The planet where it rains iron

Text by: Simon Rozé Follow

For the first time, astronomers have discovered a planet with a very particular climate. WASP-76b orbits a star 390 light years from us and it is very hot there. So hot that it’s raining iron. This discovery is the subject of a publication in the journal Nature.

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WASP-76b is what we call a gas giant. We know one not very far from Earth: Jupiter . A pale gas monster appears next to WASP-76b. If this exoplanet has the same mass as its cousin, it is twice as large and above all, there is a real hell there.

This planet in fact orbits its star in 43 hours, when Jupiter takes 12 years to complete its round around the Sun. This means that WASP-76b is very close to its star. " The day side of the planet is at 2,400 degrees, " says David Ehrenreich, of the astronomy department of the University of Geneva, who directed this work. “ It's almost the temperature of a cold star. The night side, on the other hand, is much colder: it is 1,000 degrees less.

WASP-76b has a day side and a night side. The planet always presents the same face to its star, like the Moon to the Earth. Consequently, these temperature differences cause very specific climatic phenomena on WASP-76b. “ At the border between day and night, so in the evening, we observed iron fumes. It's hot enough for the iron to vaporize. On the other side of the planet, on the morning side therefore, we no longer see these iron vapors. "

Liquid iron precipitation

The question therefore arises of knowing where this iron went during its passage through the nocturnal side of the planet. David Ehrenreich's team then assumes that “ the temperature difference allowed the iron to condense. The vapor must have formed clouds and possibly precipitation of liquid iron, and therefore an iron rain. "

To observe this distant phenomenon, astronomers were able to take advantage of a brand new instrument installed on the Very Large Telescope (VLT) of the European Southern Observatory, installed in the Atacama Desert in Chile. Called ESPRESSO, it is a spectrograph which was the first scientific observation campaign. The principle is quite simple: by studying the color of a star, we can deduce its composition. If the rays that leave this star pass through a gas, like the atmosphere of WASP-76b, part of them will be absorbed by this gas, and this will therefore change the apparent color of the star.

White sun

To take a rough example, if we look at it from space, our Sun has a rather white color. However, on Earth , it appears yellow to us: it is because our atmosphere has absorbed the blue contained in the white light of our star.

By studying this absorption, we can deduce the composition of the gas passed through, this is how we know that there is iron in the form of vapor in the atmosphere of WASP-76b.

If this discovery is spectacular, it is also interesting for astronomers. " Studying such an extreme environment is a very good test for our models, " says David Ehrenreich. The scientific community has indeed several " models " to explain how a planetary system can be formed. If these models manage to predict the existence of an object as strange as WASP-76b, where it is raining iron, then they are greatly comforted.

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• Astronomy
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