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Q&A: Sara Seager, exoplanet explorer

27 February 2019

In a field that has gone from controversial to crowded, the MIT planetary scientist is a leader in the search for biosignatures in the atmospheres of extrasolar worlds.

As a graduate student in the 1990s, Sara Seager jumped at the chance to devote herself to research in the new and controversial area of exoplanets. Her risky project—trying to understand the atmospheres of hot Jupiters—and the broader effort to find and interpret other planetary systems have exploded into a hot field, and Seager is still in the thick of it.

Sara Seager
Credit: Justin Knight

After stints at the Institute for Advanced Study and the Carnegie Institution of Washington, in 2007 she joined the MIT faculty in physics and planetary sciences. She is currently the deputy science director on the Transiting Exoplanet Survey Satellite (TESS), which is scouring the skies for small exoplanets, and a key mover behind starshade, a plan to occult stars to reveal their planets. (See the story in Physics Today, March 2019, page 24.)

For Seager, searching for exoplanets is a journey of exploration. “It’s not that I want to meet the alien—of course we all want to meet the little green humanoid—but it’s that sense of ‘What is out there? Who is out there?’ ” she says. “The fact that we can use physics and observations and tie them together to try to infer what’s out there is so exciting.”

PT: What drew you to astrophysics?

SEAGER: In high school, we did an experiment where we had to calculate the angle to shoot a spring with a measured spring constant through a hole in a big wooden board that the teacher was holding at a specified distance away. We had to choose and apply the right equation. About a third of the kids got it through. Mine was toward the end and it went through. It was a eureka moment: You can write down an equation, measure something, work it all out, and make a prediction—and it works. I was sold. I was pretty much astonished at that.

My dad always told me to “get a job, support yourself, and don’t rely on any man.” I could be a dermatologist, he said, and work part-time, make lots of money, have time for children, and do it all. I didn’t like to cross him, so I thought I had to go to medical school. But I have a terrible memory. And in medicine, at least in premed and medical school, you have to memorize whole books of stuff, so I didn’t think that was going to work out.

After my first year at the University of Toronto, I narrowed it down and took math and physics. I had always loved the stars, and I loved equations, and then I learned you could combine them as a job and solve the mysteries of the universe. I eventually realized I was lucky to have something I loved doing and was good at, and I should pursue it.

PT: How did you get into exoplanets?

SEAGER: I started graduate school at Harvard in 1994. Exoplanets were not a thing yet. For my master’s project I worked on the problem of recombination in the early universe. But when I wrapped that project up, it was solved. It was kind of a dead end. So I needed to find a new project for my PhD.

In the meantime, the first planet around a Sunlike star had been found in 1995, followed by several more. They were planets about the mass of Jupiter but with very short periods of just a few days.

Seager manages the identification of exoplanet candidates in data from the Transiting Exoplanet Survey Satellite, illustrated here. Credit: NASA-GSFC

My adviser, Dimitar Sasselov, was very excited. But like a lot of advisers, he didn’t have time to work on a new project. So he said, “Why don’t you work on the hot Jupiters?” Specifically, I was to try to understand what the atmosphere might be like. Imagine a Jupiter, which is at 5.2 astronomical units [in our solar system], suddenly at just 0.05 AU from its star. What does the atmosphere look like? Are there compositional signatures?

My adviser had Fortran code for how two binary stars interact that I heavily modified to work with a star and a hot Jupiter. It’s the radiation you care about: How does the radiation from the star impinge on the planet atmosphere and alter the atmosphere’s temperature profile or the radiation field? It was brand-new territory, open exploration. Usually people didn’t give students risky projects like that.

PT: Were you aware that the project was risky?

SEAGER: Yes. At conferences I would see students I knew from other universities and they would say, “You know, Sara, I don’t know if you should be working on this. My adviser says they are stellar pulsations, not planets.” So there was open skepticism in the community. But my adviser believed they were exoplanets. And there was another professor at Harvard in particular, Bob Noyes, who believed they were real, so I stuck with it.

PT: What made you willing to take on such a risky topic for your PhD?

SEAGER: I really didn’t like grad school. It was my first time away from home. I felt isolated. As an undergraduate, your goal is the next homework, the next step. You get a long break at Christmas, and although you work in the summer, it’s not school. Grad school is like infinity. You work for a really long time, and you don’t get much positive feedback. My programming skills were horribly weak. Now I love programming. I became really good at it.

I was always a bit of a risk taker, and I didn’t feel constrained. I thought it was a really cool project. And they tell you if have a PhD in the physical sciences, you will always be able to get a job. So, my attitude was that I had nothing to lose.

PT: When did you realize that the risk was paying off?

SEAGER: I was at the American Astronomical Society meeting, where around the time of your PhD defense you can give a 15-minute talk. A few people came up to me after my talk, and they were really excited. One wanted me to join a proposal he was working on for a small space mission to measure hot-Jupiter phase curves. It was a surprise—I had been working with my adviser in a bit of vacuum. It no longer seemed like the wrong choice when people wanted my skills.

Eventually the arguments over whether what we were observing were exoplanets went away. That is how science should work: Skepticism and predictions for one explanation or another that eventually get verified or refuted.

PT: In 2013 you were recognized with a MacArthur “genius” Fellowship. What did you do with it?

SEAGER: I was really excited to get it, and it came at a wonderful time. My first husband had died, which was a real blow to my life and to my career. All of a sudden I had to do everything, and I couldn’t keep it together. I was burning through cash, and I was pretty depressed. I had even told my dean I was thinking of quitting.

It’s very hard to be a professor. Your intellect is your capital. But you don’t have your intellect if you are focused on the laundry, and getting the kids to school, and this and that. I stopped giving invited talks, because I’d have to pay a babysitter 24/7 while I traveled. It didn’t make sense financially. Even when I was at work, I still had to think about dinner. I would be juggling all this in my head.

And that’s when the MacArthur came. It got me through those hard times, right when I needed a big cash injection. They want you to use it so you can keep thinking. That’s what I did. I used it on household help.

PT: Have you experienced sexism? Do you have advice for other women in physics?

SEAGER: As a student, I was oblivious to sexism in academia. When I was a postdoc, another team decided to use work I had published to make a proposal for the Hubble Space Telescope. Lo and behold, they discovered the first exoplanet atmosphere using my work on what to look for. The detection of sodium also confirmed that we had the basic chemistry and physics right. Less sodium than expected was detected, indicating the possible presence of clouds.

It was an early win—usually things are not found that fast after they are predicted. The negative was that the team didn’t invite me to be on the proposal. They invited an older man who hadn’t yet worked in the niche area, because they thought their proposal would be more successful; at least that’s what I was told. I was furious. It was totally the old boys’ network.

A petal of a flower-shaped starshade prototype takes shape at NASA’s Jet Propulsion Laboratory. Credit: Doug Lisman

Now I am recognized and respected, but I often see sexism in the field. I do see problems with younger women. For example, I see them come in for faculty job interviews unprepared compared with their male peers. I try to be first on their meeting list and to offer them advice, like “Has anyone told you that during your talk, there is always one jerk in the audience who will try to derail you? How are you going to handle that?” And I remind them that if we invited them, we think very highly of them so far. We want them to succeed, and they have to have confidence.

PT: Did you ever experience impostor syndrome?

SEAGER: Yes, when I first started graduate school. Later, someone gave me the advice to put my accomplishments in a binder and to flip through the binder every night. Confidence has to get deeply embedded in your consciousness. I sometimes pass on that advice to people.

PT: What are you excited about in your work now?

SEAGER: As the field matures, there are fewer new things to do. So I help manage TESS, which is an MIT-led NASA mission. My job is to manage the team that identifies the planet candidates that go out to the community for follow-up.

I have also led or co-led conceptual studies for future missions. One of my proudest accomplishments was leading a NASA-sponsored study that helped bring starshade—the idea of occulting a star to reveal its planets—from being almost a fringe concept to one that is now mainstream and a frontrunner for the next decadal survey. All of my activities are with the aim of being the starshade principal investigator, or at least making sure such a mission happens.

In parallel, I am trying to push research frontiers on biosignature gases. Those are potentially detectable gases that can be created by life and could accumulate in a planet’s atmosphere. My team decided to evaluate all the molecules that are in gas form at Earth’s surface conditions and are made from the six main life-related molecules: carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. From the list of 14 000 possible gases, about a quarter of them are made by life on Earth.

Beyond this factoid, we found something fascinating. We are starting to work through identifying the molecular fragments that life avoids. For example, there are almost no nitrogen–sulfur bonds in life’s products, despite the commonality of both atoms in life’s products. It turns out that most N–S bonds are very reactive in the presence of an S–H bond, and S–H bond–containing compounds are a key for life. It appears that life could have a metabolism based on S–H or on N–S bonds, but the two are incompatible. If we were to encounter life on another planet that for some reason relied on N–S bonds, we might actually dissolve each other. We would be poison to each other. So, in going through all the molecular fragments in gases, liquids, or solids rarely produced by life, we are hoping that it will help us understand something about the origin and evolution of life.

PT: It seems that many of your colleagues are optimistic about spotting biosignatures.

SEAGER: That’s good to hear. I’m less optimistic. I’m confident we will find some signs, but not that we will be able to robustly attribute them to life.

PT: What keeps you motivated about exoplanets?

SEAGER: The thought that out there somewhere, not too far from here, around a nearby bright star, there is another planet like ours. And this other planet may be able to support life as we know it. That’s what keeps me going.

Editor’s note, 1 March: The article was updated to correct the distance of Jupiter from the Sun.

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