Astronomers puzzled by ‘weird’ planet whose atmosphere is being ripped off by its sun

‘We were really expecting something very predictable, repeatable. But it turned out to be weird’

Astronomers are perplexed by a strange young planet so close to its parent star that its atmosphere is evaporating and its orbit is changing in unpredictable ways.

The planet AU Microscopii b (AU Mic b), according to scientists from Nasa’s Goddard Space Flight Center, is showing clear signs of atmospheric loss with the its hydrogen-rich atmosphere puffing out in front of the planet, like a headlight on a fast-bound train.

“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,” study co-author Keighley Rockcliffe said.

“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,’” Dr Rockcliffe said.

AU Mic b, the innermost planet in this planetary system, was discovered by Nasa’s Spitzer and TESS space telescopes in 2020 and is a bloated, gaseous world about four-times Earth’s size.

The study, published recently in The Astronomical Journal, found that the planet located about 32 light-years from Earth has an “extreme variability” between the changing orbits around its parent star AU Microscopii (AU Mic) – a red dwarf less than 100 million years old.

In comparison, the age of our Sun is 4.6 billion years old.

“This observation is so cool because we’re getting to probe this interplay between the star and the planet that is really at the most extreme,” Dr Rockcliffe said.

Scientists have hoped to determine if such red dwarf stars like AU Mic b can be hospitable to life since one key challenge these stars face is ferocious flares blasting out intense long-lasting radiation with about 100 to 1,000 times more energy than our Sun unleashes in its outbursts.

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

Planets forming within the first 100 million years of such a star’s birth are expected to experience the most amount of atmospheric escape which could end up completely stripping the planet of its atmosphere.

Researchers hope to find what kinds of planets can survive these environments and what they may finally look like when the star settles down.

This could help determine if there could be any chance of habitability eventually in such planets, or if they may end up being scorched planets.

The new observations of the planet’s atmosphere evaporating suggest there is swift and extreme variability in the host red dwarf’s outbursts.

Scientists propose two different theories for how hydrogen from the planet’s atmosphere may be leaving the planet during its transits.

One is that a powerful flare from the red dwarf star, seen seven hours prior, may have ionized the escaping hydrogen to the point where it became transparent to light, making it undetectable.

Another explanation scientists propose is that the flare from the sun could be shaping the hydrogen outflow from the planet, 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.

Researchers believe further observations of the planet could reveal more clues to the star and planet’s odd variability,