Planets Make it Harder to Figure out a Star’s age

Estimating stellar age has always been a challenge for astronomers. Now, a certain class of exoplanets is making the process even more complicated. Hot Jupiters – gas giants with orbital periods smaller than that of Mercury – appear to have an anti-aging effect on their stars, according to a new study. These enormous planets inflict both magnetic and tidal interference on their host star, speeding up the star’s rotation and causing them to emit X-rays more energetically, both of which are hallmarks of stellar youth. The result calls into question some of what we previously believed about stellar age, and offers a glimpse at the ongoing interconnectivity between a star and its planets long after their formation.

Determining a star’s age is hard enough on its own without taking into account planetary interference. We are quite confident of our own Sun’s age, being close enough to sample the geological leftovers of the solar system’s formation, but for other stars, our best guesses are estimates rather than exact dates. We know that certain star types go through phases at different stages of life depending on their mass (this is how we know Betelgeuse will go supernova in the next 100,000 years, give or take). If you have a whole cluster of stars of different types, you can plot their individual life stages on a Hertzspring-Russell diagram using their color and brightness, and collectively estimate the age of all the stars in the cluster. The method isn’t entirely accurate, but it’s better than nothing.

Hertzsprung-Russell Diagram showing the phases of different star types. Credit: ESO.

Things get more complicated if you want to know the age of an individual star. We do know that young stars rotate quickly, and slow down as they age. This is a useful tool for dating young stars, but is less useful for older ones, which sometimes maintain a static rotational speed for the rest of their lives. Young stars are also more prone to flares and X-ray emissions than their elderly peers. Neither metric offers a perfect dating method, but can provide a workable estimate.

Now it seems that exoplanets might throw another wrench in the works.

When Hot Jupiters were discovered (the first was seen in 1995), astronomers noticed that in some cases, the stars they were orbiting appeared younger than expected. But because our stellar dating methods are so inaccurate, it was hard to tell whether this was a real de-aging effect caused by the planets, or if the stars in question were just young to begin with.

In a paper recently accepted by the Monthly Notices of the Royal Astronomical Society, a team from the Leibniz Institute for Astrophysics Potsdam (AIP) found a way to solve this problem. The solution was to study binary star systems, where one star in the pair has a Hot Jupiter, and the other does not. Binary stars are usually the same age as each other, so they can be compared to determine if the de-aging effect is real.

“It’s almost like using twins in a study where one twin lives in a completely different neighborhood that affects their health. By comparing one star with a nearby planet to its twin without one, we can study the differences in behavior of the same-aged stars,” says Katja Poppenhäger, head of the Stellar Physics and Exoplanets section at AIP.

The study, led by PhD candidate Nikoleta Ilic, used data from the Chandra X-ray Observatory and ESA’s XMM-Newton Observatory to examine 16 binary star systems in which one of the stars had a Hot Jupiter. The findings confirm that indeed, the stars hosting massive nearby planets do appear younger than their unaccompanied twins.

Chandra data for two systems where one star is orbited by a hot Jupiter (HD189733 and WASP-77). The stars with hot Jupiters are clearly brighter than their companion stars, including a non-detection for the companion in WASP-77. Image Credit: NASA/CXC/Potsdam Univ./N. Ilic et al.

The key to this de-aging interaction is that the Hot Jupiter’s orbital period has to be faster than the rotational period of the star. In these cases, the planet transfers some angular momentum to the star via tidal forces, causing the planet to speed up and jumpstarting an increased level of X-ray emissions.

The authors also considered whether the increase in X-ray emissions was caused by magnetic fields rather than tidal forces – after all, planets like Jupiter tend to have powerful magnetic fields. But the fact that the stars also rotated more quickly in addition to the increased X-ray activity indicates a gravitational, that is, tidal, source for the de-aging process, not a magnetic one.

The results of the study indicate that stellar aging is more complex than we previously understood, and that planets – especially massive, fast-moving ones – can shape the very life cycles of their host star.

But what about Earth-like planets? Do they de-age their suns too? The answer seems to be no. In fact, in the study, stars with small distant planets actually seemed dimmer and calmer (and therefore perhaps older) than their twin. But when examined more closely, this was determined to be a fluke, explained away by the fact that those stars happened to be F/G type stars, which had reached a different, calmer stage of their lifecycle than their M type twin, which takes longer to reach a comparably calm stage. Long story short, small planets don’t have much effect on their star’s age. Only the big ones do and even then, only when they orbit very close in. The moral of the story is that if you are a star, and you want to stay young, it helps to have the company of a Hot Jupiter. Nothing less will do.

Read the paper here: Nikoleta Ilic, Katja Poppenhaeger, S. Marzieh Hosseini. “Tidal star-planet interaction and its observed impact on stellar activity in planet-hosting wide binary systems.” ArXiv Preprint.

Press Release: “NASA’s Chandra: Planets Can Be Anti-Aging Formula for Stars.”

Featured Image: An artist’s illustration showing a Hot Jupiter (lower right) closely orbiting its host star (left), with another star in the distance (upper right). The two stars are themselves in orbit with each other. Credit: NASA/CXC/M.Weiss. X-ray: NASA/CXC/Potsdam Univ./N. Ilic et al.