Valentine's Day Exoplanet Tugs at Its Star's Heart

For the first time, astronomers have spotted pulsations in a star caused by the gravitational interactions with a massive planet.

A stellar romance is blossoming in a star system 370 light-years away, causing a star to make heartbeat-like pulses as its exoplanetary partner zooms close during its short orbit.

The discovery was made by NASA's Spitzer Space Telescope that was monitoring very slight pulsations in star brightness. The star, called HAT-P-2, has a massive exoplanet - called a "hot Jupiter" - in orbit, which is around eight times the mass of Jupiter, and its orbit is causing the pulsations in the star's outer layers.

Although these pulsations have been spotted in binary stars in the past - so-called "heartbeat stars" - this is the first time a similar effect has been spotted between a star and planet.

"Just in time for Valentine's Day, we have discovered the first example of a planet that seems to be causing a heartbeat-like behavior in its host star," said Julien de Wit, postdoctoral associate at the Massachusetts Institute of Technology (MIT), Cambridge.

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The study of HAT-P-2 has been published today (Feb. 14) in Astrophysical Journal Letters.

Since 2007, astronomers have known that the star system possesses an exoplanet (called HAT-P-2b), but this is the first time that its impact on the star's brightness has been seen. The interesting thing is that HAT-P-2b is 100 times less massive than its host star, causing some surprise that its gravity can have such an impact on the star's outer layers.

"It's remarkable that this relatively small planet seems to affect the whole star in a way that we can see from far away," said planetary scientist Heather Knutson, of Caltech in Pasadena, Calif.

Hot Jupiters are a special class of exoplanet that orbit super-close to their stars. They are typically more massive than Jupiter, but complete an orbit in a matter of days, sometimes hours. In the case of HAT-P-2b, it completes an orbit once every 5.6 days and its orbit is extremely eccentric - it zooms close to its star and then swings back out again.

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When making its close approach, the exoplanet gives the star a gravitational "kiss", creating the brightness pulse.

However, after analysis, astronomers are confused. The pulses - or "heartbeats" - are of a higher frequency than expected, revealing a gap in our knowledge of gravitational interactions between stars and orbiting hot-Jupiters.

"Our observations suggest that our understanding of planet-star interactions is incomplete," added de Wit. "There's more to learn from studying stars in systems like this one and listening for the stories they tell through their 'heartbeats.'"

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