After those decades of false starts, Seager and her team have already succeeded in making the starshade seem like a real possibility. NASA recognized it as a “technology project,” which is astral-bureaucracy speak for “this might actually happen.” Today the starshade is a piece of buildable, functional hardware. Seager packs that single petal into a battered black case and wheels it, along with a miniature model of the starshade, into classrooms and conferences and the halls of Congress, trying to find the momentum and hundreds of millions of dollars that allow impossible things to exist.

“If I want the starshade to succeed, I have to help mastermind it,” Seager says. “The world sees me as the one who will find another Earth.” She has her intelligence, and her credentials, and her audience. She has her focus. But maybe more than anything else, Seager understands in ways few of us do that sometimes you need darkness to see.

Seager grew up in Toronto, wired in a way all her own. “Ever since I was a child, there was just something about me that wasn’t quite like the others,” she says. “Kids know how to sort through who’s the same and who’s different.” After her parents divorced, her father, Dr. David Seager, achieved a certain fame by becoming one of the world’s leaders in hair transplants. The Seager Hair Transplant Center still operates and bears his name a decade after his death. David Seager was besotted with his bright daughter and wanted her to become a physician.

Seager did her best to fit in. Sometimes she did; mostly she didn’t. Eventually, she gave up trying. She still talks breathlessly — “without enough modulation,” she has learned by listening to other people talk. She has never had the patience to invest in something like watching TV. “Things just move too slowly,” she says. “It feels like a drag.” She sleeps a lot, but that’s just a concession to her biology; she recognizes that she’s a more efficient machine when she’s rested. But if Seager’s apartness didn’t make her insecure, it also made her feel as though the expectations of others didn’t apply to her. “I loved the stars,” she says. When she was 16, she bought a telescope.

Friendless for most of her childhood, Seager eventually forged her way to her own vision of the good life. She found and married a quiet man named Mike Wevrick, whom she met on a ski trip with her canoe club. He had seen something in her that nobody other than her father fully saw; he saw her as special as well as strange. Later, she graduated from Harvard, an early expert in exoplanets. (51 Pegasi b was discovered just when she was searching for a thesis topic. “I was born at the perfect time,” she says.) She and Wevrick had Max and Alex; Seager was hired by M.I.T., and she and Wevrick and the boys moved into a pretty yellow Victorian in Concord, Mass. She took the train to work. Wevrick, a freelance editor, managed just about everything that didn’t involve the search for intelligent life in the universe. Seager never shopped for groceries or cooked or pumped gas. All she had to do was find another Earth.

Then, in the fall of 2009, Wevrick got a stomachache that drove him to bed. They figured it was the flu. Wevrick didn’t have the flu, but a rare cancer of the small intestine. They were told that the initial prospects were good, and he fought the cancer sufferer’s systematic fight. But while laws govern astrophysics, cancer is an anarchist. About a year after Wevrick’s diagnosis, he and Seager went cross-country skiing, and he couldn’t keep up. A few more terrible months passed, and he began writing out a methodical three-page list, practical advice for Seager after his death. It wasn’t a love letter; it was an instruction manual for life on Earth. By June 2011, he was 47 and in home hospice. Seager asked him how to get the roof rack that carried his canoes off the car. “It’s too complicated to explain,” Wevrick said. That July, he died.

The first couple of months after Wevrick’s death were weird. Seager felt a surprising sense of relief from the uncertainties of sickness, a kind of liberation. She didn’t care about conventions like money, which she had never needed to manage, and she took the boys on some epic trips. There are pictures of them smiling together in the deserts of New Mexico, on mountaintops in Hawaii. Then one day, she went into Boston for a haircut, got turned around and accidentally walked into a lawyer’s office next to the salon. Seager ended up talking to a woman inside. That woman was also a widow, and she told Seager that there would be a moment, as inevitable as death itself, when her feelings of release would be replaced by the more lasting aimlessness of the lost. Seager walked back outside, and just like that, the world came out from under her feet. She fell into an impossible blackness.