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The Hubble Space Telescope captured an unexpected observation of a nearby planet that’s having its atmosphere blasted away by energetic outbursts from its star. It’s a dramatic change from the last time Hubble checked up on the planetary system and saw nothing amiss.
The red dwarf star, called AU Microscopii, or AU Mic, is located outside of our solar system 32 light-years from Earth, which is relatively close (astronomically speaking). It’s home to one of the youngest planetary systems ever observed, and the star is less than 100 million years old, a mere fraction of the age of our 4.6-billion-year-old sun.
The system was discovered by NASA’s now-retired Spitzer Space Telescope and the Transiting Exoplanet Survey Satellite during observations in 2020, as a slight dip in the star’s brightness revealed the presence of a gaseous world orbiting in front of it.
When the Hubble Space Telescope observed one orbit of the exoplanet, which takes 8.46 days, everything seemed normal. Then, the telescope revisited the system for another look a year and a half later. Astronomers were surprised to see that AU Mic b, the closest planet to the star, is bearing the brunt of the star’s radiation, which is evaporating the planet’s hydrogen atmosphere. There are at least two known exoplanets in the system, and more may await discovery.
The findings are part of a study that has been accepted for publication in a future edition of The Astronomical Journal.
“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 study author Keighley Rockcliffe, a doctoral candidate in physics and astronomy at Dartmouth College in Hanover, N.H., 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.'”
A STRANGE OBSERVATION
Red dwarfs are the most common type of stars in the Milky Way, and many have been found to host planets. These stars are smaller and cooler than our sun, but they are known to release strong stellar flares for much longer than sun-like stars.
These damaging waves of radiation are smacking into AU Mic b, which is located just 6 million miles from the star — a tenth of distance between our sun and its closest planet, Mercury.
As the planet is blasted by radiation, its atmosphere heats to the point that it escapes the planet’s gravity and puffs out into space.
“This frankly strange observation is kind of a stress-test case for the modelling and the physics about planetary evolution,” Rockliffe said. “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.”
The stellar flares come from tangling within the star’s magnetic field that occur due to the motions of the stellar atmosphere. If the magnetic fields become too tangled, they break and reconnect. The breaks release flares between 100 and 1,000 times more energetic than outbursts from our sun.
The planet gets hit with flares, X-rays and stellar wind, or a charged stream of particles released from the star.
“This creates a really unconstrained and frankly, scary, stellar wind environment that’s impacting the planet’s atmosphere,” Rockcliffe said.
THE CHANCE FOR HABITABLE PLANETS
Given how many red dwarf stars are known to host planets, astronomers are trying to determine whether the planets closely orbiting them have a chance of retaining their atmospheres in the face of such radiation — and if they can be habitable for life.
“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?” Rockcliffe said. “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.”
Astronomers are trying to determine whether the differences in the planet’s atmospheric loss between Hubble observations are due to the star’s variability or if the stellar wind is causing the evaporating atmosphere to be visible at certain times and hidden from view at others.
The brightness of the star prevents direct observations of the planet by Hubble, but the telescope can measure changes in the star’s luminosity as hydrogen escapes the planet and causes the starlight to dim.
Astronomers will conduct more follow-up observations of the system with Hubble to track how the planet changes in the future.
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