Hubble telescope captures enigmatic planet undergoing atmospheric changes with every orbit

This stark variability in atmospheric loss between orbits has perplexed scientists.

"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. 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,'" said Keighley Rockcliffe of Dartmouth College in Hanover, New Hampshire, in an official release.

Possible reasons behind atmospheric variability

In 2020, NASA's Spitzer and TESS (Transiting Exoplanet Survey Satellite) space telescopes discovered the planet, AU Mic b.

AU Microscopii is a youthful star, with an age of under 100 million years (while our Sun, by comparison, is 4.6 billion years old).

In this young system, the planet has an orbital period of 8.46 days — located six million miles away from the star. 

Hubble observed variations in the star's brightness corresponding to the hydrogen "escaping" from the planet's atmosphere. Because of the planet's near closeness to the host star, the atmosphere was subjected to high radiation and heat, thereby making hydrogen escape outwards. 

The star's fiery eruption of highly charged flares might be the cause of the hitherto unseen atmospheric alterations. 

“One possible explanation for the missing hydrogen during one of the planet's transits is that a powerful stellar flare, seen seven hours prior, may have photoionized the escaping hydrogen to the point where it became transparent to light, and so was not detectable,” explained the release. 

The authors put forth another possible explanation for this odd behavior, suggesting that the extreme stellar wind could be shaping the planetary outflow. As a result, the atmosphere is observable at times, while not at other times by the Hubble. 

The extreme nature of red dwarf stars

Red dwarf stars, such as AU Microscopii, make up the majority of stars in our home galaxy. These stars are significantly colder and less massive than the Sun. 

Space scientists have long hypothesized whether such stars could pave the way to the formation of potentially habitable worlds in their system. 

However, young red dwarfs are extremely eruptive, frequently bombarding any planets orbiting close to the star. 

There are certain exceptionally high period events in which the star unleashes tremendous energy that may persist considerably longer than that of other stars such as the Sun.

This harsh stellar environment would make it impossible for any nearby planet’s atmosphere to flourish, especially within the first 100 million years of the star's formation. That’s why, the planet may end up without an atmosphere forever. 

For these reasons, it has been hard to derive any definitive conclusion on finding habitable worlds around the red dwarf stars.

"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," he concluded.

The results have been published in The Astronomical Journal.

Study Abstract:

Photoevaporation is a potential explanation for several features within exoplanet demographics. Atmospheric escape observed in young Neptune-sized exoplanets can provide insight into and characterize which mechanisms drive this evolution and at what times they dominate. AU Mic b is one such exoplanet, slightly larger than Neptune (4.19 R⊕). It closely orbits a 23 Myr pre-main-sequence M dwarf with an orbital period of 8.46 days. We obtained two visits of AU Mic b at Lyαwith Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph. One flare within the first HST visit is characterized and removed from our search for a planetary transit. We present a nondetection in our first visit, followed by the detection of escaping neutral hydrogen ahead of the planet in our second visit. The outflow absorbed ∼30% of the star's Lyα blue wing 2.5 hr before the planet's white-light transit. We estimate that the highest-velocity escaping material has a column density of 1013.96 cm−2 and is moving 61.26 km s−1 away from the host star. AU Mic b's large high-energy irradiation could photoionize its escaping neutral hydrogen in 44 minutes, rendering it temporarily unobservable. Our time-variable Lyα transit ahead of AU Mic b could also be explained by an intermediate stellar wind strength from AU Mic that shapes the escaping material into a leading tail. Future Lyα observations of this system will confirm and characterize the unique variable nature of its Lyα transit, which, combined with modeling, will tune the importance of stellar wind and photoionization.