The stars seem to have an anti-aging tip that humans can only dream of: their family.
Astronomers have found that having the good-sized planet orbiting its parent relatively quickly seems to slow the star’s aging process. Specifically, a nearby gas giant can transfer angular momentum to the star’s spin, countering the gradual loss of spin resulting from the braking effect of the star’s magnetic field.
The effect of exoplanets on the rotation of their host stars is a bit tricky to study. If you look at a star and a planet, you have almost no way of knowing if the rotational speed was influenced by the planet.
However, many stars in the Universe are found in multi-star systems. Binary stars are usually stellar twins, born in the same nebula, from the same dust and gas cluster. As such, their properties are often very similar. They are similar in color, size, brightness, and yes, rotation.
So, to take a closer look at the planets’ influence on stellar rotation and activity, a team of astronomers led by Nikoleta Ilic of the Leibniz Institute for Astrophysics in Potsdam (AIP) in Germany conducted the equivalent of a twin study for the stars.
They looked for binary star systems where one star has orbiting exoplanets and the other does not. They then used the planetless star as a control to look for changes potentially caused by the planet orbiting the other star.
But a single star system would not be enough to establish a model; in their study, the team looked closely at 34 binaries.
“In medicine, you need a lot of patients enrolled in a study to know if the effects are real or some kind of outlier,” says Ilic. “The same may be true in astronomy, and this study gives us confidence that these hot Jupiters are really causing orbiting stars to act younger than they are.”
Hot Jupiters are eccentrics. They’re gas giants, like Jupiter, but they hang incredibly close to their host stars — with orbital periods of days or even hours. At this closeness, they are overheated by the star, earning them their title.
Ilic and his colleagues studied X-ray data on their 34 binary systems, obtained by NASA’s Chandra Observatory. Because faster-spinning stars exhibit more X-ray activity than slower-spinning stars, they were able to find a distinct difference in rotational speed between the two stars in their binary pairs.
Indeed, the fastest stars were those with hot Jupiters; giants that orbited a greater distance from their star had no observable effect. This excludes our own Sun (sorry, Sun).
Since the rotation of stars gradually slows over time, younger stars tend to rotate faster than older stars with similar properties. This suggests that hot Jupiters are a bit like wrinkle cream for the stars. The interaction between the star and the exoplanet results in a transfer of spin from the exoplanet to the star which amplifies the spin of the latter.
The finer details of this interaction are currently somewhat of a mystery. For for the sake of their analysis, the researchers assumed that it was a tide based on the gravitational attraction between the two bodies. But magnetic forces could play a role. Further investigation might help figure out the background details.
“In previous cases there were some very intriguing clues, but now we finally have statistical evidence that certain planets do indeed influence their stars and keep them young,” says AIP astronomer Marzieh Hosseini.
“Hopefully, future studies will help uncover more systems to better understand this effect.”
The research was published in the Royal Astronomical Society Monthly Notices.