For the first time, astronomers have observed a star pulsating in response to an orbiting exoplanet.
Located 370 light-years away, the star HAT-P-2 was captured by NASA's Spitzer Space Telescope reacting to the exoplanet HAT-P-2b each time it got near the star. Pulsations from stars can be caused by various factors and have been observed before in binary systems (two stars revolving around each other or a common center) but never between a planet and a star.
"We thought that planets cannot really excite their stars, but we find that this one does," said Julien de Wit, one of the authors for a study detailing the findings published in the Astrophysical Journal Letters.
Cosmic Love Connection
With eight times Jupiter's mass, HAT-P-2b is one of the most massive of exoplanets that have been identified today. It follows a highly eccentric orbit around HAT-P-2, flying extremely close when it is near the star then hurtling out before circling back around eventually. The exoplanet flies close to the star every 5.6 days and during that time receives a lot of light, which is equivalent to an amount up to 10 times as much of what it gets per unit area at its farthest orbit point.
But while HAT-P-2b is massive for an exoplanet, it is tiny compared to its host star HAT-P-2, which is about 100 times more massive than it. It's this size difference that NASA says made the pulsation effect observed more unusual.
"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 Heather Knutson, also one of the study's authors.
Every time HAT-P-2b swings by, it appears to be giving HAT-P-2 a "kiss" as gravitational forces from the two interact. This results in the star beating like a heart as the exoplanet makes an orbit around it.
An Unexpected Discovery
This interaction between HAT-P-2 and HAT-P-2b was actually discovered by chance. De Wit, Knutson and colleagues were originally analyzing over 350 hours of observations made by the Spitzer Telescope to create a precise map of temperature distribution in an exoplanet as it orbits a star, helping astronomers keep track of how energy circulates in planet atmospheres. The HAT-P-2 system was seen as ideal because of the planet's eccentric orbit around the star, which also coincides with temperatures shifting depending on HAT-P-2b's location from the star.
It's natural for stellar pulsations to occur because star surfaces boil and turn over. However, the pulsations the astronomers detected were occurring in tandem with the orbit of the exoplanet. According to co-author Victoria Antoci, their findings are exciting because it shows that it's possible for planets to significantly impact physical phenomena on host stars.
"In other words, the star ‘knows' about its planet and reacts to its presence," she said.
The astronomers have several theories to explain how HAT-P-2b is able to affect its host star like that but they have not settled on just one, saying that further studies have to be carried out to better understand the "love connection."
