Hubble watches extreme exoplanet being stripped by its star

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Of the many strange exoplanets discovered to date, one of the most extreme has to be a world called AU Mic b. This Neptune-sized planet orbits close enough to its star that a year there lasts just over a week, and it is bombarded by dramatic flares from its host star which cook the planet with radiation.

Recently, Hubble observed this system to learn more about the relationship between the exoplanet and its star, technically called AU Microscopii.  The planet’s hydrogen atmosphere is blown away by radiation from the star, but there were confusing findings that seemed to show that no atmosphere was being lost at some times, but significant amounts of atmosphere were lost at other times.

This artist's illustration shows a planet (dark silhouette) passing in front of the red dwarf star AU Microscopii.
This artist’s illustration shows a planet (dark silhouette) passing in front of the red dwarf star AU Microscopii. The planet is so close to the eruptive star a ferocious blast of stellar wind and blistering ultraviolet radiation is heating the planet’s hydrogen atmosphere, causing it to escape into space. Four times Earth’s diameter, the planet is slowly evaporating its atmosphere, which stretches out linearly along its orbital path. This process may eventually leave behind a rocky core. The illustration is based on measurements made by the Hubble Space Telescope. NASA, ESA, Joseph Olmsted (STScI)

“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 lead researcher Keighley Rockcliffe of Dartmouth College in a statement. “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.’”

This strange variability could be due to changes in the flares given off by the star. The star is very young to host planets, at less than 100 million years old, and its flares are caused by the interaction of magnetic fields and the stellar atmosphere. This creates an extreme stellar wind effect which blows away the atmosphere of the nearby planet.

The researchers suggest that the strange variation in the planet’s atmospheric loss could be due to a particularly strong solar flare occurring shortly before Hubble took its measurements which could have ionized the hydrogen coming off the planet, making it invisible to Hubble’s instruments. Or it could be that the stellar winds do actually cause the atmospheric loss to vary considerably.

The researchers are interested in learning about how exoplanets fare in extreme radiation environments like this one.

“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?” said Rockcliffe. “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 research is published in The Astronomical Journal.

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