Gravitational Tug of 'Invisible' Exoplanet Discovered

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NASA's Kepler space telescope finds planets beyond the solar system by looking for dips in starlight caused by planets parading past their parent stars, relative to Kepler's point of view.

But there's another method by which scientists can find sibling Kepler worlds beyond the telescope's eye.

David Nesvorný, with the Southwest Research Institute in Boulder, Colo., and colleagues parsed through Kepler data on a sunlike star designated "Kepler Object of Interest 872," or KOI-872, and found something interesting — the planet's transit was late.

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"It was showing enormous time variations in transits, exceeding two hours," Nesvorný told Discovery News.

"At that point, we were sure there is something important in the system that is causing these perturbations," he said.

Turns out the candidate planet identified by the Kepler team actually is two Saturn-sized worlds, one transiting in Kepler's field of view, the other beyond it.

Transit timing variations, or TTVs, had not been used before to find planets, notes astrophysicist Norman Murray, with the University of Toronto.

"The use of TTVs to find unseen planets, although predicted some seven years ago, has not yielded secure detections before this work,"€ Murray writes in a related paper posted Thursday in the online journal Science Express.

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Another unseen world called Kepler-19c was apparently detected through its gravitational interactions with an exoplanetary sibling in 2011, so this most recent KOI-872 "unseen world" discovery seems to validate the method.

Nesvorný actually had been on the hunt for exoplanet moons, but the data on KOI-872 just didn't match the computer models developed to simulate orbital patterns of moons.

The new method has some advantages over previous planet-hunting techniques, notes Nesvorný.

"Because it’s based on the interaction between planets, it tells you a lot about the planetary system — much more than the radial velocity or the transits in a sense," Nesvorný said, referring to the two most widely used planet-hunting techniques.

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The "radial velocity" method looks for gravitational tugs of orbiting planets in a star's light spectrum. The "transit" method, such as what Kepler uses, watches for planets crossing their parent star's face, relative to the telescope's point of view.

Nesvorný and colleagues also found a third, transiting planet around KOI-872.

Their research is reported in this week's Science.

Image credit: David A. Aguilar (CfA)