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 shows planet KELT-9b orbiting its host star, KELT-9. With a temperature of more than 7,800 degrees Fahrenheit (4,600 kelvin), KELT-9b is the hottest gas giant planet discovered so far. / Photograph by NASA / JPL-Caltech

Within the billions of minutes of light measurements, Cool Worlds researchers can drill down to search for an eclipse-like event — a transit. That occurs when an exoplanet passes between its parent star and Kepler. “A planet passing by will block some of the star’s light,” says Moiya McTier, a second-year graduate student. “And the data will show that as a dip in the light.” That dip, should it repeat at precise intervals, suggests an exoplanet is there.

From that dip, researchers can create a transit light curve, a graph replete with additional data. “By studying the light curve,” McTier says, “we can figure out the physical characteristics of an exoplanet.” As technology develops, they will be able to detect specific surface features, like oceans, vegetation, or polar ice caps. “Finding out things through the transit method, the limit is your imagination,” says Kipping.

Turning the raw data into decipherable computer files is a formidable process. “It’s just a long list of meaningless numbers,” says Sandford. “You can’t scroll through it and learn anything. To comb through the entire volume of data by hand would take ten years.” Sandford’s days are spent coding — converting the gibberish into information both manageable and coherent. “It’s basically computer programming,” she says. Write good code and your computer could take hours to crack what otherwise takes a decade. But writing good code isn’t easy. “For someone starting out,” says Jansen, “a lot of time is spent just figuring out what’s wrong with your code.”


Nearly nothing is known about the topographical features of exoplanets; Cool Worlds is just now scratching those surfaces. Unquestionably, space theoreticians have long ruminated on exoplanet landscapes. They simply calculate a planet’s density and then take a smart guess. Molten glass rains down on HD 189733b; flurries of rubies and sapphires fall on HAT-P-7b; boiling lava coagulates on Kepler-78b; diamonds cover WASP-12b. None are outlandish claims, but they are all speculations. Iron, for instance, is one of the densest common elements in the universe. Hence, a dense planet might contain an abundance of iron. But maybe not. “There’s lots of leeway in the interpretation of data,” says Sandford. “It’s possible to infer a planet is partially made of carbon. And carbon is what makes up diamond. So a diamond planet is a possibility. But it’s not the only possibility. The uncertainties are quite large.”

What is known is that exoplanets come in two basic categories. “We can say some exoplanets are gaseous, and some are rocky,” says Jansen. “But that’s really the extent of our knowledge.” The rocky planets, like Earth and Mars, have solid surfaces. The gas giants, like Jupiter, Saturn, Uranus, and Neptune, may not sustain any solid surface at all. Plunge past their atmospheres, and instead of hard ground one could conceivably find a nebulous, plasma-like interior.

“The exhilaration you feel from an act of discovery is joyous.”

Jansen is surveying both types of exoplanets. Looking at their light curves, she searches for one thing: reflectivity. A highly reflective exoplanet suggests a surface that could be coated with ice. That presents the scientists with a fun extrapolation: ice signals the presence of liquid water, the quintessential biosignature, “a molecular fingerprint for life,” says Jansen. So let evolution augment the water with organic molecules, wait while they mingle and wallow for a billion years, and microbes might emerge. That’s extraterrestrial life, but Jansen cautions about getting carried away. Deducing the presence of water (much less life) based only on a planet’s reflectiveness is “very difficult,” she says. After all, trees and asphalt, two very different substances, mirror light at about the same intensity. Before reaching anything close to certainty, Cool Worlds must learn more.

McTier, meantime, will spend the next several years looking for mountains. “It’s going to be really difficult,” she says. “No one has ever found a mountain on a planet outside our solar system.” But mountains cast shadows; McTier theorizes that shadows should show in the data. “They will cause jaggedness in the light curve,” she says.

If McTier discovers a mountain, the suppositions will begin. Here’s one: on Earth, plate tectonics made mountains possible — also hills, canyons, valleys, lakes, rivers, and oceans — all topographical antecedents that led to life. Perhaps a mountainous exoplanet has tectonic plates? If so, does that mean life is more likely there? “We can learn a lot,” says McTier, “just based on shadows.”


Exoplanets give off one-billionth the light of a star. Exomoons orbiting them are even fainter. No astronomer has officially discovered one, but Cool Worlds has a candidate — a possible exomoon orbiting Kepler-1625b, a gas giant at least six times the size of Earth and about four thousand light years away. The Kepler data is promising but inconclusive; Kipping obtained time on NASA’s Hubble space telescope this past October, hoping to corroborate his findings and proclaim a discovery. A final confirmation won’t happen until spring. Beyond that he says little else: “We’re being very cautious. We’re waiting to see what the data gives us.”

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