Planets can “rejuvenate” their stars

Artist’s rendering of a hot Jupiter orbiting its host star closely. The “twin star” of the double system can be seen in the background. © NASA/CXC/M. White

Large planets may have an "anti-aging effect" on their parent star, study shows. Through their gravitational influence, they can counteract the decline in rotational speed and thus activity normally associated with the stellar aging process. This shows an examination of binary star systems in which only one partner has a large planet. The information can now be used to better estimate the age of stars, say the researchers.

In the universe there are processes with clear parallels to our living environment: stars are also born, age and finally die. The aging process is reflected in a slowing down of the rotation and the activity of a star and there are fewer and fewer outbursts. How these signs of aging appear depends on various characteristics of the star. For some time there have been assumptions and indications that it could also play a role whether a star has a major planet. It is specifically about the so-called hot Jupiters - massive gas planets that orbit their host star at Mercury's distance or closer.

Twin systems as “subjects”

It seemed possible that such a hot Jupiter could influence its parent star through its tidal forces in such a way that it spins faster than if it had no such planet. This faster rotation could in turn cause the host star to become more active and produce more X-rays, making it appear younger than it actually is. Since it is difficult to accurately determine the age of most stars, in some cases it has remained unclear whether a star is unusually active because it is influenced by its nearby planet or because it is actually relatively young. A team of astronomers from the Leibniz Institute for Astrophysics in Potsdam (AIP) has now for the first time systematically investigated the extent to which the assumed anti-aging effect of hot Jupiters can actually be proven on a broad basis.

The researchers examined about three dozen binary star systems in which only one of the two is orbited by a hot Jupiter. As they explain, stars in binary systems are known to form simultaneously, just like human twins. They are therefore of the same age and often have similar characteristics. In addition, in the binary star systems selected, the distance between the stars was so great that they could not affect each other in any relevant way or hot Jupiter could affect the other star. That meant they could use the planet-free star in the system as a kind of "control subject," the scientists explain.

Rejuvenated Gemini with planet

"It's almost like using twins in a study where one child lives in a completely different environment that affects their health," explains co-author Katja Poppenhäger from the AIP. "By comparing a star with a nearby planet to its twin without one, we can examine the differences in the behavior of stars of the same age," says the astronomer. The researchers analyzed the X-rays emitted by the stars using the Chandra and XMM-Newton space telescopes to determine how "young" they appear. In this way, they were now able to confirm the assumptions about the effects of hot Jupiters on their host stars: The numerous examples showed that the stars with the "weighty children" tended to shine brighter in X-ray light and were therefore more active than their companion stars without hot ones Jupiter.

As the researchers emphasize, they have now been able to demonstrate the phenomenon on a broad data basis for the first time: "In medicine, you need many people to take part in a study in order to know whether the effects are real or some kind of outlier," says Erst- Author Nikoleta Ilic from AIP. AIP senior author Marzieh Hosseini concludes: “There have been some very intriguing clues in previous cases, but now we finally have statistical evidence that some planets are actually affecting their stars and keeping them young. We hope that future studies will help discover more systems to better understand this effect," says the astronomer.

Source: NASA, Leibniz Institute for Astrophysics in Potsdam, specialist article: Monthly Notices of the Royal Astronomical Society, doi:10.1093/mnras/stac861

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