Sara Seager is a planetary scientist at MIT, known for her work on the search for exoplanets.

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OUTLINE:
0:00 - Introduction
5:32 - Falling in love with the stars
9:55 - Are we alone in the universe?
15:27 - Seager equation for number of habitable planets
27:48 - Exoplanets
34:44 - Earth-like exoplanets
40:43 - Intelligent life
52:34 - Number of planets per star
55:09 - Space exploration
57:36 - Traveling to Proxima Centauri
1:00:52 - Starshade
1:07:34 - Using the sun as a gravitational lens
1:09:44 - Starshot
1:12:45 - Rogue planets
1:15:44 - The Smallest Lights in the Universe
1:30:15 - Book recommendations
1:37:48 - Advice for a young person
1:39:29 - Meaning of life

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The following is a conversation with Sarah Seager, a planetary scientist at MIT known for her work on the search for exoplanets, which are planets outside of our solar system.

She's an author of 2 books on this fascinating topic.

Plus, in a couple days, August 18th, her new book, a memoir, called The Smallest Lights in the Universe, is coming out.

I read it and I can recommend it highly, especially if you love space and are a bit of a romantic like me.

She weaves the stories of the tragedies and the triumphs of her life with the stories of her love for and research on exoplanets, which represent our hope to find life out there in the universe.

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As a quick side note, let me say that extraterrestrial life, aliens, I think represent our civilization longing to make contact with the unknown, with others like us, or maybe others that are very different from us, entities that might reveal something profound about why we're here.

The possibility of this is both exciting and at least to me, terrifying, which is exactly where we humans do our best work.

If you enjoy this thing, subscribe on YouTube, review it with 5 Stars on Apple Podcast, support it on Patreon, or connect with me on Twitter at Lex Friedman.

As usual, I'll do a few minutes of ads now, and never any ads in the middle that could break the flow of the conversation.

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And now, here's my conversation with Sarah Seeger.

I remember watching the moon, and I was in the car with my dad, who my parents were divorced, and he was driving me and my siblings to his house for the weekend.

Yeah, so like looking up at the sky and there's this glowing thing.

How do you make sense of the moon at that age?

At age, like age 5, there's just no way you can.

I think it's 1 of the great things about being a kid.

It's just that curiosity that all kids have.

You know, I was thinking, because there's these almost out there ideas of that our earth is flat, floating about on the internet.

And it made me think, you know, when did I first realize that the earth is like this ball that's flying through empty space?

It's hard because we live in our frame of reference here on this planet.

None of us are lucky to go to see the curvature of Earth.

I mean, do you remember when you realized, understood like the physics, like the layout of the solar system?

Is it, Was it like, did you first have to take physics to really, like high school physics to really take that in?

I grew up in Canada where French is supposedly taught to all of us English speaking Canadians.

And it was this book in French, but it was about the solar system.

It's hard to say how much you or I understand when we're kids, but it was really a great book.

When did you first learn about the stars?

Like I do have this very incredible distinctive memory.

Now my dad was from the UK, and he was the type who you'd find wearing a tie on weekends.

So camping was not in his fear, his comfort zone.

And we went camping, because Tom said camping's the thing, we should try it.

And I just remember, I didn't aim to see the stars, but I walked out of my tent in the middle of the night and I looked up and wow, so many stars.

The dark night sky and all those stars just like screaming at me.

Honestly, like my first thought was this is so incredible, mind-blowing.

Why wouldn't anyone have told me this existed?

Have you had an experience like that with anything?

I mean, I don't know if maybe you can tell me if it's the same.

There's a few robots I've met where I just fell in love with this.

Is anyone else seeing that here in a robot is our ability to engineer some intelligent beings, intelligent beings that we could love, that could love us, that we can interact with in some rich ways that we haven't yet discovered.

Like almost like when you get a puppy instead of a dog.

And there's this immediate bond and love.

And on top of that ability to engineer it, it was, you know, I had to just pause and hold myself.

I imagine, I don't have kids, I imagine there's a magic to that as well, where it's a totally new experience.

Well, yeah, the stars though, unlike kids or the puppy, it's only a good thing.

Like, to me when I look at the stars, it's almost paralyzingly scary how little we know about the universe, how alone we are.

I'm not sure if it's just a matter of perspective, but it feels like, wow, there's billions of them out there and we know nothing about them.

And then also immediately to me somehow mortality comes into it.

I mean, how did that make you feel at that time?

I think as a child without articulating it, I felt that same way.

You're a scientist, an ex-world-class scientist, planetary scientist, astronomer.

Now, I'm a bit of an idiot who likes to ask silly questions.

So some questions are a little bit in the realm of speculation, almost philosophical, because we know so little.

And 1 of the awesome things about your work is you've actually put data and real science behind some of the biggest questions that we're all curious about, but nevertheless, many of the questions might be a little bit speculative.

So on that topic, just in your sense, do you think we're alone in the universe, human beings.

Well, Lex, the funny thing is, is that as a scientist, I so don't even want to answer that.

Yeah, We naturally resist that because we want numbers and hard facts and not speculation.

It's a great question and it's 1 we all wonder about.

But I have to give you the scientist's answer first.

Which is, we'll have the capability to answer that question.

What do you so much happened in the last hundred years?

Right, right, and there's a difference right if it's 10 years or 20 years or a hundred years.

Yeah, there's a difference in that well soon could be a decade or 2 decades.

And the journalists usually don't like that or the people want like tomorrow they want the news.

But what it's going to take is telescopes, space telescopes, or very sophisticated ground or space telescopes to let us study the atmospheres of other planets far away and to look what's in the atmospheres and to look for water, which is needed for life as we know it to look for gases that don't belong that we might attribute to life.

So we have to do some really nitty gritty astronomy.

So the promising way to answer this question scientifically is to look for hints of life.

That's where like many of your ideas come in of what kind of hints might we actually see about this life.

And I like the word you chose, hint, because it's gonna be a hint.

It's not gonna be a 100% yay we found it.

And then it will take future generations to do more careful work to hopefully even find a way to send a probe to these distant exoplanets and to really figure this out for us.

The zoomed out big picture is yes, I believe absolutely there is life out there somewhere Because the vastness of the universe is incredible.

When we look at the night sky, if you can go to that dark sky, you can see many, many hundred or even if you have good eyesight and you're somewhere very dark, you could see thousands of stars.

But in our galaxy, we have hundreds of billions of stars, and our universe has hundreds of billions of galaxies.

So think about all those stars out there.

And even if planets are rare, even if life is rare, just because the number of stars is so huge.

Things have to come together somewhere, someplace in our universe.

CBT Yeah, it's so amazing to think that somebody might be looking up on another planet in a distant galaxy.

have to interrupt your reverie and get back to in our lifetime at least, the short term.

We only have the nearest stars to look at.

It's true that there are so many stars, so many hosts for planets that might have life.

But in the practical question of will we find it, it has to be a star quite close to Earth, like a few light years, tens of light years, maybe hundreds of light years.

And by the way, you've introduced me to a tool of Eyes on Exoplanets, I think, that NASA's put together.

But anyway, can you give a sense of who our neighbors are?

We're talking about 5, 10 stars that we might actually have a chance to zoom in on?

I'm talking about maybe a dozen or 2 dozen stars.

with planets that look suitable for us to follow up in detail.

But 1 thing that's really exciting in this field is that the very nearest star to Earth called Proxima Centauri, It's part of the Alpha Centauri star system.

But Proxima Centauri appears to have a planet around it that's about an Earth-mass planet in the so-called habitable zone, or the Goldilocks zone of the host star.

Like out of all the stars out there, even the very nearest star has planets and has a planet of huge interest to us.

What does it mean to be maybe possibly habitable?

How does, you know, what we know about gases and what kind of things are necessary for life, what are the factors that make you think that it's habitable?

And by the way, I mean, maybe 1 way to talk about that is people know about the Drake equation, which is a very high level, almost framework to think about what is the probability that, correct me if I'm wrong, that there's life out there.

And intelligent life, I think, I don't know.

But then you have an equation named after you now, which I think nicely focuses in on the more achievable and interesting part of that question, which is on whether there is habitable planets out there, or how many, I guess, habitable planets are.

the funny thing is was 1 time I met Frank Drake, and I asked if he minded if I took his equation and kind of revamped it for this new field of exoplanet astronomy.

I'm not sure if he'd actually read the stuff about my equation, but he was cool with it.

Okay, so I just said like 15 different things, but maybe, can you tell from your perspective what is the Drake equation and what is, sorry, the Seeger equation?

Well, the Drake equation, as you said, it's a framework.

It's a description of the number of civilizations out there of intelligent beings that are able to communicate with us by radio waves.

So if you think of the movie Contact, you've seen Contact, right?

It's an active field of research, listening to other stars at radio wavelengths, hoping that some intelligent civilizations are sending us a message.

And the Drake equation came like at the start of that whole field to put the factors down on paper to sort of illustrate what is involved to kind of estimating.

And there's no real estimate or prediction of how many civilizations are out there, but it's a way to frame the question and show you each term that's involved.

So I took the Drake equation, and I called it a revised Drake equation, and I recast it for the search for planets by more traditional astronomy means.

We're looking at stars, looking for planets, looking for rocky planets, looking for planets that are the right temperature for life, looking for planets that might have life that outputs gases that we might detect in the future.

It's the same spirit of the Drake equation.

It's not gonna give us any magic numbers.

It's not gonna say, hey, here's exactly what's out there.

It's meant to kind of guide, guide of where we're going.

Although the Drake equation did, I mean the initial equation proposed actual numbers for those variables, right?

Oh yes, the equation proposed numbers, and you can still plug your own numbers in.

And there's this really cute website that lets you, for both the Drake and my revised equation, plug in some numbers and see what you get.

So okay, so what are the variables, but maybe also what are the critical variables?

So in my equation, I set out to what are the numbers of inhabited planets that show signs of life by way of gases in the atmosphere that can be attributed to life.

first thing I say is what are the number of stars available?

And it's not those trillions and trillions of stars everywhere.

It's what are available to like a specific search.

And so, for example, the MIT-led NASA mission TESS is surveying the sky looking for all kinds of planets, but it can also, It also has stars.

So we just take a number of stars that a given survey can access.

Then I wanted to know what kind of stars are quiet.

I called it fraction of those stars that is quiet.

In the case of TESS, the way it's looking for planets is planets that transit the star.

They go in front of the star as seen from the telescope.

But it turns out that some stars are very active, they're variable, And they brighten and dim with time, and that interferes with our observation.

So the transiting planet, so you're really looking for a black blob, essentially, that blocks the light.

We're looking for a black blob that blocks the light.

And then trying to say something about the size of the planet from the frequency of that black blob's appearance and the size of that black blob, that kind of thing.

CM1 Yeah, but let's just say that out of all the stars there are accessible to whatever telescope, some of them are just bad.

For whatever reason, you're not gonna be able to find planets around them.

So I need to know the fraction of those that are good.

So again, we have the number of stars, the fraction of them that we can actually find planets around.

1 describes how quiet they are, and 1 is if we can find a planet around that star.

These transiting planets, for example, not all planets transit because the planet would have to be orbiting that star in this kind of plane as viewed from you.

But if a star is, for example, orbiting in the plane of the sky, it will never transit.

So in that case, we have to have a fraction that takes into account that kind of geometric factor.

LRW And hopefully, I mean, you can assume that it's uniformly distributed, hopefully.

LS Yes, we can assume and there's evidence that it's uniformly distributed, yes.

So then the next, so all of these factors so far, number of stars accessible to whatever telescope you're thinking about, how many stars are quiet, fraction of stars that are quiet, fraction that are observable in this case for the geometric factor.

There's 1 more term in the Seeger equation we can measure.

I call it fraction of planets in the habitable zone.

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Sara Seager: Search for Planets and Life Outside Our Solar System | Lex Fridman Podcast #116