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There are some planets that scientists speculate may be capable of supporting life. However, these planets are hundreds of light years away. How can we be sure they are still capable of supporting life, that they are still there, or even have life?

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maybe I am being a snob from the Skeptics.stackexchange site, but this question is very unclear. Please expand your question a bit more in terms of clarity. This isn't Yahoo! answers, and the questions in general are meant to be answerable instead of esoteric philosophical questions. Are you specifically asking about any earthlike planets we have found? Because as of today, no one has made the assertion we have found any planets capable of supporting life. –  Larian LeQuella Jun 17 '11 at 21:31
Related question: physics.stackexchange.com/questions/25135/… –  Larian LeQuella Jun 17 '11 at 21:31
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2 Answers

(I just performed a bit of an edit on your question to have it align more with what I think you are asking as opposed to the overly speculative sentences you had. I hope you don't mind. :) And I will build on Andrew's answer (I hope he doesn't mind).)

This is probably related to the question What are the prerequisites for considering any other planet to be habitable?.

Currently, no planet has been determined to be habitable with any certainty. There is a list of planets that are CANDIDATES as I highlighted in this answer. The definitive site though is http://exoplanet.eu/, which is touted as The Extrasolar Planets Encyclopaedia (The Interactive Catalog probably has the data you are looking for.). I would say that there are other sites as well, but this one is public and interactive. Many other sites require membership in the IAU for instance, and others don't seem to have been updated in a while. EDIT: I just found a nice table by The Planetary Society. Notice that in no case have we performed any sort of spectral analysis of the atmospheres of these planets, although we have done that for other exoplanets. (Actually, there are a lot of scholarly papers on this subject.)

As Andrew correctly states, life will leave a very recognizable spectral signature on a planet's atmosphere as its sun's light passes through its atmosphere. Although there is always the danger of a false positive, so I would guess that any announcement would be done with caution by the scientific community (but wild abandon by the popular media). Again, there are quite a few scholarly papers on this subject as well.

Now, you mention that these planets are far away (true to a degree). We have (as a species) only explored for exoplanets in a relatively small neighborhood of the Milky Way. In general, most planets we have been able to detect (due to our technological limitations) are within a few hundred light years. This is only a very small fraction of the galaxy (which is over 100,000 light years across). Also, a couple of hundred of years in geologic terms (or biological evolution terms) is absolutely no time at all. It is but an eye blink. Barring something catastrophic, should the day come when we detect something worth speculating about the data indicating exo-life, it is highly probable that it will survive for a few hundred years. After all, life has been on this planet for almost 3.5 BILLION years, even with several major extinction events.

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That phrasing is a little optimistic, but the thrust of the question is still valid.

As for the timing issue: Special relativity implies that, if something is, say, 200 light-years away from us, we can not possibly get any information about that object that is any less than 200 years old, because the speed of light is as fast as information can travel. We would have to just hope that life wasn't wiped out by some disaster in the mean time.

As for determining life: This question is related to What are the requisites for considering any other planet to be habitable? but since that question emphasizes the necessities of life and this one asks about observing life, I think there's enough room.

Life has a radical effect on the chemical composition of the surface of the planet it inhabits. Observing the atmosphere of a planet is therefore the best place to find a "smoking gun."

Particularly, with abundant life-processes going on all over a planet, you can have the simultaneous presence of two different highly reactive molecules that would annihilate each other if they were not being produced all the time. Oxygen and methane are examples of reactive molecules that must be produced continuously, although methane can be made by geology in addition to biology.

If your physics tells you that the planet is just warm enough for liquid water and you detect H20, oxygen, and some other telltale biochemical signatures, especially complex organic (carbon-containing) molecules, that would be a pretty airtight case for the ongoing presence of abundant life on that planet.

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