The Life in Our Stars
Seager figured that one day not too far in the future, astronomers would be able to spot small, rocky planets like Earth orbiting at a comfortable distance from their stars. When they did, Seager wanted to make sure that scientists had a reliable way to scour the planets’ atmospheres for signs of life. Taking Earth as our one and only example, she knew that living organisms need liquid water—and that they either produce or expire molecular oxygen, carbon dioxide, and methane. (Life created all the oxygen in Earth’s atmosphere by photosynthesis.) If scientists found these elements—known as biosignatures—in the atmospheres of other planets, they’d have good reason to believe that life was there, too.
But back in the late 1990s and early 2000s, devising ways to study exoplanet atmospheres seemed premature at best. At worst, it sounded ridiculous. Other astronomers derided exoplanet research as “stamp collecting,” academic slang for small-time, useless pursuits. “I was on the faculty market and nobody would hire me,” Seager says.
Still, Seager forged ahead. She credits her father, a physician, with fostering her intellectual curiosity and giving her the determination to follow it. Her parents had divorced when Seager was in grade school, and her dad used his weekends with his children to augment their education in the arts and sciences. He was the first one to show her the stars and the moon through a telescope at a star party hosted by an amateur astronomy club. “When I was small, my dad took me and my siblings on a camping trip. I stepped out of the tent in the middle of the night and looked at the sky,” Seager says. She gasps a little, remembering, and clasps her hand to her chest. “I couldn’t believe it. I never conceived there could be so many stars!”
When she was a teen, Seager attended an Astronomy Day at the University of Toronto, which cemented her ambition. But when she told her dad she wanted to be an astronomer, he surprised her by trying to talk her out of it. How could she make a living looking at the stars? He wanted her to be able to take care of herself—maybe become a doctor. Astronomy, he thought, was better as a hobby.
Her father’s disapproval didn’t deter her. After spending a few years as a senior researcher at the Carnegie Institution in Washington, DC, Seager returned to Cambridge in 2007 to become a professor at MIT, hoping to probe exoplanet atmospheres for biosignatures. Her research was vindicated when the Kepler Space Telescope—launched on March 6, 2009, with Latham, Sasselov, Seager, and Charbonneau all members of its science team—began sending data on new exoplanets back to Earth. Suddenly, Seager and other exoplanet hunters had a remarkable number of planets to study. “That’s when the floodgates opened,” Sasselov says.
Over its six-year mission—handicapped by a breakdown in the spring of 2013—Kepler found some 1,000 planets and more than 4,000 planet candidates awaiting confirmation. The vast majority of them are huge and sweltering, places where life as we know it wouldn’t stand a chance. But as scientists continue to analyze Kepler’s data, they’ve uncovered and confirmed a handful of Earth-sized planets in habitable orbits (three of these were announced in January 2015). In November 2013, scientists published a paper concluding that a whopping 22 percent of Sunlike stars may host Earth twins. Just like that, the possibilities have exploded. Our galaxy may harbor 40 billion planets that could support life. Billions more may be circling smaller, cooler M-dwarf stars, also known as red dwarfs, which outnumber stars like our sun by 10 to one.
“I get asked all the time, why I care so much about this,” Seager says. “It’s the drive to have meaning.” About a year before her 2011 Media Lab conference, Seager’s husband was diagnosed with terminal cancer. He would die that July, just days after her birthday, when their two boys were six and eight years old. Four years earlier, cancer had also taken the life of her father. Seager says she counts herself as somebody “who has a lot of experience with death,” and those devastating encounters with mortality have motivated her quest.
“Everybody wants to have meaning in their lives when they wake up and face their day,” she says. “For some people, it might be raising the next generation or working in a hospital and healing people. For me, it’s finding an Earthlike world.”
That search will be aided by the next generation of space- and land-based telescopes, larger than any before them, and equipped with special technology to counteract the blur of Earth’s atmosphere and block out the glare of stars. How to analyze the data they collect in search of life is the domain of yet another Harvard astronomer.
For five chilly, gray days in late November 2014, Harvard astronomer Mercedes López-Morales huddled with a few dozen top scientists in a Bavarian castle south of Munich to talk about the problem of looking for life over incredible distances: The nearest confirmed exoplanets are some 87 trillion miles from Earth. It would take our fastest rockets thousands of lifetimes to reach them.
At the castle, López-Morales and her colleagues were plotting the exploration of these new worlds. The biggest item on their agenda: How should we weigh evidence for extraterrestrial life? Just because life on Earth is responsible for certain molecules, like oxygen, doesn’t mean that on a strange new planet, these molecules couldn’t be created by some volcanic or other geological phenomenon. “Just a year ago, we were thinking that if we detected oxygen, that would be proof,” she says. “Now we’re thinking there has to be methane, oxygen, and some other things, too.”
López-Morales, 40, has short, graying hair and speaks quietly with the accent of her native Spain. She’s a few years younger than Seager, and her reasons for joining the ranks of exoplanet researchers, as a postdoc in 2004, shows how quickly the field changed from a risky proposition to one ripe with opportunity. “A lot of it was job security and the prospects for future work,” she says. “But most of all, it was that the science was really cool. I want to spend my life doing something that has meaning to me, and for society as well.”
Like Seager, López-Morales studies exoplanet atmospheres, but with ground-based telescopes. She’s on a team designing an optical instrument for the Giant Magellan Telescope (GMT), an enormous new instrument in Chile that, beginning in 2024, will stare at celestial phenomena through an 80-foot-wide eye. Harvard is a partner in developing the GMT, one of three planned “extremely large telescopes” worldwide. It will have a resolution 10 times better than the Hubble, even though it must peer through Earth’s atmosphere. And it will take advantage of the most sophisticated optics in history, using seven of the world’s largest mirrors.
While her group hasn’t yet found any critical molecules in an exoplanet atmosphere, López-Morales is optimistic. When we spoke a few days after her return from Germany, one of her postdocs had just measured the first ground-based transit of a planet closer to Earth’s size. “We are slowly paving the way toward the detection of biosignatures in Earthlike planets,” she said in a press release about the achievement.
But that release brought an admonishment from a senior colleague. “[He] sent me an email saying, ‘Biosignatures are still a dream,’” she says. “My answer was, ‘Well, 30 years ago, it was a dream to detect the wobble of a planet around a star. And look at where we are now.’ So I’m just dreaming on.”
Then, on December 16, 2014, well before any of the new telescopes came online, news flashed around the world that the Mars rover team had detected bursts of one such biosignature—methane—on the Red Planet. While it was just a trace detection of the gas—less than one part per billion—there could be only two explanations. One was geological: Methane could be released by carbon deposits in rocks reacting with either ultraviolet radiation or a mix of heat and water. That’s right: water. The other was biological: Methane could be made by microbes living somewhere beneath the planet’s surface.
“It’s one of the most exciting announcements we’ve heard,” Seager told me when I called her the next morning. “For an exoplanet, we want to see gases that don’t belong. And methane is a nice one, because it’s so short-lived in the atmosphere,” she said—meaning that methane would not be detected unless something keeps continually generating it, possibly life. “If it ends up being attributed to life on Mars, it means life can get a hold in more than one location. It would mean a lot.”
If we do find signs of life out there, the discovery will require a grinding process of replications and other studies to eliminate any other possible explanations. Even then, we may never be 100 percent certain. Unless, of course, that life source decides to say hello, something that Harvard astronomer and longtime SETI researcher Paul Horowitz is confident will happen someday.
The only telescope still being used at Harvard’s Oak Ridge Observatory is Horowitz’s Optical SETI project, which began operating in 2006. Rather than search for radio signals, it scans the night sky for nanosecond pulses of super-bright light, basically waiting for a flash from ET’s high beams. “There are signals out there. That’s guaranteed,” Horowitz says, “but they’re at some plane of technology that we have not reached.”
SETI may be a long shot compared with looking for primitive life on exoplanets, he says, but it’s also much cheaper. And with SETI, when we know, well, we’ll know.
Seager, meanwhile, is currently working with a team to develop the Kepler telescope’s $87 million successor, called TESS—the Transiting Exoplanet Survey Satellite, scheduled to launch in 2017— which is being led by MIT astronomer George Ricker. The science team includes several other astronomers from MIT and Harvard, including Latham, Sasselov, and Charbonneau. While the Kepler focused its gaze on a narrow band of outer space, TESS will scan the entire sky for two years. It’s expected to find about 500 planets close to Earth’s size, orbiting bright stars. These planets will be targeted for closer observations by the Hubble telescope’s successor, the James Webb Space Telescope, which is scheduled for launch in 2018.