COVER STORY

Brave New Worlds

Columbia astronomers are going beyond our solar system to understand exoplanets, find exomoons, and explore all sorts of surreal estate.

by Bill Retherford '14JRN Published Winter 2017
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This artist's concept allows us to imagine what it would be like to stand on the surface of the exoplanet TRAPPIST-1f, located in the TRAPPIST-1 system in the constellation Aquarius. The TRAPPIST-1 planets are remarkably close to each other. The star they orbit, an ultra-cool dwarf, would appear about three times larger than our own sun does in Earth's skies. / Photograph by NASA / JPL-CALTECH / T. PYLE (IPAC)

The sun, veiled by the mist of a soggy Manhattan morning, was undetectable outside David Kipping’s thirteenth-floor office window in Pupin Hall on the Morningside Heights campus. But Kipping, an assistant professor of astronomy, was contemplating another celestial object anyway, one far more obscure — TrES-2b, the darkest world in the galaxy.

“It absorbs 99 percent of its sun’s light,” Kipping says. “That’s less reflective than black paint.” An artistic rendering of TrES-2b, hanging on the wall opposite the window, reveals a disk coal-dark, streaked with scorched red swirls.

In space, nothing is close, but TrES-2b is unthinkably distant, 750 light years away, about four and a half quadrillion miles from Earth.

TrES-2b is an exoplanet, a world outside our solar system, one of 3,529 verified by NASA as of October 2017. “But that,” says Kipping, “is the tip of the iceberg. There are so many worlds, it’s mind-blowing.” Exoplanets are everywhere, scattered through the Milky Way like Motel 6 on the freeway. Scan the night sky, pick out any star, and it probably has planets; most all of the two hundred billion stars in our galaxy do. “On average, it’s a few planets per star,” says Kipping. That’s approximately a trillion exoplanets.

Since the fall of 2015, Kipping’s Cool Worlds lab, part of the Department of Astronomy at Columbia, has studied these extrasolar planetary systems. Cool Worlds searches for exoplanets, exomoons, and all sorts of surreal estate; six graduate students currently work in the lab. “We look for things mostly within the habitable zone,” says Kipping. That’s the sweet spot in any system, where planets are not too hot or too cold — a place where temperatures might be right for liquid water, a place where Earth-like worlds might exist. A place, Kipping says, “where life starts to become possible.”

Kipping was eleven when astronomers found the first exoplanet orbiting a sun-like star — 51 Pegasi b — in 1995. Before that, most scientists regarded exoplanet searches as pseudoscience, like looking for UFOs, crop circles, or Roswell aliens; exoplanet hunters were considered eccentrics. “They were mavericks and radicals,” Kipping says, “and not accepted for the science they were doing.” Still, over the next decade, astronomers would discover a few hundred exoplanets, and in 2009 NASA launched the Kepler space telescope to find more. From seventy-five million miles out in space, Kepler surveyed the stars between the constellations Cygnus and Lyra, and swiftly spotted exoplanets all over the place.

Suddenly a fringe specialty became the most intriguing subject in astronomy. Initially, scientists looked for firsts: the first oxygen-rich exoplanet (Gliese 1132b), the first potential water world (Gliese 1214b), and the first Earth-size planet in the habitable zone (Kepler-186f). In March 2016, the Cool Worlds lab announced the discovery of Kepler-167e, one thousand light years away, the size of Jupiter and about as cold — 220 degrees below zero Fahrenheit. “The exhilaration you feel from an act of discovery is joyous,” says Kipping. “Whether it’s some little thing nobody else knew or an entirely new planetary system, you want to go out in the world and tell everyone. It’s like being in love.”

But the obsession with discovering new exoplanets — “a gold rush,” in Kipping’s words — has already died down. “We’re past that,” he says. “Now we’re trying to fill out the missing details. We want to understand the stories of these exoplanets.” Certainly astronomers already know basic things about them — their size, how long they take to orbit their stars — but little else. “We’re now asking more meaningful questions,” says Kipping. “What are their atmospheres? How much water do they have? What would it be like to walk on the surfaces of these worlds?” And of course, maybe the most tantalizing question ever, and surely the tease of the galaxy: do they have life?

 

The Kepler telescope, the size of an SUV and fortified with forty-two camera sensors, has scrutinized about 150,000 stars — an astoundingly slight patch of sky, about one ten-thousandth of 1 percent of the Milky Way. Go outside, make a fist, and extend your arm skyward; your fist represents roughly the portion of sky Kepler has seen.

Even from such a trifling sample, Kepler has generated a daunting assortment of data, as many as two million data points per star. Columbia is among the hundreds of universities and research centers worldwide that download and decode the data. “It’s a treasure trove of discoveries,” says Emily Sandford ’17GSAS, a fourth-year graduate student on the Cool Worlds team. “It’s more data than we know what to do with.” Discovery is within the data — the exoplanet’s radius, mass, shape, reflectivity, temperature, orbital period, atmosphere — markers that may signal if life is possible. The secrets of the galaxy, or at least some of them, are now stowed on Cool Worlds desktops. “Sometimes I think, ‘My eyes are the only ones that have ever seen this,’” says Tiffany Jansen, a second-year graduate student.

Seeing is one thing. Deciphering is quite another. Simply put, Kepler collects the data by photographing stars and recording pixels of their light. Viewed on a computer screen, the stars appear as inscrutable bright globs. Orbiting exoplanets, if there are any, are not visible. But the pixels contain a critical piece of information that kickstarts every discovery: “They measure the star’s brightness,” says Sandford.

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