Hello this is Hank Green, welcome to this SciShow Dose. There are trillions of trillions of stars and I didn't say trillions and trillions - I said trillions of trillions. Trillions of trillions people!

It'd be almost insane to think that with this many star systems in the universe, life only occurred on one of them and yet, so far, we have seen no proof of extraterrestrial life. But that is why we have astrobiology, the study of life in the universe - where it is, how it got there and what it's going to do next.

Unsurprisingly astrobiology is a big interdisciplinary field with scientists from physics, biology, geology, astronomy, chemistry, ecology and more all working together to answer what may be the biggest question we have ever asked. The way I see it there are two major groups in astrobiology. One is focused on actually discovering life and settling the question once and for all, and the other is hypothesizing what life would be like on other worlds, how life generates and spreads and whether it could survive, say, on a world without sunlight or without water, or in sub-zero temperatures.

We're going to start out with the second one. It might seem crazy to imagine life in cold, dark methane seas but astrobiologists must imagine it. We're almost certain that some sort of liquid has to be present for life to form, and though water is by far the most promising candidate for this role astrobiologists have suggested several other possibilities, like ammonia, or liquid methane or even sulfuric acid or formaldehyde.

Now it's hard to believe that those toxic compounds could harbour life but they're only toxic to us because we evolved on a water world - maybe to other organisms water would be the poison. That being said, water is for a lot of reasons considered by many to be a prerequisite for life. Add that to the fact that water is extremely common throughout the universe and that we already know liquid water exists in three distinct bodies in our solar system and definitely used to exist on a fourth and fifth, and it's obvious that water becomes the most common thing for astrobiologists to search for off earth.

Of the 800 exoplanets that have been discovered thus far only about a dozen are likely to harbour liquid water on the surface. But as current methods of detecting exoplanets are much better at detecting large planets like gas giants than they are at detecting smaller, rocky planets, it's likely that the proportions are more favorable than they seem.

Amazingly enough, astrobiologists can actually measure the atmospheres of some of these planets as they cross their star. The trick is that as the light from the star shines through the atmosphere certain chemicals absorb light at certain wavelengths, so we can even from light-years away detect whether there's water in the atmosphere. Pretty frickin' cool.

Now that we know that we should be searching for water and how to search for it let's move on to prebiotic chemistry - the study of how lifeless chemicals somehow become alive. As I speak the Curiosity rover is digging up samples of Martian soil to look for traces of organic compounds including amino acids and nucleobases - the four key molecules found in DNA. Two of those bases adenine and guanine have already been discovered on meteorites that fell to earth, including, one from Mars.

But enough of the hypothesizing, how do we actually find out once and for all whether or not there is life out there? Well first, we just go out and look. There are several good candidates for extraterrestrial life in our solar system and of course the Curiosity rover is exploring one of them as we speak. But several of the moons of Jupiter and Saturn are either thought or known to have liquids water under their surface, and hopefully, we get to take a peek some day.

We can also take a listen. There are only about 1000 sun-like stars within 100 light-years of the sun, and those are the star systems that we concentrate our listening on. And when I say listening, I mean it. SETI is about a lot more than just detecting life - SETI aims to detect intelligent life by hearing radio transmissions from other star systems. Intelligent life of course is bound to be far more rare than life, but also far more interesting.

Of course, astrobiology has more questions than answers at this point but it's nice to have so many great minds searching for answers to one simple question - are we alone in the universe? Logic certainly says that we are not but thus far we have no proof. If ever we do, you can assume that I'll be freaking out right here at SciShow and if you don't want to miss that you can go to youtube.com/SciShow and subscribe, and of course I'll be freaking out on Facebook and Twitter as well, ciao.