In December 2013, astronomers reported that the Hubble Space Telescope had detected water vapor in the atmospheres of 5 planets outside of our solar system. Before you get too excited, these are not really candidates for life. They are all hot Jupiters - gas giants that orbit very close to their stars. But you should be excited that new now have proof that water exists in atmospheres outside of our solar system, and that we have a method of detecting it.

Here's how that works. When a planet's orbit takes it between us and its star, that star's light passes through the halo of the planet's atmosphere. Using an instrument called a spectrograph, scientists can split that light into a spectrum.
This is the same electromagnetic spectrum you've seen your whole life - it goes from infrared light, through the colors of visible light, up through ultraviolet, et cetera.

Now, when nothing is between us and a star, we can see the whole spectrum as the star emits it, and when something is blocking that light, we can't see any of it. But when a planet's atmosphere is transparent, it lets some light through, but not all of it.

So the spectrum created by the spectrograph has these dark bands running through it from where different gases in the planet's atmosphere block or absorb different frequencies of light. They're called absorption lines.
And the really amazing thing is, different chemicals create different patterns of absorption lines. So a scientists can look at the information from the spectrograph and figure out which gases the light has passed through. 

At the moment, we don't have a telescope powerful enough to read the atmospheres of anything smaller or darker than those hot Jupiters, but in the future, we will. Probably as soon as 2018, when NASA launches the James Webb Space Telescope. And once we can read the atmospheres of terrestrial planets like Earth, made of metal and rock instead of gas, we can start looking for signs of life outside our solar system. 

What will we be looking for? Well, definitely water vapor. Water in the air means there might be water on the ground - especially if the planet is the right distance from the sun. Life as we know it couldn't happen without liquid water. 
We'll also be looking for free, or molecular oxygen. Molecules of oxygen - O_2. Oxygen atoms are relatively abundant in the universe, in fact the earth is 45% oxygen by weight, and about 85% oxygen by volume. But almost all of it is bound up in molecules with silicon, magnesium, and iron, making up the rocks under our feet. 

It's a really friendly atom, always looking to pick up an electron from any other atom that can spare one, but when it bonds with itself to form O₂ molecules, it's not particularly stable, so it tends to get converted into other molecules. 
But our atmosphere is more than 20% O₂, because it's always being made by living things, particularly plants and other photosynthetic organisms which use sunlight, water, and carbon dioxide to create massive amounts of atmospheric oxygen.

So, if an alien astronomer were looking at our atmosphere, they would see something way chemically out of balance - the planet has too much free oxygen. Something is alive down there. And once you have water and oxygen, the way is open for the development of cellular respiration, which is how aerobic organisms use oxygen to fuel their metabolic processes. And aerobic metabolism is much more efficient than anaerobic metabolism - the kind the doesn't use oxygen. Something with an aerobic metabolism can extract up to 16 times more energy out of the same amount of fuel. You need that kind of energy to support multi-cellular creatures. Animals, and plants, and eventually, people-type things. 

So, finding water vapor in the atmosphere of these hot Jupiters is just the next step toward finding habitable, and potentially inhabited planets. Identifying planets like ours isn't just the stuff of science-fiction anymore - some time in the next 10 years we're gonna be able to a point a telescope into deep space, look at a far-away planet and know if there is any chance that there might be someone out there looking back. 

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