Greg at systemic makes a bold, and probably accurate prediction
For what it is worth, I think he is right.
We're not at the point of seeing precise Earth analogues right now, but we are close, and we are at the point where we can detect either Earth like planets around nearby low mass stars, or "super-Earths" around Sun like stars.
The super-Earths are rocky planets with masses several times those of Earth. They are likely to have substantial volatiles, ie water and air, on their surface, and they can have liquid water on the surface persist over astronomical time scales.
We won't know for sure until we actually see biosignatures, but it is a good bet that by 2010 we will have candidates where we genuinely think we might see liquid water and signatures of life in the atmospheric spectra, if such things happen commonly in locales where they may happen.
Exciting times.
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Is anyone taking bets on what proportion of these will actually be habitable?
See, the problem is that when you get right down to it, we can only really detect carbon-based life (presumably an oxygen-based atmosphere is not strictly necessary, though). But what other chemistries can we hope to find, given that any of them would be hypothetical?
When you put a time frame on it like this, it seems so much more exciting than the fast progress in detection sensitivity and discoveries in exoplanets.
Hm. I'm a layman here, but my impression was that the main indicators may be non-equilibrium atmospheric chemistries and radiation absorption.
In some cases, like the methane on Mars, it may be hard or impossible to distinguish those indicators from those sourced by other natural processes such as volcanism. But combined perhaps there is a chance.
Where does carbon chemistry come into it (except from being the most likely)?
That is the big question, and one of the first ones brought up when we can get enough fresh and uncynical astrobio grads mixed with some pitchers of beer or margaritas...
There are some ideas on how to detect non-carbon/oxygen life, none of which are terribly robust.
Dis-equilibrium chemistry is probably necessary, but definitely not sufficient - stellar radiation can set up strong disequlibria in upper atmospheres, for example.
Some general things being played with are isotope fractionation and chiral chemistry;
both are harder still to detect, and since biogenesis is abiotic there is always the residual worry that such must also come about through inorganic chemistry, although arguably not in detectable abundances without reproduction.
This will be one of the more fun things to find out.