A process called 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. 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 about 1.5 years later showed clear signs of atmospheric loss.
AU Microscopii is a red dwarf located 31.9 light-years away in the southern constellation of Microscopium.
Also known as AU Mic, Gliese 803 and HD 197481, the star is approximately 22 million years old.
AU Mic is a member of nearby collection of stars called the Beta Pictoris moving group, which takes its name from a bigger, hotter A-type star that harbors two planets.
The star hosts a young Neptune-sized exoplanet, AU Mic b, and a relatively rare edge-on disk of debris extending from about 35 to 210 AU (astronomical units) from the star.
First discovered by NASA’s Spitzer and TESS space telescopes in 2020, the planet has a radius of 0.4 Jupiter radii and a mass of less than 0.18 Jupiter masses.
AU Mic b orbits its parent star once in every 8.5 days at a distance of only 0.07 AU.
All planets with an atmosphere lose some gas as they orbit their stars, either subtly like Earth or in dramatic plumes like AU Mic b. But astronomers have never before seen atmospheric escape stop and start between orbits.
“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,” said Dartmouth College astronomer Keighley Rockcliffe.
“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’.”
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