A young planet orbiting a red dwarf star is changing in unpredictable ways with every orbit.
The planet named ‘AU Mic b’ is so close to its parent star that it experiences a consistent, torrential blast of energy, which evaporates its hydrogen atmosphere.
However, during one orbit observed with Nasa’s Hubble Space Telescope, the planet 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 or ‘hiccups’ between orbits has shocked astronomers.
‘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 Keighley Rockcliffe of Dartmouth College.
‘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”.’
Rockcliffe was equally puzzled to see the planet’s atmosphere puffing out in front of the planet, like a headlight on a fast-bound train.
‘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,’ she said.
Researchers believe the ‘hiccups’ in AU Microscopii b’s atmosphere is caused by changes in the magnetic field of the parent star, which can affect the amount of stellar wind and ultraviolet radiation that the planet is exposed to.
Another explanation is that the amount of atmosphere escaping from the planet can vary depending on the strength of stellar wind.
When the stellar wind is strong, it can cause the planetary outflow to be compressed and diverted around the planet, making the outflow less observable.
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, takes 8.46 days to orbit once around its Sun, just 6 million miles from the star. That’s about 1/10th of Mercury’s distance from our Sun.
The bloated, gaseous planet is about four times Earth’s diameter.
AU Mic b was discovered by Nasa’s Spitzer and TESS (Transiting Exoplanet Survey Satellite) space telescopes in 2020.
Red dwarfs like AU Microscopii are the most abundant stars in our Milky Way galaxy but due to their withering flares planets orbiting them aren’t hospitable to life.
The flares from these stars are 100 to 1,000 times more powerful than our Sun’s solar flares. It’s a blistering fireworks show of torrential winds, flares, and X-rays blasting any planets orbiting close to the star.
Under these torrid conditions, planets forming within the first 100 million years of the star’s birth experience the worst of it, completely stripped of its atmosphere.
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