The exoplanet AU Microscopii b (AU Mic b), orbiting a young red dwarf star 32 light-years from Earth, is being battered by its host star with such intensity that it periodically loses much of its atmosphere. This peculiar behavior gives scientists a rare opportunity to study the tumultuous early stages of planet formation.
The Neptune-sized exoplanet, AU Mic b, exhibits a unique pattern of atmospheric escape. Instead of a constant leakage of atmosphere, it rapidly turns on and off within a single orbit. Moreover, when the atmosphere is detected, it appears to leak out in front of the exoplanet rather than lagging behind it – a phenomenon that initially perplexed astronomers.
“This frankly bizarre observation is a kind of stress test for modeling and physics of planetary evolution. This observation is great because we get to study the interaction between a star and a planet in the most extreme case,” said astronomer Keagley Rockcliffe of Dartmouth College, who led the study.
The planet AU Mic b was first discovered in 2020 and immediately caught the attention of scientists due to its size being slightly larger than Neptune. With a mass of 20.12 Earth masses and a radius of 4.19 Earth radii, AU Mic b is in close proximity to its star, making one orbit in just 8.5 days.
This proximity exposes the exoplanet to the aggressive behavior of its young host star. Young stars are known to be more active than their older counterparts, and AU Mic is no exception. The star bombards AU Mic b with intense stellar winds, flares, and hard X-rays, producing more than 6 flares per day.
Rockcliffe and her team are focusing their study of AU Mic b to shed light on a puzzling gap in the detection of exoplanets – planets with radii 1.5 to 2 times larger than Earth’s, located close to their stars. AU Mic b is at the larger end of this gap, and if it is indeed losing its atmosphere and shrinking in size, it could provide insight into the mechanisms responsible for this gap. Violent interactions between young planets and turbulent stars can cause atmospheric loss and compression, leaving behind barren rocky worlds.
“We want to find out what types of planets can survive in such conditions,” Rockcliffe explains. “What will they look like when the star calms down? … We don’t know what the final composition of the planets will be, since we don’t have anything like that in our solar system.”
The exact processes taking place in AU Mic b’s atmosphere remain unclear, but the intensity of the phenomena suggests that several atmospheric collapse mechanisms are operating simultaneously. This makes AU Mic b an ideal laboratory for studying the stability of exoplanets and the various problems they face.
While scientists continue to unravel the mysteries surrounding AU Mic b, this unusual exoplanet serves as a fascinating case study to provide valuable insights into the early life of planets and the complex interactions between stars and their celestial satellites.
