Astronomers discover exoplanet where it rains iron every evening

Extreme exoplanet where it rains molten iron and temperatures reach 4,400F with 11,000mph winds is discovered 640 light years from Earth

  • It is a tidally locked planet called WASP-76b orbiting a star 640 light years away
  • It was discovered by astronomers using the Very Large Telescope array in Chile
  • The exoplanet has staggeringly hot surface temperatures of 4,400F

An extreme exoplanet where it rains iron and experiences staggering 4,400F temperatures has been discovered by astronomers 640 light years from Earth.

It was discovered by researchers from Geneva University in Switzerland using the European Southern Observatory (ESO) Very Large Telescope in Chile.

The planet, named WASP-76b, is a gas giant about twice the size of Jupiter.

It is tidally locked, meaning one side always faces its star and so becomes hot enough to vaporise metal, ripping their molecules apart into atoms.

The atoms then evaporate in the atmosphere and are carried to the night side on ‘savage winds’ caused by the extreme temperature differences across the planet. 

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This illustration shows a night-side view of the exoplanet WASP-76b. The ultra-hot giant exoplanet has a day side where temperatures climb above 4,400F, high enough to vaporise metals

Professor David Ehrenreich, an astronomer at Geneva University in Switzerland, said: ‘One could say that this planet gets rainy in the evening – except it rains iron.’ 

WASP-76b is a planet of extremes – there are significant differences between its 4,400F day side that always faces its star and the 1,800F cooler night side.

The permanent day side is ‘roasted’ and is so hot all clouds are dispersed as the molecules are turned into their individual atoms. 

The staggering difference between the two portions of the planet mean winds can reach upwards of 11,184 miles per hour. 

The boiling iron particles are bought over from the day side on these winds.

As they reach the cooler nightside they condense into clouds full of iron droplets that then ‘rain down’ on the gas giant. 

It’s parent star – WASP-76 – is a main sequence star about one and a half times the size of the Sun and a surface temperature of about 10,000 degrees Fahrenheit. 

The only planet in the system is the tidally locked WASP-76b and it is incredibly close to its star, orbiting every 1.8 days.

For comparison the closest planet to the Sun, Mercury, orbits our star every 88 days. 

One of the scientists on the discovery team, Professor Don Pollacco told BBC News it is hard to imagine world’s so different to our own.

‘This thing orbits so close to its star, it’s essentially dancing in the outer atmosphere of that star and being subjected to all kinds of physics that, to put it bluntly, we don’t really understand,’ he said.

‘It will either end up in the star or the radiation field from the star will blow away the planet’s atmosphere to leave just a hot, rocky core.’

On its day side, it receives thousands of times more radiation from its parent star than the Earth does from the Sun – in part also due to how close it is.

Not only does WASP-76b have different day-night temperatures, it also has distinct day-night chemistry, according to the study.

This is how they were able to determine that the iron ‘rain’ is being carried from one side of the planet to the other – as it isn’t present on the day side. 

The extreme temperature difference between the day and night sides results in the vigorous winds that carry the particles. 

A new light scanner on the Very Large Telescope called ESPRESSO enabled chemical variations to be identified on a planet outside the solar system for the first time.

It’s parent star – WASP-76 – is a main sequence star about one and a half times the size of the Sun and a surface temperature of about 10,000 degrees Fahrenheit

A strong signature of iron vapour was detected at the evening border that separates the planet’s day and night side.

‘Surprisingly we do not see the iron vapour in the morning,’ said Ehrenreich.   

ESPRESSO stands for the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations and was originally designed to hunt for Earth-like planets around Sun-like stars. 

However, it has proven to be much more versatile than just finding rocky worlds. 

‘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.

This comic-book-style illustration by Swiss graphic novelist Frederik Peeters shows a close-up view of the evening border of the exoplanet WASP-76b

WASP-76b is a planet of extremes – there are significant differences between its 4,400F day side that always faces its star and the cooler night side

Ultra-hot giant exoplanets receive thousands of times the Earth’s solar radiation – often due to their close proximity to their parent star.

The high temperatures are ideal laboratories for studying their extreme climates and chemistry, said Ehrenreich.

Maria Rosa Zapatero Osorio, an astrophysicist at the Centre for Astrobiology in Madrid and chair of the ESPRESSO team, added: ‘The observations show 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.’ 

Planets with an extremely hot day side and colder night side would have a gigantic condensation front in the form of a cloud cascade at its evening border.   

The research has been published in the journal Nature.


Distant stars and their orbiting planets often have conditions unlike anything we see in our atmosphere. 

To understand these new world’s, and what they are made of, scientists need to be able to detect what their atmospheres consist of.  

They often do this by using a telescope similar to Nasa’s Hubble Telescope.

These enormous satellites scan the sky and lock on to exoplanets that Nasa think may be of interest. 

Here, the sensors on board perform different forms of analysis. 

One of the most important and useful is called absorption spectroscopy. 

This form of analysis measures the light that is coming out of a planet’s atmosphere. 

Every gas absorbs a slightly different wavelength of light, and when this happens a black line appears on a complete spectrum. 

These lines correspond to a very specific molecule, which indicates it’s presence on the planet. 

They are often called Fraunhofer lines after the German astronomer and physicist that first discovered them in 1814.

By combining all the different wavelengths of lights, scientists can determine all the chemicals that make up the atmosphere of a planet. 

The key is that what is missing, provides the clues to find out what is present.  

It is vitally important that this is done by space telescopes, as the atmosphere of Earth would then interfere. 

Absorption from chemicals in our atmosphere would skew the sample, which is why it is important to study the light before it has had chance to reach Earth. 

This is often used to look for helium, sodium and even oxygen in alien atmospheres.  

This diagram shows how light passing from a star and through the atmosphere of an exoplanet produces Fraunhofer lines indicating the presence of key compounds such as sodium or helium 

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