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Permalink Reply by Gary Burk on July 2, 2010 at 7:34pm

Of the approximately 300 extra-solar planets we've detected so far, none are earth like planets (ELP) . This does not mean that earth like planets are very rare, rather it suggests that our detection methods may not be sensitive enough to detect an ELP. Frank Drake recognized that there is a "habitable zone" where life with similar chemistry to our might exist. The habitable zone for any star is the area around it where water is in its liquid phase. Liquid water is necessary but not sufficient for life to exist. A planet would also need a liquid conducting core and perhaps "guard planets", such as Jupiter, to sweep the area of meteors, comets, etc.

We now detect extra-solar planets by one of two methods. The photometric method relies on the tiny reduction in brightness of the parent star when a planet transits it. The spectroscopic method uses the doppler shift of the stars color as it wobbles as the planet orbits it. Most of the planets we've found so far have be accomplished by the spectroscopic method.

For either the photometric or the spectroscopic method to work, a planet must be massive (relative to the earth) and close to its star. So, most of the planets we've discovered so far are called "hot Jupiters"
meaning a huge planet that is near its star. Recently a planet of only 4-5 earth masses was detected, so far that holds the record for the smallest one we've found.

There may be lots of ELPs. We do not yet have methods sensitive enough to detect them. A standard question for Astronomy undergrads is "If we were on the closet star to the Earth, would we be able to detect the Earth orbiting around the sun? " The answer is "Not yet, but we're getting closer. We would need instruments 5 times more sensitive than we now have available."

We have no means now to estimate the probability of life with different chemistry (say, silicon based).

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Permalink Reply by Apeman Jim on July 2, 2010 at 11:09pm

Hmm...

I am fairly (but not intimately) familiar with the methodology of finding planets. I know the liklihood of finding an ELP is pretty low, and I dont expect us to start doing that for some time. (This is, of course, not helped by any stars whose planets may orbit in a more perpendicular direction to our own) What I do want to know is if there has been any changes to predictions relating to the frequency of planet formation.
Prior to last decade speculation was all that we had, and I was under the impression that there was a sense of infrequency about the arrangement. The more likely planets are, the more likely there will be ones which will be favorable for life.
As you mentioned earlier, it is desirable for the planet to be in the "goldilocks" zone for the arrival of life, but I am sure that is with earth-type life in mind. I often think of the sorts of life forms which live around sulfur vents at the bottom of the sea.
In any event, I was just curious as to whether or not any exo-biologists had laid any new odds on the table for neighbors, great or small, in our galaxy.

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