Most projections for the future of our exoplanet hunt have us finding and, we hope, imaging one by 2020. Our first images will tell us little more than that it is real and possibly some information about its atmosphere and/or composition.

But then what?

Obviously if we start finding other Earths within imaging range we will want to do as much as possible to find out more, but what will it take and by when?

Will the OWLs being planned today (up to 100m) be enough, or will we have to start building OWLs in space to achieve the resolution to see landmasses and water? (Is there a physical limit to what we will be able to see?)

And then, what if we detect signs of life or even intelligence (via industrial gases in the atmosphere)? How far do you think we will go to find out all we can about our neighbours?

How far we'll go depends in large part on how close the planet is. If it's within 10-20 l.y., I think we'll do whatever we can to find out as much info as possible. To my knowledge, we're limited only by how much money we want to spend, and the technology at our disposal. The hurdles will be significant, though.

Nothing short of the mother of all interferometers could resolve an Earthlike planet into a disk; I've calculated that at 1 l.y. (~63,241 AU), Earth would have a diameter of .000278 of an arcsecond. No telescope that is in the planning stages--not the GMT or even the OWL--could pull that off. I'd want the baseline for any serious exoplanet imaging interferometer to be no less than 10 kilometers, for utility out to ~20 l.y. To actually resolve continents, I'm guessing the basline would have to five to ten times greater still.

<<And then, what if we detect signs of life or even intelligence (via industrial gases in the atmosphere)? How far do you think we will go to find out all we can about our neighbours?>>

Finding out which planets have intriguing life signatures will be accomplished by missions such as Darwin, long before we get the technology to image "exocontinents." However:

1.) Scientists are notoriously skeptical, and rightly so. Discovering a planet with an Earthlike atmospheric planet does not mean that the planet will actually be like Earth, and it will be almost impossible to prove without more powerful instruments--in short, it would be hotly contested. Yet the very uncertainty that the planet is Earthlike might make people unwilling to spend the R&D to make a more powerful instrument, and so on...

2.) Detecting intelligence could probably only be done by means of a received signal, which is a whole 'nother ball of wax, IMO. I don't think we could detect any civilization without some kind of EM leakage; keep in mind that even a person in geostationary orbit would find it impossible to see any signs of the human race on the planet, short of seeing lights on Earth's night side.

All that aside, if we found an Earth analog (a term I like better than "Earthlike") within 10 parsecs, I think we would go for broke in learning everything we could about it. It would be, IMO, the most monumental thing that could happen to the human race short of actual contact.

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The "then what?" question has always been intriguing. My guess is that we'll have found at least 5 Earth-like planets by 2020. They will be deemed low priority by NASA (and *coff- the ESA), and most observation will be done by amateurs.

Yes, there is a big limitation, given today's technology. A speck orbiting a very bright speck is about the best image we'll see, in most EM ranges. The distances likely will preclude improving resolution, even if OWLs are built in space.

I'm not sure halocarbons or other industrial gases could be detected reliably, using current technology. Likely, people would freak out if an Earth-like exoplanet was found to have both water vapor and CO2, or methane- though all these gases can be produced by non-biotic means. But the possibly will always be intriguing, and the technology of detection will only improve.

Pretty much so. Imaging an extrasolar planet's surface in any detail would require immense space interferometers. One example is the Planet Imager, which consists of five four 8-m telescope arrays. Fortunately, finding life does not necessarily require spacecraft larger than TPF because detecting abovementioned signals of life requires only spectroscopic measurements.

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I can imaging an interferometer consisting out of a number of
large telescopes all over the world.
As they do now with radio-telescopes in projects like Merlin and the VLBI.

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Unfortunately, that can't be done with optical telescopes.

Imagine (or calculate!) what resolution an Earth-wide optical interferometer would have!

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I wonder what they would consider an "Earth analogue"? What happens if we catch an otherwise Earth identical planet in an iceball phase? Or if its having a rough run of volcanism like the stretch that created the Great Rift Valley? Would we know the difference from so far away?

I believe that an "Earth analogue" would be a planet that humans could survive on with limited difficulty.

Mass and gravity approximating Earth (perhaps 0.7 to 1.4 Earth masses)
Liquid water available at all times
An Oxygen-Carbon Dioxide biosphere

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Yet!
I'm an optimist.

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Then we're not going to know if its an Earth analogue for a LONG time.

Maybe so. But I think that when organizations trump up a big frozen iceball as a "Earth twin" (like a few weeks ago), then that makes it hard for the public to take scientists seriously.

An "Earth analogue" should, at a minimum, be able to support Earth-style biology.

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Well, I wouldn't be surprised if an Earth-mass planet orbiting at one AU from its star is discovered soon by the gravitational lensing method. Nothing will be known about its actual habitability, however.

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Well, based on the solar mass and orbital distance, we should be able to infer whether liquid water is possible. And, if it is, that's a good indicator of life (since water is practically everywhere)

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I guess how much we will be willing to spend investigating Earth analogues depends on what we find. If we find a planet within a system's habitable zone a few light years away the pressure will be on to find out more. If that planet turns out to have indicators of life, the pressure only increase in an effort to find out what sort of life it could be.

Of course, there's always that virtually insurmountable problem of distance, and there will be those who think that it's a waste of money to build bigger and better instruments to probe something we'll never get to visit (at least for many decades or centuries). On the other hand, perhaps such a discovery will be the impetus needed to drive space exploration forward and achieve seemingly impossible goals.

If we do, it will be both a great achievement and very frustrating, since all such discoveries are one-off events. Nonetheless, the method could be useful in telling us how many such planets are out there.

Actually, we don't know that, since the only data point we have to go on is Earth itself. Mars and Europa may be the only other comparatively accessible sources of liquid water (now or in the past) and we might at some point know if life arose in either or those two places, but until then we simply do not know if liquid water equals life.

But hopefully the discoveru of liquid water on an exoplanet will spur us to find out more.

Right. Which is exactly why I called it a "good indicator" of life, not proof. However, I think that evidence is starting to build that, given the appropriate conditions, abiogenesis is an inevitability. Water is everywhere in the universe; the only catch is that life needs it to be in that narrow temperature range in which is it liquid.

I think that it will

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I don't want to nitpick but... I will anyway . My point was that I don't think we DO know yet if liquid water is a "good indicator". It might very well be that life is only found on one in a million places water is found, or less. Maybe water is a necessary precursor to life but water's presence, in itself, doesn't mean life there is likely (as far as we know).

There's an interesting thought...would a planet need to be Earth's size to have a sustainable biosphere? Why couldn't a planet two or three times Earth's mass manage one?