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We're at a tipping point in human history,

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a species poised between gaining the stars[br]and losing the planet we call home.

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Even in just the past few years,[br]we've greatly expanded

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our knowledge of how Earth fits[br]within the context of our universe.

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NASA's Kepler mission has discovered

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thousands of potential planets[br]around other stars,

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indicating that Earth is but one[br]of billions of planets in our galaxy.

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Kepler is a space telescope

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that measures the subtle dimming of stars[br]as planets pass in front of them,

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blocking just a little bit[br]of that light from reaching us.

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Kepler's data reveals planets' sizes

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as well as their distance[br]from their parent star.

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Together, this helps us understand[br]whether these planets are small and rocky,

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like the terrestrial planets[br]in our own Solar System,

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and also how much light they receive[br]from their parent sun.

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In turn, this provides clues as to whether[br]these planets that we discover

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might be habitable or not.

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Unfortunately, at the same time[br]as we're discovering this treasure trove

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of potentially habitable worlds,

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our own planet is sagging[br]under the weight of humanity.

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2014 was the hottest year on record.

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Glaciers and sea ice that have[br]been with us for millennia

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are now disappearing[br]in a matter of decades.

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These planetary-scale environmental [br]changes that we have set in motion

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are rapidly outpacing our ability[br]to alter their course.

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But I'm not a climate scientist,[br]I'm an astronomer.

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I study planetary habitability[br]as influenced by stars

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with the hopes of finding[br]the places in the universe

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where we might discover[br]life beyond our own planet.

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You could say that I look for[br]choice alien real estate.

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Now, as somebody who is deeply embedded[br]in the search for life in the universe,

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I can tell you that the more[br]you look for planets like Earth,

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the more you appreciate[br]our own planet itself.

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Each one of these new worlds[br]invites a comparison

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between the newly discovered planet[br]and the planets we know best:

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those of our own Solar System.

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Consider our neighbor, Mars.

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Mars is small and rocky,[br]and though it's a bit far from the Sun,

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it might be considered[br]a potentially habitable world

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if found by a mission like Kepler.

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Indeed, it's possible that Mars[br]was habitable in the past,

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and in part, this is why[br]we study Mars so much.

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Our rovers, like Curiosity,[br]crawl across its surface,

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scratching for clues as to the origins[br]of life as we know it.

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Orbiters like the MAVEN mission[br]sample the Martian atmosphere,

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trying to understand how Mars[br]might have lost its past habitability.

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Private spaceflight companies now offer[br]not just a short trip to near space

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but the tantalizing possibility[br]of living our lives on Mars.

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But though these Martian vistas

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resemble the deserts[br]of our own home world,

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places that are tied in our imagination[br]to ideas about pioneering and frontiers,

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compared to Earth

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Mars is a pretty terrible place to live.

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Consider the extent to which[br]we have not colonized

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the deserts of our own planet,

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places that are lush[br]by comparison with Mars.

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Even in the driest,[br]highest places on Earth,

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the air is sweet and thick with oxygen

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exhaled from thousands of miles away[br]by our rainforests.

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I worry -- I worry that this excitement[br]about colonizing Mars and other planets

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carries with it a long, dark shadow:

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the implication and belief by some

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that Mars will be there to save us[br]from the self-inflicted destruction

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of the only truly habitable planet[br]we know of, the Earth.

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As much as I love[br]interplanetary exploration,

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I deeply disagree with this idea.

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There are many excellent reasons[br]to go to Mars,

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but for anyone to tell you that Mars[br]will be there to back up humanity

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is like the captain of the Titanic[br]telling you that the real party

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is happening later on the lifeboats.

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(Laughter)

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(Applause)

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Thank you.

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But the goals of interplanetary[br]exploration and planetary preservation

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are not opposed to one another.

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No, they're in fact two sides[br]of the same goal:

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to understand, preserve[br]and improve life into the future.

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The extreme environments[br]of our own world are alien vistas.

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They're just closer to home.

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If we can understand how to create[br]and maintain habitable spaces

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out of hostile, inhospitable[br]spaces here on Earth,

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perhaps we can meet the needs[br]of both preserving our own environment

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and moving beyond it.

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I leave you with a final[br]thought experiment:

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Fermi's paradox.

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Many years ago, the physicist Enrico Fermi[br]asked that, given the fact

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that our universe has been around[br]for a very long time

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and we expect that there[br]are many planets within it,

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we should have found evidence[br]for alien life by now.

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So where are they?

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Well, one possible solution[br]to Fermi's paradox

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is that, as civilizations become[br]technologically advanced enough

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to consider living amongst the stars,

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they lose sight of how important it is

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to safeguard the home worlds that fostered[br]that advancement to begin with.

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It is hubris to believe[br]that interplanetary colonization alone

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will save us from ourselves,

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but planetary preservation[br]and interplanetary exploration

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can work together.

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If we truly believe in our ability

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to bend the hostile environments of Mars[br]for human habitation,

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then we should be able to surmount[br]the far easier task of preserving

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the habitability of the Earth.

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Thank you.

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(Applause)