(the following text is adapted from a byline by Jennifer Chu, MIT News Office)

 For the first time, astronomers from USA, Canada and Aarhus have observed a star pulsing in response to its orbiting planet. The results are published 14 February 2017 in The Astrophysical Journal Letters in a paper titled: "Planet-induced Stellar Pulsations in HAT-P-2's Eccentric System"

 The star, which goes by the name HAT-P-2, is about 400 light years from Earth and is circled by a gas giant measuring eight times the mass of Jupiter — one of the most massive exoplanets known today. The planet, named HAT-P-2b, orbits its star in a highly eccentric orbit, coming extremely close to the star, and then hurtling far out before eventually circling back around.

 The researchers analyzed more than 350 hours of observations of HAT-P-2 taken by NASA’s Spitzer Space Telescope. They found that the star’s brightness appears to oscillate ever so slightly every 87 minutes. In particular, the star seems to vibrate at exact harmonics, or multiples of the planet’s orbital frequency — the rate at which the planet circles its star.

 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 gaseous surface flare, or pulse, in response. 

 “It’s assumed at this [orbital] frequency, the planet cannot really excite the star, but we find that it does,” says Julien de Wit, a postdoc in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “There is a physical link between the two, but at this stage, we actually can’t explain it. So these pulsations induced by the star’s companion remain unexplained.” De Wit is a the lead author of the paper detailing the results.

 “This is really exciting because, if our interpretations are correct, it tells us that planets can have a significant impact on physical phenomena operating in their host-stars,” says co-author Victoria Antoci, a postdoc at Stellar Astrophysics Centre, Aarhus University in Denmark. “In other words the star ‘knows’ about its planet and reacts to its presence. The exciting part for me, as an asteroseismologist, is that in highly eccentric systems the presence of planets can help to ‘sound’ the interior of stars, which will ultimately help to better understand stellar structure and evolution.”

The orbit of exoplanet HAT-2Pb has an eccentricity of 0.51

 Getting a pulse

The team came upon the stellar pulsations by chance. Originally, the researchers sought to generate a precise map of an exoplanet’s temperature as it orbits its star. Such a map would help scientists track how energy is circulated through a planet’s atmosphere. This again can give clues to the wind patterns and composition of the atmosphere of the planet.

 With this goal in mind, the team viewed HAT-P-2 as an ideal system: Because the planet has an eccentric orbit, it seesaws between temperature extremes, turning cold as it moves far away from its star, then rapidly heating as it swings extremely close.

 “The star dumps an enormous amount of energy onto the planet’s atmosphere, and our original goal was to see how the planet’s atmosphere redistributes this energy,” de Wit says.

 The researchers obtained 350 hours of observations of HAT-P-2, taken intermittently by Spitzer’s infrared telescope between July 2011 and November 2015. The dataset represents one of the largest ever taken by Spitzer, giving de Wit and his colleagues plenty of observations to allow for detecting the incredibly tiny signals required to map an exoplanet’s temperature.

 The team processed the data and focused on the window in which the planet made its closest approach, passing first in front of and then behind the star. During these periods, the researchers measured the star’s brightness to determine the amount of energy, in the form of heat, transferred to the planet.

 Each time the planet passed behind the star, the researchers 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 about 90 minutes, that happened to be exact multiples of the planet’s orbital frequency.

 “They were very tiny signals,” de Wit says. “It was like picking up the buzzing of a mosquito passing by a jet engine, both miles away.”

 Lots of theories, one big mystery

 Stellar pulsations can occur constantly as a star’s surface naturally boils and turns over. But the tiny pulsations detected by the international team of astronomers seem to be in concert with the planet’s orbit. The signals, they concluded, could not be due to anything in the star itself, but to either the circling planet or an effect in Spitzer’s instruments.

 The researchers ruled out the latter after modeling all the possible instrumental effects, such as vibration, that could have affected the measurements, and finding that none of the effects could have produced the pulsations they observed.

 “We think these pulsations must be induced by the planet, which is surprising,” de Wit says. “We’ve seen this in systems with two rotating stars that are supermassive, where one can really distort the other, release the distortion, and the other one vibrates. But we did not expect this to happen with a planet — even one as massive as this.”

The team has some theories as to how the planet might be causing its star to pulse. For example the planet’s transient gravitational pull might be disturbing the star just enough to tip it toward a self-pulsating phase. There are stars that naturally pulse, and perhaps HAT-P-2b is pushing its star toward that state, the way adding salt to a simmering pot of water can trigger it to boil over. De Wit says this is just one of several possibilities, but getting to the root of the stellar pulsations will require much more work.

 “It’s a mystery, but it’s great, because it demonstrates that our understanding of how a planet affects its star is not complete,” de Wit says. “So we’ll have to move forward and figure out what’s going on there.”

 Victoria Antoci adds: " From theoretical point of view it is a mystery, indeed, but I think this is fun! It is from these cases that we learn the most. Modelling stars and planets independently from each other is already very tricky. Even with new and fast computers, realistic full-scale stellar and planetary models are not feasible and we have to simplify or neglect complex physical phenomena. I guess that by doing this we might be missing a very important piece of information about the physical interaction which the HAT-P-2 system can help to understand. In order to solve this puzzle we need to study the detailed stellar structure in the outer layers of this star. The only way to do so is with more observational data and the help of asteroseismology."

 The paper can be downloaded here.