[The article below was originally posted on the BBC Future blog, and was titled "Will we ever… find life elsewhere in the universe?" I'm reposting it here because, oddly, the BBC page is only readable for people outside the UK! It has to do with the BBC rights and all that. But they gave me permission to post it here, and since I thought it was fun and provocative, I figure y'all would like it. Enjoy.]
Will we ever... find life in space? One of the reasons I love astronomy is that it doesn't flinch from the big questions. And one of the biggest is: are we alone? Another reason I love astronomy: it has a good shot at answering this question. Even a few decades ago hard-headed realists pooh-poohed the idea of aliens. But times change, and so does science. We've accumulated enough data that makes the question less far-fetched than it once was, and I'm starting to think that the question isn't "Will we find life?" but rather "Which method will find it first?" There are three methods that, to me, are the front-runners for finding life on other worlds. And I have an idea as to which one may find it first.
Life on Mars? The first method follows the principle that when you're looking for something, it's best to start close to home. We know of one planet that has life: Earth. So it makes sense to look for other places with Earth-like conditions: that is, liquid water, oxygen in the air, nutrients for growth, and so on. The most obvious place to look is Mars. At first glance it appears dry, cold and dead. But if you can see past that, things start to look up. The polar caps, for example, have lots of frozen water, and we've directly seen ice at lower latitudes on the Red Planet as well – meteorite impacts have left behind shiny craters, digging up fresh ice from below the surface. Several Mars rovers and landers have uncovered tantalising evidence that liquid water might flow just beneath the surface, but we still lack any conclusive evidence. However, if you broaden your timescale a bit, there is excellent evidence that in the past – perhaps a billion years or so ago – our neighbouring planet had oceans of liquid water and thicker air. In fact, conditions were pretty good to develop life as we know it even before it popped up here on Earth.
It's entirely possible that life got a toehold (or pseudopod hold) there long ago, and died out. If that's the case, we may yet find fossils in the Martian rocks. Again, there's no conclusive evidence yet, but we've literally barely scratched the surface there. Now that it has successfully landed on Mars, we have the exciting possibility that the plutonium-powered, car-sized Curiosity rover will soon use its on-board laser and other tools to crack open and examine rocks in the Gale Crater, which were laid down billions of years ago in the presence of liquid water. And Mars isn't the only possibility in our solar system. Liquid water exists inside Saturn's moon Enceladus, where geysers of liquid water erupt from deep canyons at its south pole. Energised by the gravitational tug of the giant ringed planet itself, the interior of Enceladus may be a vast ocean of liquid water even while the surface is frozen over. That doesn't guarantee we'll ever find alien fish swimming that moon's seas, of course. But it's an interesting place to look. Europa, a moon of Jupiter, almost certainly has an undersurface ocean as well. If you relax your constraints even more, Saturn's moon Titan has lakes of liquid methane and ethane on its surface, too. The chemistry for life would be different there – it's a rather chilly -180C on the surface – but it's not impossible to suppose life might arise there too. Finding out whether this is the case means getting up close and personal. We're doing that for Mars; however, the likes of Europa and Enceladus may have to wait a decade or four.
Phone home But maybe we don't have to go anywhere. Instead, we might be able to sit here and wait for alien beings (of whatever form) to message us.
BBC Future has created an interactive Drake Equation calculator. How many alien civilizations are out there? Plug in some numbers and find out!
SETI is the Search for Extraterrestrial Intelligence, and its name tells you its story: it's a group of astronomers looking for signs of intelligent life in space. They use various methods to look for advanced aliens, but the most promising one is to listen for any messages sent across the skies. The basic SETI assumption is that aliens are out there and want to contact us. If that's the case, there's a good way they can signal us: radio waves. They're the perfect medium: they're cheap, easy to make, easy to encode with information, they travel across the whole galaxy unimpeded, and they move at the speed of light, the fastest thing we know. So SETI scours the skies looking for radio signals from ET. They haven't found anything yet, but as SETI astronomer Seth Shostak points out, we've just started looking. There's a lot of galaxy and a lot of radio wave frequencies to sift through. But our technology gets better all the time, allowing for more sensitive searches. According to Shostak, if they're out there and currently sending signals our way, we should have an answer one way or another in about 25 years given the way things are going. I think SETI is a good idea. In a practical sense their engineers are advancing our radio technology and signal processing, and philosophically I think it's interesting to listen for alien signals. But I do wonder about the basic assumption that aliens are out there and want to contact us – it's a big leap, and based on our own human motivations. So while this is certainly worth the effort, it's hard to know if it'll pay off, and the 25-year deadline reflects that. But I suspect another method may have the edge.
New world order For a long time, we only knew of nine planets (including Pluto, though this was downgraded from its planet status five years ago), and only one that could support life. Then, in 1995, astronomers found the first planet to orbit another sun-like star. The planet wasn't like ours at all – more massive than Jupiter, and orbiting so close to its parent star its temperature is over 1,000C. But it was a watershed moment. We finally knew that other planets exist.
Since then, NASA's Kepler space telescope, the European Space Agency's Corot mission and ground-based instruments have found nearly 800 other planets, and that number grows every week. We know of enough planets orbiting other stars that we can actually start to extrapolate some numbers: it looks like approximately half of all stars in the galaxy have planets, and planets may in fact outnumber the 200 billion stars in the Milky Way. We still don't know how many of these worlds are like ours, but it seems like it's a good bet the number is in the million, if not billions. We're finding smaller and smaller planets all the time, and statistically speaking Earth-sized planets should be fairly common. The big question is how many of these have life? We don't know. But consider this: we have evidence that life on Earth started almost immediately after its surface was cool and stable enough to allow it. For three billion years that earthly life consisted of one-celled organisms, and it's only relatively recently that these evolved into the type of multi-celled creatures that now inhabit every niche of this blue planet. It means that any Earth-like planets we find may be populated by... yeast. But that counts. It's life. And life does something special: it ingests chemicals and excretes other chemicals.
Tell-tale signs One such chemical is oxygen. On Earth, we breathe it in, but plants breathe it out. There's a lot of it in our air; our atmosphere is more than 20% oxygen. If we found a planetary atmosphere with lots of oxygen gas, that would almost certainly be an indicator of life. As it turns out, we're on the verge of being able to do just that. Planets are dim and huddle close to their stars, but there are techniques to separate the light from the two objects. Oxygen has a signature, like a fingerprint, that can be detected in that light. It will take an extremely sensitive telescope and very clever techniques to see it, but we have the technology now to build such machines. One such is the James Webb Space Telescope, due to launch in 2018. It should be able to detect oxygen in an alien planet's air. And many other instruments are being planned and built that can carry out similar observations. Our technology is getting so good so quickly that finding alien biological atmospheric signatures is probably our best bet. To me, the numbers add up better than for the other strategies: there must be lots of this type of planet out there, life seems to arise easily, and biology messes with a planet's chemistry in a detectable way. We don't know if Mars or those watery moons have life at all, and even if they do it could take a long time to find it. And who knows if smart aliens are out there, and want to talk to us? But it may only be a few more years until we point a telescope at a fleck of light, absorbing those photons one by one, sifting through them, and finding in them – literally – the breath of life. So when will we find life in space? If it's out there, then my hope is: very soon.