A young planet, roughly four times the size of Earth, orbiting a petulant red dwarf star. 32 light-years away, this planet — known as AU Mic b — experiences a consistent torrent of energy from its host star, causing its hydrogen atmosphere to puff off the planet. What’s truly remarkable is that the planet’s atmosphere appears to be extremely variable. Astronomers have observed AU Mic b with the Hubble Space Telescope and have seen drastic changes from one orbit to the next. It’s almost as if the planet is “hiccupping” ahead of itself.
The cause for this variability is still unknown, but astronomers have posited two explanations. The first is that a powerful stellar flare may have photoionized the hydrogen in the atmosphere to the point where it became undetectable. The second is that the stellar wind from AU Mic’s star is shaping and altering the planetary outflow, making it visible at some times and invisible at others.
This discovery is incredibly exciting because it provides us with a unique opportunity to study how planets interact with their parent stars. It also provides us with insight into how planets form and evolve in different environments, and how their atmospheres respond to extreme conditions like those found around a red dwarf star. The next step is for scientists to continue to observe AU Mic b and try to determine what’s causing its atmosphere to fluctuate so drastically. Who knows what other remarkable discoveries await us? This young planet may just be the tip of the iceberg when it comes to planetary science.
Hubble follow-up observations of further AU Mic b transits can be highly beneficial to the understanding of exoplanetary atmospheric escape and evolution. The paper published on 27th July 2023 in The Astronomical Journal presents the results of such observations, which should offer additional clues to the star and planet’s odd variability, further testing scientific models. By observing the silhouette of AU Mic b passing in front of its much larger red star, heat from the star is evident in the evaporation of the planet’s atmosphere, a phenomenon that could potentially provide further insight into exoplanetary atmospheric escape and evolution.