Neptune-Sized Exoplanet Erupts Hydrogen in Intermittent Bursts While Orbiting Its Star

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.

This artist’s illustration shows the Neptune-sized exoplanet AU Mic b passing in front of its host red dwarf star. Image credit: NASA / ESA / Joseph Olmsted, STScI.

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’.”

Red dwarfs like AU Mic are the most abundant stars in our Milky Way Galaxy. They therefore should host the majority of planets in our Galaxy.

But can planets orbiting red dwarf stars like AU Mic b be hospitable to life?

A key challenge is that young red dwarfs have ferocious stellar flares blasting out withering radiation. This period of high activity lasts a lot longer than that of stars like our Sun.

The flares are powered by intense magnetic fields that get tangled by the roiling motions of the stellar atmosphere.

When the tangling gets too intense, the fields break and reconnect, unleashing tremendous amounts of energy that are 100 to 1,000 times more energetic than our Sun unleashes in its outbursts.

It’s a blistering fireworks show of torrential winds, flares, and X-rays blasting any planets orbiting close to the star.

“This creates a really unconstrained and frankly, scary, stellar wind environment that’s impacting the planet’s atmosphere,” Dr. Rockcliffe said.

“Under these torrid conditions, planets forming within the first 100 million years of the star’s birth should experience the most amount of atmospheric escape. This might end up completely stripping a planet of its atmosphere.”

“We want to find out what kinds of planets can survive these environments. What will they finally look like when the star settles down? And would there be any chance of habitability eventually, or will they wind up just being scorched planets?”

“Do they eventually lose most of their atmospheres and their surviving cores become super-Earths? We don’t really know what those final compositions look like because we don’t have anything like that in our Solar System.”

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.

There is so much variability because the star has a lot of roiling magnetic field lines.

One possible explanation for the missing hydrogen during one of the planet’s transits 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, and so was not detectable.

Another explanation is that the stellar wind itself 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.

“This is predicted in some models, but this is the first kind of observational evidence of it happening and to such an extreme degree,” the authors said.

Their paper was published in the Astronomical Journal.

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Keighley E. Rockcliffe et al. 2023. The Variable Detection of Atmospheric Escape around the Young, Hot Neptune AU Mic b. AJ 166, 77; doi: 10.3847/1538-3881/ace536