Astronomers Observe First Exoplanet-Induced Stellar Pulsations

Astronomers have detected strange pulsations in the outer shell of a star called HAT-P-2, and their best guess is that a closely orbiting exoplanet causes these vibrations each time it gets close to the star in its orbit.
HAT-P-2 is located in the constellation Hercules and is approximately 385 light-years from Earth.
Also known as HD 147506 and HIP 80076, this star is circled by a hot Jupiter, an exoplanet that is extremely warm and orbits its host star tightly.
The planet, named HAT-P-2b, is 8 times the mass of Jupiter.
Known to the exoplanet community since 2007, HAT-P-2b was initially interesting to astronomers because of its highly eccentric orbit.
The planet spends most of its time relatively far from HAT-P-2, but comes around for a close encounter every 5.6 days.
Those are indeed hot dates for this planet, as it receives as much as 10 times the amount of light per unit area at closest approach than at its farthest point in the orbit.
MIT astronomer Julien de Wit and co-authors analyzed more than 350 hours of observations of HAT-P-2 taken by NASA’s Spitzer Space Telescope.
Each time HAT-P-2b passed behind its host star, they saw something unexpected: instead of a flat line, representing a momentary drop as the planet is masked by its star, they observed tiny spikes — oscillations in the star’s light, with a period of 87 min, that happened to be exact multiples of the exoplanet’s orbital frequency.
“They were very tiny signals. It was like picking up the buzzing of a mosquito passing by a jet engine, both miles away,” Dr. de Wit said.
The precisely timed pulsations have lead the researchers to believe that, contrary to most theoretical model-based predictions of exoplanetary behavior, HAT-P-2b may be massive enough to periodically distort its star, making the star’s molten surface flare, or pulse, in response.
“We thought that planets cannot really excite their stars, but we find that this one does,” Dr. de Wit said.
“There is a physical link between the two, but at this stage, we actually can’t explain it. So these are mysterious pulsations induced by the star’s companion.”
“We had intended the observations to provide a detailed look at HAT-P-2b’s atmospheric circulation,” said co-author Dr. Nikole Lewis, an astronomer at Space Telescope Science Institute.
“The discovery of the oscillations was unexpected but adds another piece to the puzzle of how this system evolved.”
The astronomers have some theories as to how HAT-P-2b might be causing its star to pulse.
For example, perhaps the planet’s transient gravitational pull is disturbing the star just enough to tip it toward a self-pulsating phase.
There are stars that naturally pulse, and perhaps the planet is pushing its star toward that state, the way adding salt to a simmering pot of water can trigger it to boil over.
“This is just one of several possibilities, but getting to the root of the stellar pulsations will require much more work,” Dr. de Wit said.
“It’s a mystery, but it’s great, because it demonstrates our understanding of how a planet affects its star is not complete.”
“So we’ll have to move forward and figure out what’s going on there.”


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