MadSci Network: Astronomy

Re: Which solar systems could favour the development of life forms?

Date: Wed Apr 21 14:11:25 1999
Posted By: John Dreher, Staff Astronomer, SETI Institute
Area of science: Astronomy
ID: 924647511.As

Life exists here on Earth in a very wide range of habitats, from boiling springs to antarctic lakes, from mountain tops to deep rock layers. The one common denominator seems to be the availability of liquid water, at least some of the time.

If this local observation were to be generally true, then the "best" planetary systems for life would be those that have planets in the so-called "habitable zone" -- a range of distances from the parent star that allows liquid water to exist. In our own solar system, this zone runs from a little outside the orbit of Venus, to (perhaps) the orbit of Mars. Of course, it (probably) takes more than just the correct distance from the star; size and atmosphere matter too. The Moon is in the habitable zone, but is without liquid water (because it is too small to hold an atmopshere). Alas, we don't know anything about where and what kind of small planets (like Earth) that may or may not exist around other stars. All the planets detected so far around other stars are very massive "gas giants", because that is the only kind of planets that current searches can detect. (I except from consideration, here, planets around pulsars.)

But we know very little, as yet, about the kinds of life that might exist. Life that is similar to our Earth life might not be able to evolve or survive in or on a gas giant planet, but that does not prove that there is not some, to-us-exotic form of life that might. Our knowledge is so limited, at present, that we should be humble and not make too many assumptions. We have not yet explored our own solar system in any detail as far as searching for life. To the very limited extent we can, it would be wise of us to examine ANY other solar systems for signs of life. As an example, the SETI project that I work on, Project Phoenix, has added all of the known extrasolar planetary systems to our search list, despite the guess on our part that many are probably not suitable to life-as-we-know-it.

So I can't really answer your excellent question as well as I'd like. In another decade or two, we will have learned more -- maybe even enough to provide a good answer! Should be fun trying anyway. You can learn a lot about extrasolar planets and the search for life starting here:

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