An unexpected observation of a neighboring planet whose atmosphere is being
destroyed by violent outbursts from its star was made by the Hubble Space
Telescope. Since Hubble last examined the planetary system and found nothing
wrong, a lot has changed.
The red dwarf star known as AU Microscopii, or AU Mic, is situated 32
light-years from Earth, which is rather close (astronomically speaking),
outside of our solar system. The star is less than 100 million years old,
just a tiny portion of the lifetime of our 4.6 billion year old sun, and it
supports one of the youngest planet systems yet discovered.
During studies in 2020, the now-retired NASA Spitzer Space Telescope and
the Transiting Exoplanet Survey Satellite found the system. A modest
decrease in the star's brightness indicated the presence of a gaseous planet
circling in front of it.
Everything appeared normal until the Hubble Space Telescope viewed the
exoplanet's single orbit, which lasts 8.46 days. A year and a half later,
the telescope returned to the system to take another look. The fact that AU
Mic b, the star's nearest planet, is receiving the majority of the star's
radiation, which is dissolving the planet's hydrogen atmosphere, startled
astronomers. The system has at least two confirmed exoplanets, and others
might yet be hidden from view.
The research involved in the results has been approved for publication in a
subsequent issue of The Astronomical Journal.
Keighley Rockcliffe, a doctoral student in physics and astronomy at
Dartmouth College in Hanover, New Hampshire, wrote the study. "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," she
said in a statement. "We had high expectations for a repeated, predictable
event. But it ended up being strange. That can't be right, I thought when I
first saw this.
Unusual observation
The most prevalent kind of stars in the Milky Way are red dwarfs, and
several planet-hosting red dwarfs have been discovered. Despite being
smaller and colder than the sun, these stars have a reputation for producing
powerful stellar flares for a lot longer than solar-like stars.
AU Mic b, which is only 6 million miles from the star and is a tenth of the
distance between our sun and its nearest planet, Mercury, is being struck by
these harmful radiation waves.
The planet's atmosphere warms up as a result of radiation exposure to the
point that it defies gravity and expands out into space.
According to Rockliffe, "This genuinely odd observation is sort of a
stress-test case for the modeling and the physics about planetary
evolution." This finding is remarkable because it allows us to examine the
most severe interaction between the star and the planet.
The magnetic field of the star tangles as a result of the stellar
atmosphere's movements, which causes stellar flares. The magnetic fields
break and re-connect if they get too twisted. Between 100 to 1,000 times
more intense flares than our sun's outbursts are released during the
pauses.
Flares, X-rays, and stellar wind—a charged stream of particles expelled
from the star—all strike the planet.
The atmosphere of the Earth is being impacted by a highly unrestrained and,
to be honest, frightening stellar wind environment, according to
Rockcliffe.
The possibility of habitable worlds
Astronomers are attempting to discover if the planets that circle red dwarf
stars closely have a chance of keeping their atmospheres in the face of such
radiation — and if they can be suitable for life.
"We're interested in learning what sorts of planets can endure these
conditions. When the star settles, how will they eventually appear? And is
there any hope that they may become habitable in the future, or will they
just become burnt planets? stated Rockcliffe. Do they ultimately shed the
majority of their atmospheres and transform into super-Earths with the
remaining cores? Since there is nothing comparable in our solar system, we
actually don't know what those ultimate compositions look like.
Researchers are attempting to ascertain if variations in the planet's
atmospheric loss between Hubble views are caused by the star's variability
or whether the stellar wind is responsible for the evaporating atmosphere
appearing in view at some times and being obscured at other times.
Hubble cannot observe the planet directly due to the star's brightness, but
it can track fluctuations in the star's luminosity as hydrogen leaks off the
planet and the starlight dims.
To monitor the planet's future developments, astronomers will do further
follow-up surveys of the system with Hubble.
