busy. Why am I writing right now? Well, my friend, it has to do with that procrastination/amount of work to do correlation.

Anyway, today I thought I'd talk about Gliese 581 g, an unconfirmed rocky planet orbiting the red dwarf star Gliese 581. The planet got everyone excited back in September since it was discovered in the habitable zone of its sun, that is, that perfect distance from the star where water can exist in liquid form. This would be the first time a planet outside our solar system has been discovered in that special region around its sun. Here is an image of the Gliese solar system compared to our own (from Wikipedia):

We shouldn't, however, start packing our bags to go visit this extrasolar planet. Besides the fact that interstellar travel is currently just a pipe dream, Gliese 581 g probably wouldn't be as hospitable as one might first think. Even though it's considerably closer to its sun than the Earth is to our own sun, Gliese 581 is much less massive than the Sun and therefore much less luminous. Thus, the average temperature on this planet is about 37 degrees (Celsius) colder than on Earth. It's possible that since it's 3 to 4 times as massive as the Earth, it would be able to sustain a thicker atmosphere with an increased greenhouse effect, but that's pure speculation at this point.

The other issue is that since the planet is so close to its sun, about one tenth the distance that Earth is from our sun, there's a good chance that it's tidally locked, that is, its rotation has slowed so much that one side of the planet is always facing the sun. This would imply that it would be incredibly hot on the side of the planet facing the star and very very cold on the side facing away from the star. This happens when the gravitational gradient, that is the difference in gravitational force which decreases with distance from the massive object (the star in this case), is large it exerts a torque on the planet which causes its rotation to slow down, eventually causing it to rotate at the same rate as it's orbiting the star. This is probably best described in a diagram (from Wikipedia, of course):

I don't know how much that really helped, but the point is, if there is a large difference in temperatures on the planet, it would be more difficult for liquid water to form, and therefore for life to exist. This problem might be solved if the atmosphere of this planet is thick enough to distribute the heat around the planet. Venus, for example, is also almost tidally locked to the Sun and because of its thick atmosphere, the temperature is more or less uniform on all sides. However, since our instruments still aren't good enough to be able to detect the atmospheric composition of Gliese 581 g, it's hard to say anything more on this subject.

The story is further complicated by the recent announcement by another research group that they aren't able to detect the new planet in another data set. The planet was discovered using 122 measurements from the HIRES instrument on the Keck 1 telescope in Hawaii, and 119 measurements from the HARPS spectrograph at La Silla Observatory in Chile taken over 11 and 4.3 years respectively. All the planets in that system (there are 6 in total) were discovered using the radial velocity technique, which I briefly described in a previous post. Basically, the planets all cause the star to wobble slightly in its orbit, and by subtracting the wobbles of the closer and more massive planets, one is able to detect the fainter signals from the other planets in the system. Let's take another look at that wobble (if only because I love animated gifs):

So this other group added 60 data points onto the HARPS measurements and says that they can't find the signal for Gliese 581 g. It has something to do with the error bars on the previous measurements, and assumptions that are made about the shape of the orbits of the planets in the system. On one side of the argument, you have astronomers saying that this new planet doesn't exist at all, and on the other side, the astronomers who originally detected it maintain that you need the data sets from both instruments to be able to see it.

At any rate, it will be interesting to see how the argument is settled, which I suppose will happen in the next year or so when even more measurements are taken. If this planet does indeed exist, it's certainly an exciting prospect for discovering life elsewhere in the universe!

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