Planet that triggers stellar flares is slowly destroying itself

A planet orbiting a young star far beyond our solar system is behaving in a strange way. Instead of just soaking up radiation from its star, this planet may actually be the one triggering powerful energy blasts.

The stellar bursts are so intense, they strip away the planet’s atmosphere bit by bit. Scientists think this could be the first time we’ve seen a planet play such a dangerous game with its host star.

Stirring up deadly flares

Until recently, this kind of interaction was just a theory. Now, astronomers finally have the evidence to back it up.

Using data from the European Space Agency’s Cheops mission, a team of scientists from the Netherlands Institute for Radio Astronomy (ASTRON) has found a planet that might be stirring up deadly flares on its parent star.

“We hadn’t seen any systems like HIP 67522 before; when the planet was found it was the youngest planet known to be orbiting its host star in less than 10 days,” said study co-author Dr. Ekaterina Ilin of ASTRON.

A young star emerges

The star at the heart of this drama is HIP 67522. It’s slightly bigger and cooler than our Sun, but far younger – just 17 million years old. Compare that to our Sun’s 4.5 billion years, and HIP 67522 is basically an infant in stellar terms.

HIP 67522 has at least two planets, with the inner one, called HIP 67522 b, completing a lap around the star in just seven days.

Because the star is so young, it spins rapidly and churns with energy, which turns it into a powerful magnetic engine. This is in stark contrast to our calmer, middle-aged Sun.

Astronomers already knew that magnetic stars can produce sudden energy flares when their magnetic field lines snap and realign. These flares can emit all kinds of radiation, from visible light to powerful gamma rays.

Planets that trigger stellar flares

Scientists have debated since the 1990s whether a planet could disturb a star’s magnetic field and trigger flares – but only now have they confirmed it.

When NASA’s TESS spacecraft picked up signs of flaring activity from HIP 67522, scientists saw a rare opportunity. They used ESA’s Cheops telescope for a closer look.

“We quickly requested observing time with Cheops, which can target individual stars on demand, ultra precisely,” Ilin said. “With Cheops, we saw more flares, taking the total count to 15, almost all coming in our direction as the planet transited in front of the star as seen from Earth.”

The fact that so many of the flares occurred when the planet passed in front of the star suggests a direct link. The team believes HIP 67522 b may actually be setting off these outbursts.

Whipping the star into a frenzy

Here’s how it might work: as the planet orbits very close to the star, it could be pushing energy into the star’s magnetic field lines.

It’s a bit like snapping a towel. When the wave of energy travels down the magnetic line and reaches the surface of the star, it sets off a flare. This is a surprising role reversal. Usually, we think of stars affecting planets – not the other way around.

Not only is HIP 67522 b causing these flares, but it’s also taking the full brunt of the energy. That means it’s getting blasted with six times more radiation than normal.

Puffy planet triggering energetic flares

HIP 67522 b rivals Jupiter in size but matches cotton candy in density, making it one of the puffiest planets scientists have discovered. That makes it especially vulnerable to the star’s radiation.

Each flare strips away more of the planet’s feathery atmosphere. If this keeps up, HIP 67522 b could shrink from a Jupiter-sized planet to something closer to Neptune in just 100 million years.

“The planet seems to be triggering particularly energetic flares,” noted Ilin. “The waves it sends along the star’s magnetic field lines kick off flares at specific moments.”

“But the energy of the flares is much higher than the energy of the waves. We think that the waves are setting off explosions that are waiting to happen.”

A planet bombarded with radiation

HIP 67522 b is no longer alone. Since its discovery, scientists have identified a few similar star-planet systems and believe dozens more may still be out there.

“I have a million questions because this is a completely new phenomenon, so the details are still not clear,” Ilin said.

“There are two things that I think are most important to do now. The first is to follow up in different wavelengths – Cheops covers visible to near-infrared wavelengths – to find out what kind of energy is being released in these flares. Ultraviolet and X-rays, for example, are especially bad news for the exoplanet.”

“The second is to find and study other similar star-planet systems; by moving from a single case to a group of 10–100 systems, theoretical astronomers will have something to work with.”

Cheops’ unexpected contribution

ESA’s Cheops telescope wasn’t even built to study flares. Its main job is to look at the size and atmospheres of exoplanets. But that hasn’t stopped it from contributing to this unexpected discovery.

“Cheops was designed to characterize the sizes and atmospheres of exoplanets, not to look for flares. It’s really beautiful to see the mission contributing to this and other results that go so far beyond what it was envisioned to do,” said Maximillian Günther, Cheops project scientist at ESA.

In the future, ESA’s upcoming Plato mission will take a closer look at stars like HIP 67522. With its ability to detect smaller flares, it might give us a better picture of what’s really going on in these volatile star-planet relationships.

The full study was published in the journal Astronomy & Astrophysics.

Image Credit: Janine Fohlmeister (Leibniz Institute for Astrophysics Potsdam)

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