Researchers using ESO’s Very Large Telescope (VLT) have observed an
extreme planet where they suspect it rains iron. The ultra-hot giant exoplanet
has a day side where temperatures climb above 2400 degrees Celsius, high enough
to vaporise metals. Strong winds carry iron vapour to the cooler night side
where it condenses into iron droplets.
“One could say that this planet gets rainy in the evening, except it
rains iron,” says David Ehrenreich, a professor at the University of Geneva in
Switzerland. He led a study, published today in the journal Nature,
of this exotic exoplanet. Known as WASP-76b, it is located some 640 light-years
away in the constellation of Pisces.
This strange
phenomenon happens because the ‘iron rain’ planet only ever shows one face, its
day side, to its parent star, its cooler night side remaining in perpetual
darkness. Like the Moon on its orbit around the Earth, WASP-76b is ‘tidally
locked’: it takes as long to rotate around its axis as it does to go around the
star.
On its day side,
it receives thousands of times more radiation from its parent star than the
Earth does from the Sun. It’s so hot that molecules separate into atoms, and
metals like iron evaporate into the atmosphere. The extreme temperature
difference between the day and night sides results in vigorous winds that bring
the iron vapour from the ultra-hot day side to the cooler night side, where
temperatures decrease to around 1500 degrees Celsius.
Not only does
WASP-76b have different day-night temperatures, it also has distinct day-night
chemistry, according to the new study. Using the new ESPRESSO instrument on
ESO’s VLT in the Chilean Atacama Desert, the astronomers identified for the
first time chemical variations on an ultra-hot gas giant planet. They detected
a strong signature of iron vapour at the evening border that separates the
planet’s day side from its night side. “Surprisingly, however, we do not see
the iron vapour in the morning,” says Ehrenreich. The reason, he says, is that
“it is raining iron on the night side of this extreme exoplanet.”
“The
observations show that iron vapour is abundant in the atmosphere of the hot day
side of WASP-76b,” adds María Rosa Zapatero Osorio, an astrophysicist at the
Centre for Astrobiology in Madrid, Spain, and the chair of the ESPRESSO science
team. “A fraction of this iron is injected into the night side owing to the
planet’s rotation and atmospheric winds. There, the iron encounters much cooler
environments, condenses and rains down.”
This result was
obtained from the very first science observations done with ESPRESSO, in
September 2018, by the scientific consortium who built the instrument: a team
from Portugal, Italy, Switzerland, Spain and ESO.
ESPRESSO — the
Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations
— was originally designed to hunt for Earth-like planets around Sun-like stars.
However, it has proven to be much more versatile. “We soon realised that the
remarkable collecting power of the VLT and the extreme stability of ESPRESSO
made it a prime machine to study exoplanet atmospheres,” says Pedro Figueira,
ESPRESSO instrument scientist at ESO in Chile.
“What we have
now is a whole new way to trace the climate of the most extreme exoplanets,”
concludes Ehrenreich.
Journal article: https://www.nature.com/articles/s41586-020-2107-1
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