In the constellation Pisces, hot Jupiter lies 640 light-years from Earth.
The gas giant WASP-76b orbits its star in a dizzying orbit of just 1.8 days, at temperatures exceeding 2,400 degrees Celsius – hot enough for iron to evaporate.
But when day turns to night, the temperature drops quickly enough for the iron vapor to condense again into a scorching liquid, which then flows into the interior of the planet.
“You could say that it rains on this planet in the evening, rains of iron,” said astrophysicist David Ehrenreich of the University of Geneva in Switzerland.
The planet WASP-76b, which was announced back in 2016, is a type of planet known as hot Jupiter. It is slightly less than the mass of Jupiter, but more bloated and 'fluffy', about 1.8 times the size of Jupiter.
It is located 5 million kilometers from its star, which is larger and hotter than our Sun – 1.5 times the mass of the Sun, 1.8 times hotter, with a temperature of about 6055 degrees (the Sun is 5504 Celsius).
Thus, the planet is not only subject to scorching radiation, thousands of times higher than the radiation of the Earth from the Sun, but also tidal bound. This is when one side of an orbiting body is always facing the object around which it revolves – for a close example, the Moon is tidally tied to the Earth.
In the case of WASP-76b, this means that one side is in the eternal day, and the other is in the eternal night, with a significant temperature difference between them. On the day side, 2400 degrees Celsius, and on the night side, about 1500 degrees Celsius.
It's not the hottest exoplanet ever discovered – this corona is worn by KELT-9b, an exoplanet so hot it literally evaporates – but it's definitely larger in scale.
Simulations suggest that on planets like WASP-76b, extreme temperature differences between the two sides should cause strong winds. This and the rotation of the planet should push the iron vapor around the planet, and the atoms on the day side should recombine into molecules on the night side.
However, evidence to support this expectation – such as a chemical gradient – has not been obtained. So Ehrenreich and his team decided to take a closer look. Specifically, they wanted to study terminators – the lines between night and day – to see if they displayed asymmetric chemistry. This would also confirm the metal rain theory.
They used highly dispersed spectroscopy to analyze light around the edge of the planet, looking for signatures in the spectrum indicating that the element is blocking some of the light. And they found them. On the evening terminator – the border where day turns into night – they found a strong signature of iron vapor.
On the morning terminator – the border where night turns into day – this signature was missing. This is pretty strong evidence to support iron rain, as liquid iron is the most stable high-temperature ferrous condensate.
“Observations show that the atmosphere on the hot day side of WASP-76b contains a lot of iron vapor,” says astrophysicist Maria Rosa Zapatero Osorio of the Center for Astrobiology in Spain.
'Some of this iron is injected into the night side by the planet's rotation and atmospheric winds. There, iron meets a much cooler environment, condenses and rain falls. '
Then, since iron has dropped out of the upper atmosphere, it does not appear as vapor on the morning terminator.
Now that the team's observations have yielded results, it might be possible to make similar observations of other hot Jupiters, looking for signs of metallic rain. And, of course, everyone has high hopes for the high-tech James Webb Space Telescope's ability to peer into the atmosphere of various exoplanets. The telescope is due to start working next year.
Astronomers have already discovered exoplanets with clouds of corundum – the building block of rubies and sapphires – and others that have iron clouds. We can't wait to see what other weather exists in the universe.
The study was published in the journal Nature.
Sources: Photo: ESO / M. Kornmesser
