Sara Seager

Like here on our planet, we have oxygen. We all humans all need oxygen to survive. Oxygen fills our Earth's atmosphere to 20% by volume. Without plants and photosynthetic bacteria, we would have virtually zero oxygen.

Like here on our planet, we have oxygen. We all humans all need oxygen to survive. Oxygen fills our Earth's atmosphere to 20% by volume. Without plants and photosynthetic bacteria, we would have virtually zero oxygen.

So if there's an intelligent civilization on a planet orbiting a nearby star looking back at us, they'll suspect there's life here, not because they can see animals or not from big things like the Great Wall of China, but from oxygen, a gas that's so reactive, it shouldn't be here at all unless it's continually replenished. And so that's what we're looking for.

So if there's an intelligent civilization on a planet orbiting a nearby star looking back at us, they'll suspect there's life here, not because they can see animals or not from big things like the Great Wall of China, but from oxygen, a gas that's so reactive, it shouldn't be here at all unless it's continually replenished. And so that's what we're looking for.

And it'll be tricky, very tricky to assign unusual gases to life.

And it'll be tricky, very tricky to assign unusual gases to life.

We have no real evidence that there has, and people look. We have a whole other category of search for life called technosignatures, signs of a technologically advanced civilization. And we really don't have evidence of any yet. However, to speculate, our universe is so vast. We have 100 billion stars in our Milky Way galaxy. And in our universe, there are hundreds of billions of galaxies.

We have no real evidence that there has, and people look. We have a whole other category of search for life called technosignatures, signs of a technologically advanced civilization. And we really don't have evidence of any yet. However, to speculate, our universe is so vast. We have 100 billion stars in our Milky Way galaxy. And in our universe, there are hundreds of billions of galaxies.

So there's untold numbers of possibilities. Surely there has to be advanced life somewhere. Our question really is, is it close enough to us that we could someday somehow make contact?

So there's untold numbers of possibilities. Surely there has to be advanced life somewhere. Our question really is, is it close enough to us that we could someday somehow make contact?

Well, light has a finite speed. So when we look at stars, the light from those stars was emitted a while back. So when we look at a star, for example, if a star is 100 light years away, the information we're measuring from that star came from that star 100 years ago. It just took that long to reach us. So we're time travelers in the sense that what we see are things that happened a long time ago.

Well, light has a finite speed. So when we look at stars, the light from those stars was emitted a while back. So when we look at a star, for example, if a star is 100 light years away, the information we're measuring from that star came from that star 100 years ago. It just took that long to reach us. So we're time travelers in the sense that what we see are things that happened a long time ago.

We know that our universe formed suddenly in a cosmic Big Bang event because we see the leftovers from that in the form of just uniform background radiation. Like after a fire dies down, you see coals burning out of lower temperature. We actually can almost see back to when the first stars and galaxies were born.

We know that our universe formed suddenly in a cosmic Big Bang event because we see the leftovers from that in the form of just uniform background radiation. Like after a fire dies down, you see coals burning out of lower temperature. We actually can almost see back to when the first stars and galaxies were born.

Only there's a little glitch right now because it appears that stars and galaxies formed sooner than we expected them to. They're very massive galaxies that are kind of bigger than for their age than they should be. We know that there's so many galaxies out there and that every galaxy appears to have a massive black hole at its center.

Only there's a little glitch right now because it appears that stars and galaxies formed sooner than we expected them to. They're very massive galaxies that are kind of bigger than for their age than they should be. We know that there's so many galaxies out there and that every galaxy appears to have a massive black hole at its center.

We know that even in between galaxies and in between stars, in between galaxies, in between galaxy clusters, it's not totally, totally empty. There's still some little tiny small amount of material everywhere.

We know that even in between galaxies and in between stars, in between galaxies, in between galaxy clusters, it's not totally, totally empty. There's still some little tiny small amount of material everywhere.

Very large. I think it's like tens of billions of light years across, like almost 100 billion light years. So big.

Very large. I think it's like tens of billions of light years across, like almost 100 billion light years. So big.