A big part of science is asking the right questions. We've been looking for signs of life elsewhere in the universe, but how do we know we'd be able to detect them even if life is out there? One way to answer that question is to send a probe far into space... 
...and see if it detects signs of life on the one place we know for certain it exists in the universe—Earth! That was one of the goals of the Venus Express mission. Launched in late 2005, it was sent millions of miles away to study Venus. But while it was there, astronomers...
... turned it towards Earth to see if it could detect signs of habitability on our own planet. It detected certain molecules associated with life in Earth's atmosphere, but the problem is, Venus shows the same signs and it's not habitable. Astronomers hope to fine-tune...
...their observations to detect more definitive signs, like that of photosynthesis—the process of converting starlight to chemical energy for plants and other organisms—on Earth and apply that to the search for life on planets outside our solar system. Pretty clever!
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