Stars often whip their planets with stellar winds and radiation, pull them ever closer with gravity and sear them with heat. But a newfound planet exerts an unexpectedly strong—and ultimately self-destructive—influence on its star in return.
The star, HIP 67522, is slightly larger than our sun and shines roughly 408 light-years away in the Scorpius-Centaurus association. It’s 17 million years old, a youngster by stellar standards, and has two orbiting planets that are even younger. The innermost of these two planets, a Jupiter-size gas giant called HIP 67522 b, orbits HIP 67522 at a distance of less than 12 times the star’s radius—about one-seventh Mercury’s distance in solar radii from our own sun. This in-your-face proximity, combined with HIP 67522’s volatile teenage nature, creates a spectacle that astronomers have never seen before: a planet that triggers powerful flares on the surface of its host star, leading to the planet’s own slow destruction.
“In a way, we got lucky,” says Ekaterina Ilin, an astrophysicist at the Netherlands Institute for Radio Astronomy (ASTRON), who led a study published in Nature on the HIP 67522 system. “We took all the star-planet systems that we knew of and just went ahead looking for flares—sudden, intense bursts of radiation coming from the star’s surface.” Parsing the data gathered by two space-based telescopes, nasa’s TESS (Transiting Exoplanet Survey Satellite) and the European Space Agency’s CHEOPS (Characterizing Exoplanet Satellite), Ilin’s team noticed that HIP 67522’s flares seemed to be synchronized with its closest planet’s orbital period. And those flares were gigantic—“thousands of times more energetic than anything the sun can produce,” Ilin says.
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The planet HIP 67522 b suffers severe consequences from flares on its host star: the high-energy radiation blasts atmosphere off of the low-density planet, threatening to eventually bring it down from a Jupiter-size world to a Neptune-size one.
Janine Fohlmeister/Leibniz Institute for Astrophysics Potsdam
The orbiting gas giant most likely sparks these powerful flares by perturbing the star’s strong magnetic field lines as it passes by. This disruption sends waves of energy downward along the lines—and when those waves meet the star’s surface, a flare bursts out. The star’s magnetic loops are “almost like a spring waiting to be let go,” Ilin says. “The planet’s just giving it this last push.” Based on the team’s observations, HIP 67522 b sets off a flare once every Earth day or two.
This action has severe consequences for the planet: Ilin estimates the unlucky gas giant is hit by six times more flares than it would be if it weren’t triggering extras, and it’s blasting away its own atmosphere. At this pace, Ilin’s team says, HIP 67522 b will shrink from Jupiter’s size to Neptune’s or smaller in about 100 million years. “Flaring might cut the lifetime of the planet’s atmosphere in half,” she says.
Researchers had suspected this type of star-planet interplay might occur, but they had never previously confirmed it, says Antoine Strugarek, an astrophysicist at the Institute of Research into the Fundamental Laws of the Universe, which is part of the French Alternative Energies and Atomic Energy Commission. “This is the first time we see very convincing evidence such interaction has been actually detected,” says Strugarek, who was not involved in the study.
It is too early to draw far-reaching conclusions from this first example of the phenomenon. As a next step, Ilin says, researchers can compare HIP 67522 b with the other planet in the system (orbiting a bit farther from the star) to calculate how much mass the more closely orbiting world loses through this process compared with the more distant one, which is probably hit with only random flares.
Another unanswered question is exactly how the flare triggering works. “Is it a wave [of magnetic energy] that propagates from the planet?” Ilin wonders. She suggests that what happens could be like an effect seen on the sun: smaller solar flares sometimes perturb nearby magnetic loops and tip them over the edge to snap and produce a larger flare.
But perhaps the most important question is how common the phenomenon is. For now Ilin wants to try to find more systems in which planets induce stellar flares that scientists can study. “Once we figure out how it works, we can turn it into a planet-detection technique,” she says. Instead of searching for the planets themselves, researchers could find stars that flare following a certain pattern—suggesting they, too, might have planets with a self-destructive bent.
