A young gas giant exoplanet appears to be causing its host star to emit energetic outbursts. This finding, which comes from astronomers at the Netherlands Institute for Radio Astronomy (ASTRON) and collaborators in Germany, Sweden and Switzerland, is the first evidence of planets actively influencing their stars, rather than merely orbiting them.
“Until now, we had only seen stars flare on their own, but theorists have long suspected that close-in planets might disturb their stars’ magnetic fields enough to trigger extra flares,” explains Maximilian Günther, a project scientist with the European Space Agency’s Cheops (Characterising ExOPlanet Satellite) mission. “This study now offers the first observational hint that this might indeed be happening.”
Stars with flare(s)
Most stars produce flares at least occasionally. This is because as they spin, they build up magnetic energy – a process that Günther compares to the dynamos on Dutch bicycles. “When their twisted magnetic field lines occasionally snap, they release bursts of radiation,” he explains. “Our own Sun regularly behaves like this, and we experience its bursts of energy as part of space weather on Earth.” The charged particles that follow such flares, he adds, are responsible for the aurorae at our planet’s poles.
The flares the ASTRON team spotted came from a star called HIP 67522. Although classified as a G dwarf star like our own Sun, HIP 67522 is much younger, being 17 million years old rather than 4.5 billion. It is also slightly larger and cooler, and astronomers had previously used data from NASA’s Transiting Exoplanet Survey Satellite (TESS) to identify two planets orbiting it. Denoted HIP 67522 b and HIP 67522 c, both are located near their host, but HIP 67522 b is especially close, completing an orbit in just seven Earth days.
In the latest work, which is detailed in Nature, ASTRON’s Ekaterina Ilin and colleagues used Cheops’ precision targeting to make more detailed observations of the HIP 67522 system. These observations revealed a total of 15 flares, and Ilin notes that almost all of them appeared to be coming towards us as HIP 67522 b transited in front of its host as seen from Earth. This is significant, she says, because it suggests that the flares are being triggered by the planet, rather than by some other process.
“This is the first time we have seen a planet influencing its host star, overturning our previous assumptions that stars behave independently,” she says.
Six times more flaring
The ASTRON team estimate that HIP 67522 b is exposed to around six times as many flares as it would be if it wasn’t triggering some of them itself. This is an unusually high level of radiation, and it may help explain recent observations from the James Webb Space Telescope (JWST) that show HIP 67522 b losing its atmosphere faster than expected.
“The new study estimates that the planet is cutting its own atmosphere’s life short by half,” Günther says. “It might lose its atmosphere in the next 400‒700 million years, compared to the 1 billion years it would otherwise.”
If such a phenomenon turns out to be common, he adds, “it could help explain why some young planets have inflated atmospheres or evolve into smaller, denser worlds. And it could inform how we see the demography of ‘adult planets’.”
Astrobiology implications
One big unanswered question, Günther says, is whether the slightly more distant planet HIP 67522 c shows similar interactions with its host. “Comparing the two would be incredible, not only doubling the sampling size, but revealing how distance from the star affects magnetic interactions.”
The ASTRON researchers say they also want to understand the magnetic field of HIP 67522 b itself. More broadly, they plan to look for other such systems, hoping to find out how common they really are.
For Günther, who was not directly involved in the present study, even a single example is already important. “I have worked on exoplanets and stellar flares myself for many years, mostly inspired by the astrobiology implications, but this discovery opens a whole new window into how stars and planets can influence each other,” he says. “It is a wake-up call to me that planets are not just passive passengers; they actively shape their environments,” he tells Physics World. “That has big implications for how we think about planetary atmospheres, habitability and the evolution of worlds across the galaxy.”
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