We don't know of course if some alien life forms might exist that have no need for water – but the problem is that we don't have any examples available to know what chemical reactions they might use. On the other hand we do know of at least one example of a family of life-forms that do require water, and we understand a good deal about their chemistry. ("At least one" because there is still some debate about whether life on Earth evolved only once – the majority view – or multiple times).

So granting that some other life forms might exist, how might we detect them? We have a pretty good idea that if we observe molecular oxygen (O2) on another planet, it's likely to be the result of photosynthesis from water-based organisms. What do we look for in order to detect non-water-based life? Since we don't have an example, we don't know – except to look for "unusual" spectra that we can't explain any other way. It will most likely be many centuries (if ever) before we will be able to detect alien life directly, say by detecting signals from intelligent beings or actually being able to image the effects that alien lifeforms might be having on their world. We should be able to get spectra from those worlds much sooner.

Finally, binary star systems are common but not likely to be hospitable to life on any planet that orbits one of them. Young yellow stars aren’t good candidates either, they need to be more than 3 billion years old, preferably in the 4-5 billion year range to be the most stable. This reduces the number of stars to no more than 1 in 100 that are worth looking for habitable planets around. The odds of a planet – one to maybe three times the size of Earth being in the habitable zone are let’s say 1 in 10. I don’t know. I’m not claiming that my numbers are accurate, but if only one in a thousand stars had the potential of life, that would still be more than a million sources of life in our galaxy. Now that is exiting! Spread them out through the arms of the galaxy and hundreds of them could be in “near spaceâ€? by my definition. I suspect that the term “nearâ€? has probably changed a lot in our life time as the number of large telescopes and optical arrays increased.

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However, nature seems to like logarithms. If the planets in planetary systems are spaced approximately logarithmically, this isn't so much of a problem, since you get more planets close to the star (in fact, scaling the habitable zone does not change its width on a logarithmic scale). In fact both Gliese 876 and Gliese 581 have two planets each which are in or close to the habitable zone.

So all-in-all, a bit of a non-story.

Small habitable zones are bad enough. Fluctuating, fast-moving habitable zones are much worse.

On behalf of red dwarfs everywhere, some of whom are my best friends, I wish to complain of their being stereotyped. You wouldn't call a tall people basket-ball players, would you?

And anyway, what research has been done to measure the mean size of red dwarf habitable zones, or to determine the statistical significance of any difference in size between their habitable zones and those of white dwarfs or those of normal people of any color.

If you are talking about the habitable zones and temperature of stars, I would remind you that stars, in general, like, fer example Britanny Spears, are HOT HOT HOT.

The Hab Zone is simply the distance at which water will be liquid due to temperature from solar radiation. As was mentioned, there are other things that can affect whether water can be liquid, like albedo and greenhouse and heck, even gravitational mucking. But we don't completely understand whatall goes into MAKING things like greenhouse.

Earth itself was very inhospitable for a long time. Only single-celled organisms could survive. A little closer or farther from the sun, and they might not have.

If Venus or Mars were ever habitable, it didn't last long enough for life to really take hold. There might still be something in either place, as Life can be rather tenacious. At most, we will find bacteria there. I suspect we'll find lots of bacteria, and that it will all be related, to whatever extent can be detected from something that evolves in hours or days rather than millenia or megayears.

The big question is, will we ever find complex life, and complex life has a fairly narrow band of tolerances, and this is what Hab Zone will come to mean when bacteria is discovered all over the place.

This doesn' take into considereation the fact that water may not be needed for alien life. This insistance on liquid water for life is tiresome.

What about crystaline life-forms? Remember "alien" means "alien." We may not even recognize alien life when we see it.

As far as sun like stars go, we have no further to look than our nearest (triple) companions, Alpha Centauri.

As far as "an exact" match, I remember one star being hailed called HIP 56948 (Google it for more info).

Either way, I wonder, what about blue stars? Are they more/less favorable for habitable planets than our regular sun?

The Fool
http://newfrontiersblog.blogspot.com

I think it is "Earth-type planets "? You mean rocky planets?

By the way, you do a remarkable job. Really. Thank you so much!