substantiate at least one of the claims of 2000 years of Christian understanding (and several hundred Jewish before that)

And anyway, we've got Chris de Burgh, the iPad, and Taramasolata. What more could you want in the universe?

But I think – given that special relativity puts a limit on how fast a spaceship can travel* – that the distances are too great to ever be able to make meaningful contact.

*Unless scientists fathom out how to remove mass from objects

I can just about buy the idea that a planet would need an atmosphere to sustain life, but I don't really understand why water is always assumed to be a pre-requisite. Just because it is on Earth doesn't mean it *has* to be elsewhere. For all we know, there might be lifeforms on gas giants that breathe gaseous mercury and are mostly made of lithium.

I mean, obviously Earth-like planets are a sensible place to *start* looking for life since we know that they can theoretically sustain it, but who's to say they're the *only* places life might arise?

Just look at some of the extremophiles that live on earth. There are creatures which live on ocean vents where the sea is at boiling point or higher and is, to all intents and purposes, concentrated acid. Or look at Pyrococcus furiosus, which grows best at 100 degrees celsius and whose cells contain tungsten. Life exists on Earth in places and forms we didn't used to think could possibly sustain it. So why shouldn't it also exist in even stranger places, and even weirder forms?

I don't think anyone has yet managed to find any truly Earth-like planets to study, let alone 500 of the things!

The universe is so damn huge it's very unlikely that any planets are within range of meaningful examination. I think that the planets the OP alludes to are possibly just exoplanets of Earth-like size, but this is a long, long way from being Earth-like as in being largely awash with lovely briny, just right for bugs 'n' stuff, sitting just the right distance from a suitable star.

on the likelihood of life, Jim, but not as we know it. Whether we'll ever come across aliens or they us is moot. It's the assumption that just because a very small number of earth-like planets can't support life, then there isn't any, I find unacceptable.

"Two possibilities exist: Either we are alone in the Universe or we are not. Both are equally terrifying."

I understand where the posters are coming from, but I think the environment will place limits on the nature of life, given a limited choice of chemical elements.

Atmosphere is a bit of a movable feast. On the early earth, the atmosphere didn't contain any oxygen at all; rather it appeared as a poisonous waste product of some bacteria who developed the ability to photosynthesise. Of course, life evolved to utilise all this oxygen and here we are today. We're currently altering the composition of the atmosphere by raising the levels of CO2 and trapping more heat. And life on earth will carry on, even if it means fewer (or none) of us.

As for water, there isn't really anything that has all the myriad and unusual properties of water. You could speculate on alternatives – ammonia, hydrogen peroxide et cetera – but their properties would be much more limiting. Science-fiction authors have speculated on silicon-based life forms, as opposed to carbon, but again, its chemical properties make this problematic.

So simple organisms maybe, but perhaps not more complex, large multicellular ones that can build spaceships.

Tharggggg345. You know him. Big bloke. Green hair. Blue eye. Nice tentacles.

One flippant, one a bit serious.

Do we have intelligent life on Earth?

How do we know we are the only intelligent life on Earth?

(a medium-sized planet in the galaxy you guys so poetically name HCG87) laughed ourselves silly when we read what that scientist said. Humans, huh?!

If the universe has other life forms, then where the hell are they? Given that we've been broadcasting for nearly 100 years, anything or any one within a hundred light year radius would have heard us by now. And vice-versa.

If you time-line out the expected life of the universe, you can see that we're pretty much still at the beginning. Maybe we really are the first.

100 years is not a long time. It takes me that long to blink.
100 light years is not a large distance. I have to go further than that when I misplace the remote control and want to change channels...
(Oh, and to be pedantic - it takes me another 100 light years to lumber back to the sofa...there and back puny human - do you see?

Our perception is Earth-based and we keep finding examples of sustainable life existing in very difficult environments. There may be life forms that are different to those we know and that have adapted to their own particular environments. If Darwin is correct, and if nature does find a way forward despite all the chaos, then it is probable that other forms of life exist out there in space. The big questions I have concern what causes life to spark and form in the first place, and whether there is more than just one way to make that happen. Life forms may be of the simplest kind, but there may also be more sophisticated forms that exist. We just don't know.

If we, on our insignificant pale blue dot, are all there is, then it seems an awful big waste of space.

is that, does life only exist on our planet because we are exactly the right distance away from the sun ?

Even on this planet, are there not places that are just too close - ie too hot, and just too far away, and therefore too cold ?

I may well be completely wrong, but it strikes me as a very fine line we exist on

Although things like greenhouse gases and water vapour in the atmosphere make the temperature higher than it would otherwise be. Also changes in the orbit of the earth relative to the sun affect the global temperature. Probably other stuff (I'm no expert).

Then again, scientists have postulated life on Saturn's moons.

This recent article by Matt Ridley is interesting. It's prompted by the discovery of the first small and rocky planet like ours outside the solar system 'Kepler 10b'. And in response to physicist Enrico Fermi's questions: "Where are they? Why have we not picked up signals from other civilizations?":

We have the moon to thank for [Earth's astonishing stability], Mr. Frenk says. Our unusually large satellite helps to keep us on an even keel. Without it, we would get occasionally tumbled by the influence of Jupiter. But the moon's birth (blasted from our own surface by an asteroid collision) was a highly improbable event. A bigger collision and the Earth would have been smithereens; a smaller one and the debris would have fallen back to Earth rather than coalesce as a single moon. So here's the flaw in the Fermi paradox: The birth of a moon just the right size to stabilize a rocky planet's orbit and rotation is very improbable.

... who am I to argue.

(Any excuse to give this another spin)

But where was the OPs original information published? I understood that up until now it has only been very large planets very near their suns that have been easily detectable. "Earth-like" planets are much further out and much smaller. They are detecting these planets by watching them pass in front of their sun, something that, in the case of our planet, only happens once each year from a fixed viewpoint. I would be very suprised if the consensus expert opinion is that there are no earth-like planets.