Hubble telescope sees an angry star and an evaporating planet

Just about 32 light-years from our solar system (which is incredibly close, cosmically speaking) an angry red dwarf star named AU Microscopii is absolutely tormenting one of its very own planets, AU Microscopii b. Piece by piece, the stellar beast seems to be eviscerating the world's atmosphere with its high-energy radiation. It's quite chaotic over there, to say the least. 

But, lucky for us space-gazers, thanks to NASA's trusty Hubble Space Telescope, scientists announced on Thursday (July 27) that they've earned an important new view on this interstellar nightmare. And in a way, these new Hubble results may have a twofold effect on the field of astronomy: They can tell us about exoplanets undergoing a sort of atmospheric evaporation while also giving us a better understanding of the stars that might ceaselessly pluck those atmospheres off.

Measuring only about four times the diameter of Earth, the terrorized planet AU Mic b had the misfortune of spawning only about 6 million miles (9.6 million km) from its star — an orb that just so happens to be classified as a red dwarf, one of the most extreme stellar bodies to exist in our universe. Research has shown, for instance, that even the quietest examples of red dwarf stars like this one are still wilder than our blistering sun. But that's not all. To make matters even more intense, AU Mic's feistiness is exacerbated by the fact that it's one of the youngest stars out there. 

"It's 23 million years old — which is very, very young, basically a toddler star," Keighley Rockliffe, first author of a study on the new Hubble findings and exoplanet researcher at Dartmouth College, told Space.com. "Stellar astronomers are excited because AU Mic is an example of what the young, tantrum-like years of a red dwarf — the most common type of star in the Milky Way — are like."

Related: The 10 most Earth-like exoplanets

The power of deduction

Basically, as Hubble is located in Earth's orbit, it can identify when a distant planet transits between its stellar host and our planet by catching rays of starlight. 

Such a passage makes the star at hand appear dimmer for a moment, alerting whatever's watching that something must have created the light dip. In fact, this mechanism of deduction was actually how scientists located AU Mic b to begin with in 2020. Instead of Hubble, however, NASA's Spitzer and TESS space telescopes get the credit on that first contact. Now, though, Hubble gets to claim its own AU Mic system discovery.

When Rockliffe and fellow researchers observed AU Mic b with Hubble, they saw far more than just regular transit starlight dimming. Some of the light waves associated with this dimming told them the material causing the dip was much deeper than the planet itself. Too deep. And, that material literally seemed to be pulling ahead of the planet. Too far. Zooming out, it almost looked like AU Mic b was transiting way earlier than expected and in a wonky format.

The only explanation? Well, they were looking at planetary material so far away from the world's core that it had to have gravitationally escaped from the planet. Or as Rockliffe puts it, it must've been "blowing away!" 

"I remember when I first created the light curve of this planet with our Hubble data and saw that the transit happened well before the predicted transit of the planet," she said. "I immediately thought 'well, I definitely made a mistake.' However, no matter how many times I redid the analysis from different methods, the result was the same."

Presumably, this wisping-away of planetary atmosphere is due to the red dwarf's super high-energy flares — think, our sun's flares times a thousand — blasting the planet with radiation.

And although such an atmospheric escape process due to an aggressive host star isn't exactly new, Rockliffe explained that what makes this observation so novel is that it's the first time scientists have witnessed such shedding on a regular old planet. 

"AU Mic b is special because it is not special at all!" she said. "It is an example of the young stage of the most typical exoplanet-star system in the galaxy, and therefore gives us important insight into what atmospheric escape looks like at this very critical and very common formative period of an exoplanetary system."

For example, the first observations of such atmospheric evaporation were with worlds known as "hot Jupiters," some of the rarest exoplanets in our galaxy. They're kind of like Jupiter doppelgangers, except very (very) hot because they're so close to their stars. But the point here is that there aren't a lot of Hot Jupiters that we know of.

To put this into perspective, Rockliffe explains that using that information to decode the mechanism would be "like using a financially affluent person's experience to extrapolate the typical experience of the middle class."

It just wouldn't be accurate. 

But now that we have these new Hubble observations about AU Mic b, perhaps we're on the path toward learning more about the daily lives, trials and tribulations of, well, normal stars.

Now what?

"Atmospheric escape," Rockliffe explained, "is potentially one of the most influential evolutionary processes for the majority of exoplanets."

Essentially, this work could help scientists decode what worlds beyond our solar system are like, where they reside and perhaps create guides that'll help us write out their lore. 

Rockliffe added that one of the questions scientists are still trying to answer concerns how exoplanets could lose their atmospheres. "For exoplanet astronomers, among many other things," she said, "what mechanism is primarily driving this atmospheric escape? Is it heating from the intense high-energy radiation from the star, or is it a hot newly-formed planetary core heating the atmosphere from the inside out?"

For instance, one of the stranger things about the recently observed atmospheric escape on AU Mic b is that it seems to be erratic. It's almost like the star sometimes gets into a bad mood and takes it out on the planet, but other times, it's just chilling.

"My opinion, or intuition, is telling me this behavior is intricately tied to the atmosphere's interaction with AU Mic's stellar winds," Rockliffe said. "AU Mic's stellar winds might be causing turbulence as it interacts with the atmosphere, causing this time-variable burping of hydrogen gas."

Interestingly, she also has some theories as to what the planet might look like billions of years from the point Hubble isolated — though, they are still preliminary. On one hand, if the world gets completely stripped of its atmosphere and broken down to its bare bones, it could end up looking something like Mercury. But on the other, if it gets to keep its lower atmosphere, we might be in store for a Neptune duplicate. 

But really, only time (and more Hubble observations, of course) will tell.

Plus, in the realm of stellar evolution, these findings could help astronomers understand what young red dwarfs have in common, and do not have in common, with stars like our sun and even their older counterparts. 

"This is the first time we've seen planetary outflow out ahead of the planet like this. Even now, there's this existential dread that I've done something completely wrong. But I trust my co-authors to catch mistakes I might make and I trust the various approaches I took to prove this result is accurate," she said, particularly calling out the role her advisor, Elisabeth Newton, had in the work. "It has meant the world to be able to do such cool science with such a great scientist and within the environment she's created."

Next up, the team hopes to study those weird variable atmospheric escape "burps" in more detail with Hubble — and potentially, the James Webb Space Telescope. 

A paper on this work was published on July 27 in The Astronomical Journal.