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Stanford astronomer catches glimpse of protoplanet


On Nov. 18, Stanford astronomer and postdoctoral scholar Kate Follette caught a glimpse of a newborn planet.

Named LkCa 15 b, the alien planet seems to be growing into a Jupiter-like world some 450 light years away. The postdoctoral researcher and her colleagues had to design a new instrument to detect the light emanating from the protoplanet.

“This is the first incontrovertible detection of a planet still in the process of forming — a so-called ‘protoplanet,’” Follette said to the Stanford News Service.

Observing protoplanets yields clues to the forming of worlds in other galaxies. The prevailing approach has been to search for clearings inside rings of debris, where planets are thought to form. Yet, astronomers have learned little about protoplanets in this way. They hover past the limits of what the best telescopes can see.

Instead, Follette and her colleagues caught the light LkCa 15 b gave off as it grew. It was no easy task — light from young planets is normally much fainter than the light from their host stars.

The difference in brightness between a star and a young exoplanet is usually comparable to the difference between a firefly and a lighthouse,” Follette said. “It’s very hard to isolate the light from the planet when it is so faint and so close to the star from our point of view.”

Luckily, the researchers knew exactly what they were looking for. LkCa 15 b gave off a special wavelength of visible light known as Hydrogen-alpha (H-alpha) light as it swelled from its core of rock or ice into gas giant.

“Because we could focus on a special color of light where the planet is glowing very brightly, the signal was significantly stronger than what we normally look for,” Follette said.

Follette and her colleagues will be publishing the sighting in their planet formation theories in the Nov. 19 issue of Nature. Follette’s co-author Steph Sallum, a graduate student at the University of Arizona, verified the observation with independent data on the same protoplanet.

The University of Arizona’s Magellan Telescope and its visible light camera first captured the light from LkCa 15 b. The Magellan is the first telescope up to the task of tackling H-alpha light.

Bruce Mackintosh, professor of physics, astronomer and advisor to Follette, said that these imaging processes help to chart the birth cycle of planets. He has tracked down a slightly older planet using the Magellan system.

“51 Eri b is an adolescent — about 20 million years old, already full-grown and still cooling off from the energy released during its formation,” he told the Stanford News. “Kate’s planet is a baby — still heating up and growing.”

The team will continue to watch LkCa 15 b grow to understand how planets form, as well as to test the theory that “clearings” in debris disks are a sign of developing planets.

“One of the fundamental human questions is whether we’re alone or unique,” Follette said. “It’s cool to look at Jupiter-like exoplanets like LkCa 15 b, but ultimately we’re trying to push the technology to be able to detect Earth-like exoplanets.”

“I’ve always been inspired by the famous ‘pale blue dot’ image of Earth taken by Voyager as it passed Saturn,” she added. “We’d really like to do that some day for a planet around another star, and this sort of work is moving us in that direction.”


Contact Fangzhou Liu at fzliu96 ‘at’

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Fangzhou Liu ’19 was Vol. 253 Executive Editor; before that, she co-led the news section. She grew up in Singapore and studies computer science and linguistics.