Hubble sees evaporating planet getting the hiccups

A young planet whirling around a petulant red dwarf star is changing in unpredictable ways orbit-by-orbit. It is so close to its parent star that it experiences a consistent, torrential blast of energy, which evaporates its hydrogen atmosphere — causing it to puff off the planet.

Located 32 light-years from Earth, the parent star AU Microscopii (AU Mic) hosts one of the youngest planetary systems ever observed. The star is less than 100 million years old (a tiny fraction of the age of our Sun, which is 4.6 billion years old). The innermost planet, AU Mic b, has an orbital period of 8.46 days and is just 6 million miles from the star (about 1/10th the planet Mercury's distance from our Sun). The bloated, gaseous world is about four times Earth's diameter.

During one orbit observed with the NASA/ESA Hubble Space Telescope, AU Mic b looked like it wasn't losing any material at all, while an orbit observed with Hubble a year and a half later showed clear signs of atmospheric loss. This extreme variability between orbits shocked astronomers. They were equally puzzled to see, when it was detectable, the planet's atmosphere puffing out in front of the planet, like a headlight on a fast-bound train.

The never-before-seen changes in atmospheric outflow from AU Mic b may indicate swift and extreme variability in the host red dwarf's outbursts. One possible explanation for the missing hydrogen 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. Another explanation is that AU Mic’s stellar wind is shaping the planetary outflow, making it observable at some times and not observable at other times, even causing some of the outflow to "hiccup" ahead of the planet itself. 

Hubble follow-up observations of more AU Mic b transits should offer additional clues to the star and planet's odd variability, further testing scientific models of exoplanetary atmospheric escape and evolution.

These results are featured in the paper published on 27th July 2023 in The Astronomical Journal.

[Image description: An illustration depicts a planet, shown in silhouette as a small dark circle, passing in front of a much larger red star on a black starry background. Heat from the star is evaporating the planet’s atmosphere, which stretches out linearly along the planet’s orbital path as dark purple gas.]