Astronomers Spy a Star Ripping a Planet’s Atmosphere to Shreds
Young, small stars are especially volatile — which is bad news for this exoplanet.
What do you get when a hot young world orbits a wildly unstable young red dwarf? For AU Microsopii b, the answer is flared from the star tearing away the atmosphere. That catastrophic loss happens in fits and starts, "hiccuping" out its atmosphere at one point and then losing practically none the next.
That frenetic activity is kind of shocking. Usually, interactions between stars and their planets are more constant. But not this one. "We've never seen atmospheric escape go from completely not detectable to very detectable over such a short period when a planet passes in front of its star," said Keighley Rockcliffe of Dartmouth College in Hanover, New Hampshire. "We were really expecting something very predictable, repeatable. But it turned out to be weird. When I first saw this, I thought, 'That can't be right.'"
Rockcliffe and her team are working on scenarios for this weirdly variable atmospheric loss. Such activity is important to understand as astronomers find more planets close to their stars, particularly red dwarfs. “We want to find out what kinds of planets can survive these environments. What will they finally look like when the star settles down? And would there be any chance of habitability eventually, or will they wind up just being scorched planets?” said Rockcliffe. “Do they eventually lose most of their atmospheres, and their surviving cores become super-Earths? We don’t really know what those final compositions look like because we don’t have anything like that in our solar system.”
How Red Dwarf Stars Affect Planet Atmospheres
AU Microscopii b (AU Mic b) is a Neptune-sized planet with a hydrogen atmosphere. It was first discovered orbiting its parent star by NASA’s Spitzer Space Telescope and the Transiting Exoplanet Survey Satellite) in 2020. They spotted the planet as it transited (passed in front of) the star.
The star and its planet lie 32 light-years from Earth. AU Microscopii (AU Mic) itself is a very young star—only about 32 million years old. This stellar baby is a red dwarf and shows an amazing amount of flaring activity and variability. That affects its planets, particularly AU Mic b, which orbits only 9.6 million kilometers away.
Red dwarfs like AU Mic are the most abundant stars in our Milky Way galaxy. It seems likely that they host many of the planets in our galaxy. But there’s a catch. These stars blast out ferocious stellar flares that blast any nearby planets with radiation. The flares stem from activity in the strong magnetic fields in the stellar atmospheres. The fields break from time to time and then reconnect. That releases huge amounts of energy that send torrential stellar winds, flares, and X-rays out to space. Any planets that get in the way get baked. “This creates a really unconstrained and, frankly, scary, stellar wind environment that’s impacting the planet’s atmosphere,” said Rockcliffe. Young planets around these young stars lose their atmosphere, sometimes completely.
What Happens to AU Microscopii’s Neptune-Sized Planet
To understand the weirdly hiccupy atmospheric loss at AU Mic b, Rockcliff’s team used observations made by Hubble Space Telescope’s Space Telescope Imaging Spectrograph (HST/STIS). It’s sensitive to ultraviolet light, which is given off as the escaping atmosphere is photoionized by the parent star’s extreme heat and flare activity. The STIS instrument captured enough data to allow the science team to at least theorize about what’s happening in this system as the star heats its immediate environment with flares and other activity.
Changes in atmospheric outflow from AU Mic b may indicate rapid extreme variability in the star’s outbursts. Astronomers attribute that variability to roiling magnetic field lines. It’s possible that a powerful stellar flare photoionized the hydrogen escaping from the planet to the point where it became transparent to light. That rendered it undetectable.
Most puzzling is the escape of hydrogen ahead of the planet as it orbits its parent star. It’s possible that the high-energy radiation from the star is shaping the atmospheric hydrogen into a “leading tail” that precedes the star. To prove that, astronomers will need to do more follow-up observations of AU Mic b as it transits its star in the future. Not only will that pin atmospheric loss to stellar variability, but further observations will help explain atmospheric loss in such planets.