Researchers using ESO’s Very Large Telescope (VLT) have observed one of the most extreme planets they suspect rains iron. The inconceivably hot giant exoplanet has a day side where temperatures climb above 2400C, high enough to vaporise metals.
Powerful winds also ferry iron vapour to the planet’s cooler night side where this condenses into iron droplets.
One could say that this planet gets rainy in the evening, except it rains iron
Professor David Ehrenreich
David Ehrenreich, a professor at the University of Geneva, who led the study, said: “One could say that this planet gets rainy in the evening, except it rains iron.”
He led a study, published today in the journal Nature, of this exotic exoplanet.
Dubbed WASP-76b, the exoplanet is located approximately 640 light-years away in the constellation of Pisces.
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This bizarre “iron rain” phenomenon happens because the planet only ever shows one face, its day side, to its parent star.
WASP-76b’s cooler night side remains in perpetual darkness.
Like the Moon orbiting the Earth, WASP-76b is “tidally locked”.
This means it takes as long to rotate around its axis as it does to go around the star.
The day side receives thousands of times more radiation from its parent star than the Earth does from the Sun.
The exoplanet is so hot 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 bringing the iron vapour from the ultra-hot day side to the cooler night side, where temperatures decrease to around 1500C.
The landmark new study also reveals WASP-76b also has distinct day-night chemistry.
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Using the new Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (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 separating the planet’s day side from its night side.
Professor Ehrenreich said: “Surprisingly, however, we do not see the iron vapour in the morning.”
The reason, he says, is “it is raining iron on the night side of this extreme exoplanet.”
María Rosa Zapatero Osorio, an astrophysicist at the Centre for Astrobiology in Madrid, and chair of the ESPRESSO science team, said: “The observations show that iron vapour is abundant in the atmosphere of the hot day side of WASP-76b.
“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 an international ESO scientific consortium who built the instrument.
ESPRESSO was originally designed to hunt for Earth-like planets around Sun-like stars.
However, it has proven to be much more versatile, explained Pedro Figueira, ESPRESSO instrument scientist at ESO in Chile.
He said: “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.”
Professor Ehrenreich added: “What we have now is a whole new way to trace the climate of the most extreme exoplanets.”
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