A young star is attacking the exoplanet in its orbit with such fury that it occasionally rips off large amounts of atmosphere.

This exoplanet is called AU Microscopii b (or AU Mic b) and orbits a very young red dwarf star just 32 light years from Earth. The system is only 23 million years old, almost an infant, and its wild behaviour offers a rare glimpse into the turbulent early life of newly formed planets.

The most interesting aspect of the Neptune-sized world is that atmospheric escape is not constant. Instead, it seems to switch on rapidly, moving from detectable to undetectable along a single orbit. And when it was detectable, the atmosphere seemed to leak in front of the exoplanet, rather than behind it.

"When I first saw it, I thought, 'This can't be right,'" says Keighley Rockcliffe, an astronomer at Dartmouth College who led the research.

"This is obviously a bizarre observation, a kind of stress test case for modelling and physics related to planetary evolution. This observation is so cool because we can really study this interaction between the star and the planet at its most extreme."

AU Mic b was discovered in 2020, and even then scientists knew it should be interesting. It is slightly larger than Neptune, with 20.12 Earth masses and 4.19 Earth radii; Neptune has 17.15 Earth masses and 3.88 Earth radii. Unlike Neptune, AU Mic b is very close to its star: An orbit is only 8.5 days.

This proximity gives the exoplanet a front-row seat to the star's belligerent behaviour, and the star is belligerent. Young stars are much more active than older, calmer, more established ones, and even old red dwarfs are quite active.

AU Mic is fully operational, spewing out more than 6 flares a day. It is a constant onslaught of wild stellar winds, flares and harsh X-ray light.

Rockcliffe and his team wanted to take a closer look at the exoplanet because it could help explain an interesting gap in exoplanet detections, worlds between 1.5 and 2 times Earth's radius that are close to their stars.

AU Mic b is on the large side of this gap, but if it is actively losing its atmosphere and thus shrinking, "we want to find out what kind of planets can survive in these environments," explains Rockcliffe.

"What will they eventually look like when the star settles down? ... We don't really know what these final compositions look like because there is nothing like it in our Solar System."

It's not clear what exactly is in AU Mic b, but whatever it is, it's a lot. All the mechanisms for atmospheric stripping seem to be happening simultaneously, which means it is the perfect laboratory for studying the weather conditions of an exoplanet.

As for planet-leading hydrogen detection, there could be several reasons for this. Stellar explosions can ionise hydrogen, making it invisible to our telescopes. It's still there, we just can't see it. Strong stellar winds could also be directing where the hydrogen goes, pushing it towards the front of AU Mic b rather than leaving it behind.

"Knowing how atmospheres evolve and which planets will have stable atmospheres is important for finding life on other planets," says Rockcliffe. "Atmospheres are crucial for understanding how life can form and persist. "It could shed light on the mechanisms that produce the void. Young worlds battered by stormy stars can cause them to lose their atmospheres and shrink. Larger worlds may have enough gravity to maintain a thick atmosphere, while smaller ones simply turn into bare rock.

 

Source: https://www.sciencealert.com/