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This is Hubble Telescope’s famous photograph, Hubble Ultra-Deep Field. 

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There are nearly 10,000 galaxies each containing as many as 

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100 billion planets in this image alone. 

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But the question has always been, out of those billions of planets, 

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how many could have life?

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Observing Earth’s global biology on a massive, planetary scale 

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has given scientists the tools to answer important questions - 

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like how can we use models of our own planet to detect signs of life on other worlds? 

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In short - the first thing we’re going to do is figure out how we’d find ourselves. 

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When I talk to people about this, 

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about the search for life on all these planets we’ve found around all these other stars,

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a very common response I get is this line from Contact 

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– well, if there isn’t anything out there, it would be a horrible waste of space – 

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which is a wonderful line, and especially was a wonderful line 20 years ago. 

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But now we’re beyond that. 

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That’s Dr. Shawn Domagal-Goldman. 

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He’s one of NASA’s many scientists heading up the search for life. 

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I’m a research space scientist and astrobiologist at NASA Goddard Space Flight Center. 

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What I do at NASA is I look for ways to look for life on other planets. 

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Ten years ago, conversations about life in the universe 

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were mainly limited to bar talk and philosophical conversations. 

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But that’s all changed. 

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We can now apply the scientific method to the question, are we alone? 

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We, based on our understanding of how life operates on Earth, 

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are starting to derive principles of the signals that life creates 

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that we could then look for on these planets around other stars. 

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But with a universe as vast as ours, 

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where do you even begin looking for these Earth-like planets? 

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NASA scientists must take an extremely calculated approach 

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when it comes to combing the universe for signs of life. 

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By studying Earth’s climate over its long history, 

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we have a pretty good understanding of how climate operates on other rocky planets. 

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And that gives us some helpful clues on the distance from a star and the size of planet 

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that could harbor a global biosphere like the one we have here on Earth.

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It all comes down to knowing where to look.

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We have a concept for this: 

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its called the habitable zone or the Goldilocks zone

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and the basic idea is, you can’t be too hot, because otherwise you’ll lose your oceans 

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– they’ll basically boil and steam away, you can’t be too cold, 

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because then your oceans will freeze over. 

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You want that that big sort of ocean reservoir

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at the surface which happens when you’re kind of in the middle and just right. 

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In our solar system, the Goldilocks zone is bound by Venus 

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which is too hot and steamy with no oceans at the surface and Mars

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which is too cold and too small. 

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And size matters too.

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To give an example, the moon is technically in just right place. 

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It’s in the middle of the Goldilocks zone just like Earth is, 

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and it gets the right amount of energy from the sun. 

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But it’s too small to hold on to an atmosphere. 

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And the same thing goes for planets that are too big, 

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like gas giants where there’s too much pressure bearing down on liquid water.

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We’re on the Goldilocks planet, and what’s really neat is 

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we found a lot of other so-called Goldilocks planets in the last few years 

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that we could then think about looking for signs of life on in the future.

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The studies Shawn is talking about have been coming out pretty consistently since the early 2000s. 

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The recent uptick in exoplanet discoveries over the past seven years or so 

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is due in large part to the Kepler Space Telescope 

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which found over thousands of exoplanets orbiting other stars.

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One cluster of planets after another, astrophysicists have discovered 

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a mind-blowing number of worlds that are the right size and distance from their star 

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to have potentially have conditions for life similar to Earth. 

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The most amazing thing that Earth has taught us 

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is that life can really exist in very dramatic environments 

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from really hot environments in the middle of a desert

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to really cold environments with little light at the very bottom of the ocean.

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Based on what we know about Earth, the fundamental cocktail looks like this: 

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You need liquid water, 

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the right atmospheric gases 

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- and if you’re lucky – specific global signals of life. 

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Everywhere we look, whether it’s a desert or Antarctica 

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or the deep ocean or the deepest parts of Earth’s crust that we’ve explored, 

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as long as there’s a little tiny speck of liquid water, there’s life. 

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And because of that, it’s been central to NASA’s search for habitable environments elsewhere. 

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It’s why scientists get excited about the water spewing up 

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from the icy moons of Europa and Enceladus in our outer solar system. 

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Not only could they have water, they could have global oceans like we have here on Earth.

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After liquid water, we’d look for atmospheric gases 

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– actually the gas we’re breathing now, oxygen.

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Find oxygen and methane together in the same atmosphere

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and you’ve got something special. 

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There are ways to build up oxygen or methane in a planetary atmosphere, 

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but the only way you get them both in the same atmosphere at the same time 

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is if you produce them both super rapidly. 

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And the only way we know how to do that is through life.

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The next thing scientists could look for is pigment 

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- the colors of life, like the chlorophyll found in plants on land 

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and algae and phytoplankton in the ocean. 

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Although there aren’t currently any outer space missions in progress to retrieve this data, 

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we could– in theory– be able to detect similar colors on a planet around another star in the future.

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But maybe one of the coolest things about this whole enterprise is how quickly we’re learning. 

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I firmly believe that one of two things is going to happen in the course of my scientific career.

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Either we’re going to find evidence that we’re not alone in the universe 

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or we’ll have so exhaustively searched for it and not found anything 

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that we’ll know that the universe is a lonely place and our place in it is more special because of that.

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Either way I can wait to find out what we uncover in the next 20 years.

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