A Jupiter-size planet as delicate as cotton candy provokes its star

A gas giant planet in distant space has kicked the cosmic hornet's nest: After the world's relentless taunting, its fiery star is after revenge, and the planet will suffer the consequences, astronomers say. 

Exoplanet HIP 67522 b, fluffed up to the size of Jupiter, circles so close to its star that it’s triggering violent flares — bursts of high-energy radiation — from its host that then blast it in the face. Those flares strip away the planet’s delicate atmosphere, bit by bit, year by year.

Researchers using the Characterising Exoplanet Satellite (Cheops) mission — run by NASA's European counterpart across the pond — spotted 15 intense outbursts from the star HIP 67522, most of them happening just as the planet crossed in front of it. That timing isn't a coincidence, according to the European Space Agency. The team suspects the planet is disturbing the star's magnetic field, spurring a torrent of explosions far more powerful than anything the sun tosses at Earth.

This is the first time scientists have caught a planet appearing to incite its star as it happens. Until now, the energy between stars and their planets was assumed to flow one way: The star radiates, the planet takes the hit. But HIP 67522 b is turning that narrative on its head, triggering flares that hit back.

"Cheops was designed to characterise the sizes and atmospheres of exoplanets, not to look for flares," said Maximilian Günther, the mission's project scientist, in a statement. "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."

Intense stellar explosions are blasting away the planet's thick atmosphere. Credit: European Space Agency infographic

About 400 light-years away, the star, HIP 67522, is young — just 17 million years old — and violent. It's slightly bigger and cooler than the sun but more energetic, spinning fast. 

HIP 67522 b orbits it at breakneck speed, completing a loop in just seven days. That kind of closeness creates tension. As the exoplanet swings around its host, it seems to inject energy into the star’s magnetic field, like a whip cracking. When the wave hits the surface, the star pummels back but with even more fury.

HIP 67522 b is already one of the least dense exoplanets ever found, with a consistency more like cotton candy than a solid. Its atmosphere is quickly deteriorating under the star's rapid fire of flares. The team has estimated the planet experiences six times more radiation than it otherwise would. The researchers' findings appear in the journal Nature.

"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," said Ekaterina Ilin at the Netherlands Institute for Radio Astronomy, who led the research. "We think that the waves are setting off explosions that are waiting to happen."

HIP 67522 b sends a wave of energy down magnetic field lines toward the surface of its host star, which then detonates a flare. Credit: Danielle Futselaar / https://www.artsource.nl/ illustration

The idea that a planet could provoke its own doom is not new. That theory has been around since the 1990s, but this is the first hard evidence. The discovery prompts more questions: How many planets out there are caught in this kind of feedback loop? What kinds of flares are they triggering? And just how fast are the planets' atmospheres evaporating?

In a cosmic blink — perhaps 100 million years — HIP 67522 b could shrink down to the size of Neptune or smaller.

Astronomers now want to expand the hunt. With tools like NASA's Transiting Exoplanet Survey Satellite, aka TESS, and the upcoming ESA Plato mission, scientists hope to find more of these ill-fated planets and figure out if HIP 67522 b is an outlier or just one of a bunch of tragic worlds out there asking for trouble.