The Smallest Lights: Sara Seager on Finding Life Beyond the Sun

One of Sara Seager's earliest memories is the Moon. Her parents were divorced, and on weekend drives to her father's house the Moon seemed to follow the car, a glowing companion she could not explain. She grew up in Canada with a French picture book about the solar system that she loved to flip through, long before high-school physics could make the layout feel real. Lex Fridman, hosting the conversation, says the moment he understood Earth as a ball flying through empty space still feels both terrifying and awe-inspiring.

The memory that actually set her course was camping. Her father was from the UK, the sort of man who wore a tie on weekends, and camping was not in his comfort zone. One summer the family babysitter, a fourteen-year-old named Tom, insisted that camping was the thing they should try. Seager crawled out of the tent in the middle of the night, looked up, and saw a dark sky full of stars screaming at her. Her first thought was that it was mind-blowing: "Why wouldn't anyone have told me this existed?"

Lex's reaction to the same sky is almost the opposite. Looking up, he feels paralyzed by how little we know and how alone we seem, and the thought of mortality arrives immediately. Seager says that as a child, without the words for it, she felt that terror too. The fork is useful. Some people enter astronomy through beauty, others through dread, and both paths tend to end at the same question: is anyone else out there?

Asked whether humans are alone, Seager first resists, because scientists want numbers rather than speculation. Her scientific answer is that the tools are coming: telescopes that can study exoplanet atmospheres for water and for gases that do not belong. "Soon," for her, means a decade or two, not tomorrow, and a first result will be a hint rather than a verdict. Zoomed out, she believes life exists somewhere, because each galaxy holds hundreds of billions of stars and the universe holds hundreds of billions of galaxies. In our lifetime, though, we can only inspect neighbors a few, tens, or maybe hundreds of light-years away.

The stars worth a close look number maybe a dozen or two dozen, Seager says, once you restrict the sample to planets suitable for detailed follow-up on life. The surprise is that the very nearest star, Proxima Centauri, part of the Alpha Centauri system, appears to have a planet. It is about an Earth mass, and it sits in the habitable zone, the Goldilocks belt where liquid water could persist. If even the closest neighbor comes with a world of huge interest, planets cannot be rarities.

Habitable, here, does not mean a twin of Earth in every respect. Size, temperature, and atmospheric gases all matter, and Proxima is a small red star unlike the Sun. Lex points to NASA's Eyes on Exoplanets visualization as a way to feel the neighborhood. The larger point is still statistical: each bright point in a dark sky is someone else's sun, and the nearest of those suns already has a planet sitting where water might be stable.

Lex brings up the Drake equation, the famous framework for estimating how many radio-communicating civilizations might share the galaxy. Seager once met Frank Drake and asked whether she could recast his equation for exoplanet astronomy; he was fine with it. Drake's original list was never meant to spit out a magic number. It was a way to put the factors on paper, the same spirit as the movie *Contact* listening for a message. Her revision aims at a more achievable target: inhabited planets that show signs of life through gases in the air.

The early terms are things a given survey can actually count. How many stars can a telescope reach? The MIT-led NASA mission TESS, she notes, has about 30,000 red dwarfs in its sample. What fraction of those stars are "quiet," rather than so variable that they drown a transit signal? What fraction have orbits oriented so a planet crosses the star as seen from Earth? Evidence suggests those orientations are roughly uniform. Kepler already supplies an estimate for how often planets sit in the habitable zone around certain kinds of stars.

Two terms cannot be measured yet, and she keeps them separate on purpose. One is the fraction of those planets that actually have life. The other, a spectroscopy term, is the fraction whose life produces a gas that accumulates enough to detect. Humans exhale carbon dioxide, but Earth already has hundreds of parts per million of it, so our breath would not help a distant astronomer. A boring gas that life emits but geology also supplies is useless as a biosignature. Splitting the terms is a warning not to let the known hide the unknown.

Asked for her gut number on whether life will produce gases we can actually see, she refuses a single figure. On the worst days she puts it at about one percent; on the best days, around eighty percent. The swing is not a change of theory. It is what she thinks honesty looks like when the last terms in the equation are still blank.

The work she calls her life's work for the next twenty years starts with a basic assumption: life uses chemistry. On Earth, organisms eat, breathe, store energy, and dump a waste product into the air. If some creature were more like a windmill, harvesting mechanical energy without storing it, it might emit nothing at all, and the method would fail. The second problem is harsher. A gas can be produced and then destroyed at once by ultraviolet light or by oxygen, which she calls incredibly destructive to many molecules.

Skepticism is part of the job. Plenty of non-living processes can make gases, and a growing field now tries to rule those out before anyone claims a detection. The idea itself is old: almost a century ago people were already talking about atmospheric gases as a life indicator, and the conversation always comes back to oxygen. Earth's air is about 20 percent oxygen by volume, and without plants and photosynthesizing bacteria it would be virtually empty of the gas.

If a nearby civilization pointed the kind of telescopes Seager wants to build at the Sun, they would see Earth's oxygen. They would not be 100 percent sure it was biological, but after exhausting abiotic scenarios they would be left with a strong hint. That word is load-bearing. A first detection will not be a victory lap; later generations will have to do more careful work, maybe even send a probe. When she started in exoplanets, people did not believe anyone would ever study an exoplanet atmosphere. Now dozens have been studied, and hundreds of researchers work on the problem.

An exoplanet is simply a planet orbiting a star other than the Sun. The first one found around a Sun-like star arrived in the mid-1990s and was immediately controversial. The only planetary system anyone knew was our own, and in that system Jupiter sits far from the Sun. The new world hugged its star so tightly that many people refused to believe it was a planet. Thousands of exoplanets are now known, and it appears that nearly every star has at least one. The line between completely crazy and mainstream research, Seager says, is constantly shifting.

When she started, exoplanets were not considered a career. Today at least a thousand people work in the field, probably more. What changed looks mundane: better detectors, computers, and digital cameras instead of film. The transit method watches a star's brightness over time and waits for a characteristic dip as a planet crosses. From the ground, small telescopes, almost glorified telephoto lenses, can reach about one percent precision, enough for Jupiter-sized worlds roughly ten to twelve times Earth's size. Smaller planets need space telescopes.

The planet itself is usually less than a pixel. Astronomers are not photographing continents; they are measuring a star's tiny flicker and inferring a companion. Transits are geometrically rare, so surveys stare at tens of thousands to millions of stars at once, which is why the computers matter. Seager served as deputy science director for TESS, with the job of getting planet candidates out to the community. NASA's policy is to make the data public as soon as possible. In practice the files are hard to work with, which is why Planet Hunters crowdsources the light curves, and amateurs often catch faint signals the team's software missed.

The planets being found are not Earth twins. They are simply the easy ones. There are worlds the size of Earth on orbits tighter than Mercury's, so close that a year can last less than a day. These hot super-Earths are heated enough, researchers think, to melt rock, leaving lava lakes instead of rivers. Molecules needed for life would not survive those temperatures.

Stranger still, the most common planet type known so far sits between Earth and Neptune, two to three times Earth's size, with no analog in our solar system. Seager's image is walking into a forest, meeting an animal nobody has ever seen, and then discovering it is the most common creature in the woods. One hopeful guess is that these mini-Neptunes are water worlds: scaled-up versions of Jupiter's icy moons, more than half water by mass.

Why the obsession with water? All life on Earth needs it, and astrobiology has long said follow the water. Zoomed out, the consensus is that some liquid is required so molecules can meet and react. There are not many candidate liquids on a planetary scale. Molten rock is too hot. Titan's ethane and methane lakes are too cold, and such worlds are too dark to study as exoplanets. Water covers the widest range of temperatures and pressures. A less intuitive corollary follows: water vapor in the air of a small rocky planet almost certainly means a liquid reservoir below, because ultraviolet light splits water and the hydrogen escapes like a child's helium balloon. Without an ocean to replenish vapor, the atmosphere would be wrung dry.

Asked which nearby systems she is most hopeful about, Seager says she is hopeful that every nearby star has a living planet, because the field almost has to believe that in order to move. She calls it the dream condition: life is abundant; a wet world with continents also has the ingredients; and if the ingredients are there, life will form. That is a working bet, not a result.

On intelligence, Seager thinks life had a chance to become smart somewhere in the huge census of stars, but she is not convinced it is nearby. If intelligence is hard, it will, by definition, be far away. Lex notes that evolution can take paths we would not recognize as minds at all. From her toolkit she still loves SETI, the search for a transmitted message. She also notes a humbling fact: Earth has built no structure, not even the Great Wall of China, that a distant civilization could see. Why would another species build a giant billboard for us?

Messaging extraterrestrial intelligence, or METI, makes some people nervous: do you want to tell them you are here, or wait until they call? Seager likens it to the dating-game pause before picking up the phone. The joke has a sting. If everyone only listens, nobody makes progress. Voyager is out there with a record of Earth, but on a cosmic scale it is barely moving. Distances in astronomy, she reminds Lex, are all relative.

Could you at least tell, from gases, how busy a planet's residents are? Seager zooms out to oxygen again. When cyanobacteria learned photosynthesis billions of years ago, they reengineered the entire atmosphere; 20 percent oxygen is planetary-scale engineering, and it was poisonous to much of what was already alive. Humans are not operating at that scale. Lex says we are destroying the planet; Seager corrects the phrasing. We are destroying the environment that many species, including us, depend on. The Earth itself will remain. Except perhaps in a nuclear war, it would be hard to tell from far away that anything had happened.

Public excitement about aliens, Seager thinks, is excitement about discovery, which she welcomes. She does not really follow the world of people who think visitors are already here. They follow her. Scientists in her field get a stream of messages: I have something you should see; the aliens are already here. She is less allergic to the topic than some colleagues. If aliens had shown up, they would be a very hard scientific phenomenon to study. Typical scientists still want major evidence, not a sketchy picture, and she agrees that the evidence is not there.

Conspiracy talk gets a dry reply from a colleague who works with NASA and with government: trust me, they could not hide it if they tried. The people she meets at NASA, in academia, even in government, are "curious descendants of apes" who want to discover things, not bury them. Most would love to leak a finding. In public talks she used to put up the *X-Files* poster that says "I want to believe." After one talk a nervous couple invited her to lunch. They were kind, sincere, and convinced that aliens had abducted the wife, left her unable to have children, and implanted something behind her ear. They only wanted to tell the story. Lex's gloss is that the human mind may be as strange as any extraterrestrial, and perhaps entangled with the same mystery.

Scale still does the philosophical work. The number of stars is on the order of 10 to the 20, a one followed by twenty zeros. Planetary systems may be dynamically full, like a coffee cup you do not leave with a single drop: disks of gas and dust form many worlds, some get kicked out, a packed system remains. Other stories have giant planets migrating inward and clearing siblings as they go. True Earth analogs, Earth-sized worlds in Earth-like orbits around Sun-like stars, are still too rare in the catalogs for a hard rate. Some people extrapolate as many as one in five Sun-like stars.

SpaceX and other commercial launchers are, from Seager's point of view, straightforward good news. Reusable rockets lower the cost of getting to space, and her field needs cheaper access. Boots on Mars is mostly a human story. Exploration is part of what it means to be human, and pure exploration has a record of dropping useful accidents: lasers in grocery scanners and surgery, GPS that nobody set out to invent as a consumer map. She supports going to Mars, but the honest motive is wanting to look. She is convinced something interesting will come of it. She just does not know what.

Visiting an Earth-like planet around another star is much harder. The nearest Earth-mass worlds orbit Proxima Centauri, more than four light-years away, around a small red star. Humans cannot travel at light speed; conventional ships would take tens of thousands of years. Lex mentions multigenerational vessels and the film *Passengers*. Seager floats a colder option: maybe we do not send people. Maybe we send raw biological materials and instructions to print humans, or other life, on the other end. Labs already print liver cells and beating heart cells. Getting a body to another planet is so hard that printing one might be easier.

Lex follows the thought to a grim image: a successful ship, perhaps carrying non-human life, finally meets another intelligence, and by the time those beings reconstruct its origin, human civilization is gone. Seager does not argue. The near-term giant project she actually wants is more modest and still enormous: finding Earth-like planets and reading their air, a billions-of-dollars job, because an Earth is so small, so light, and so faint beside its star that it is a needle in a haystack, but worse. That search needs sophisticated space telescopes.

Directly seeing an Earth-like exoplanet means blocking a star that outshines the planet by about ten billion times. Starshade is a specially shaped screen, on the order of 30 meters across, attached to a spacecraft that formation-flies with a space telescope far from Earth. The idea was conceived in the 1960s and revisited every decade until engineers began to think they could actually build it. Seager's analogy for the alignment is asking a friend to hold up a dime five miles away, perfectly on your line of sight.

Shape is the whole trick. A circular or square screen would not go dark. Starlight behaves like a wave and bends around the edge, throwing concentric ripples brighter than the planet, like a pebble tossed in a pond. The 1960s math, one family of solutions to diffraction, makes the shade a giant flower. Light that bends around the petals interferes with itself and leaves a shadow smooth to one part in ten billion, with the leftover waves pushed to the outer edges. Remaining problems were attacked as a technology to-do list: formation flying at tens of thousands of kilometers became a sensing problem, because a beacon's beam spreads so wide; petal edges have to be razor-thin so sunlight does not bounce into the telescope. Most of the hard items, she says, are now checked off. When she first evangelized the project it sat on the giggle-factor side of the crazy-versus-mainstream line.

Two other projects are not hers, and she loves them anyway. One would send a telescope to 500 times the Earth-Sun distance, farther than the Voyagers, and use the Sun itself as a gravitational lens. Einstein saw that mass warps space, so light that should go straight curves; galaxy clusters already lens background galaxies, and microlensing events magnify stars. Lined up behind the Sun, a telescope might return something like a ten-by-ten-pixel, or 100-pixel, image of a planet in another system. The flight would take on the order of 25 years, plus the time to send data home, and you need one telescope per target because each star sits behind the Sun in a different direction.

The second is Breakthrough Initiatives' Starshot, talked about as tiny StarChips. The plan is to launch thousands of smallcraft, like turtle hatchlings, knowing not all will make it. Each deploys a solar sail a few meters across. A ground-based bank of gigawatt-class lasers would flash the sails and accelerate them to about one-twentieth the speed of light, dropping the trip to the nearest star from tens of thousands of years to about twenty. They would be going too fast to slow down: a flyby, a few snapshots, a radio beam home. Breakthrough listed 19 concrete challenges, from buying land and clearing airspace for that much laser power to finding a sail the lasers will not vaporize. The money on the table will not finish the project, she says, but it plants a seed and nudges the line.

Seager's memoir opens on a rogue planet, PSO J318.5-22, that lurches across the galaxy like a rudderless ship, wrapped in perpetual darkness, its skies raining molten iron. The loneliness of that image is the point. We want to belong to a family, a community, a solar system with a warm star; a world with no sun feels like exile. Some rogue planets form as failed stars, born without a parent. Others are kicked out of their systems like billiard balls. She thinks there may be as many of them as there are stars. The book's title, *The Smallest Lights in the Universe*, has a double meaning. On the face of it, other Earths are so dim beside their massive stars that hunting them is a hunt for the faintest lights. The other meaning is the glimmer you get once you have fallen off a cliff into despair.

Her husband, Mike, got sick, and the medical system treated the early signs as nothing. He had random stomachaches that came and went, the kind of complaint doctors blow off, until he was in bed and barely able to move. Tests then showed a 100 percent intestinal blockage: nothing could get in or out. She felt like a child, unsure whether the doctors had no idea what they were doing or were watering down the truth. Her father had once had strange pains that turned out to be pancreatic cancer, a diagnosis that often arrives after the disease has already spread, because the pancreas sits where you cannot feel it. Mike was eventually diagnosed as terminally ill.

That summer she started asking everyone she knew the same question: if you had one year to live, what would you do differently? Lex's answer was unusual. He has long tried to be ready to die any day, so that if the present conversation were his last, he would still be glad it happened. Seager says she never got that answer from anyone else. People who could take action almost never did. They ran out of time in a different sense: they postponed the family hours and the career change until a pressing end of life, and then, like Mike with his bucket list, they got too sick too fast to finish. After he died, most of what she had been doing felt meaningless. She settled on a remaining purpose: devote the rest of her life to finding another Earth, and to finding out that we are not alone.

Asked what she learned about love from losing it, she says she learned to love the things and people she already has, and not to let the little things bother her as much. Grief, in her image, is an ocean. Tiny islands of light appear: children laughing one day, colleagues pulling her into a project, later a group of widows her age, all with children, in her town, gatherings that were morose and joyful at once. Eventually the ocean shrinks and the islands become continents with lakes. How do you rediscover the beauty of life? Stay open, she says, and get out there. A friend, after her father died, had asked why humans evolved to take death so harshly. A species that did not care would be a different world. Fear of loss and the wish for connection, in that light, are the same coin.

What would bring her joy while she is still here is finding another Earth: an Earth-like planet around a Sun-like star, with signs of water and gases that do not belong, enough of a door opening to fuel the next generation. She still stands behind Starshade, but she also names a nearer path. Planets around small red dwarfs are not Earth twins so much as Earth cousins, and there is a chance, in the next ten or twenty years, of establishing that some of them have water and signs of life.

Asked for books that shaped her, she names Lois Lowry's *The Giver*, a young-adult novel she first met after immigrating, when her English was still shaky. It follows a boy in a cleaned-up utopian society who is chosen to receive the harsh memories the community has deleted, and it jolted her into asking whether she was just going with her society's flow. Lex is surprised she picks a children's book; she is surprised he has heard of it. The right book at the right age, they agree, can change a life's direction, and teachers are undervalued for holding that position.

She also recommends the film *Picture a Scientist*, about women and women of color in science, which she found shocking and sobering. One message is that people tend to like people like themselves, and so they exclude a lot of people and a lot of ideas; saying you want to be more diverse does not solve a nearly impossible problem, and the film opens eyes in a way slogans do not. Third is a quieter book, *Sleeping Island* by P. G. Downes, pressed on her at a Wilderness Canoe Association gathering in Toronto when she was in her early twenties. Downes, a teacher, spent summers canoeing in northern Canada, writing in the 1940s or 1950s about a late-1930s trip through land that still had no maps. She went and did the same: to the end of the road, canoe in the water, rivers with rapids, lakes, portages. Part of the attraction was fear. She wanted to live on the edge and persevere.

Her advice for a young person is tough in a small way: find something you love doing that you are also very good at. The stars have to align, because the skill and the daily love have to overlap, or the work becomes a slog. In her own case it was a long slog first. She was good at high-school math and science and loved astronomy; the day she realized you could be an astronomer for a job is still one of the best days of her life. Physics did not come easily, and graduate school took a very long time. What carried her was later knowing she had both the skill and the love. Asked why we are here, she wishes she had a better answer. The meaning, as she puts it, is almost wishing there were a meaning, and that we already knew it.