In a surprising discovery, astronomers have observed a young planet evaporating its layers as it orbits a red dwarf star. The planet AU Mic b is located in one of the youngest planetary systems ever observed, hosted by the star AU Microscopii (AU Mic), less than 100 million years old and 32 light-years from Earth. The research team was stunned to find that AU Mic b, which orbits the star every 8 1/2 days, has been undergoing unpredictable atmospheric evaporation.
The planet is forming into a more minor, potentially rockier Neptune-like planet as it loses its larger gaseous envelope or atmosphere. But observing the atmospheric escape of such short-orbit, Neptune-sized planets is problematic because it occurs so quickly. The evaporation is expected through various mechanisms, including hydrodynamic evaporation, which involves water vapor boiling off the surface and expanding outward. However, observing this evaporation is challenging because it occurs so fast that the planetary gases are not easily detected in spectra obtained during transits.
Using the Hubble Space Telescope, researchers were surprised to see a sudden and dramatic change in the shape of the hydrogen escaping from the planet’s atmosphere. During one transit, there was no visible hydrogen, suggesting that a powerful star flare seen seven hours earlier may have photoionized the escaping hydrogen and made it transparent to light. Alternatively, the stellar wind could be shaping the planetary outflow to make it visible at certain times and invisible at others, perhaps even “hiccuping” ahead of the planet itself.
The unexpected variation in the shape of the hydrogen cloud suggests that the interaction between AU Mic and its forming planet is much more complicated than had previously been assumed. The team will continue to conduct follow-up observations of the evaporating AU Mic b and plans to use Hubble during more transits to test scientific models of exoplanetary atmospheric escape and evolution.
The study is published in the journal Science Advances. It features an illustration of a young Neptune-size planet surrounded by its rapidly evolving gaseous envelope, with the star it orbits in the background. It is part of a series of posters called “The Galaxy of Horrors,” illustrating how stars and their planets interact. The posters, which feature illustrations by artist Keighley Rockcliffe, are available online in both English and Spanish. The scientists are also working on a book based on the posters that will be out next year.
