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Planetary science

Planetary science

Liquid metal, ruby and sapphire could rain down on huge exoplanet

19 Mar 2022
Hot and cold: artist's impression of the exoplanet WASP-121b and its host star. (Courtesy: NASA, ESA, and G Bacon (STSci))

Liquid metal, ruby and sapphire could rain down on one hemisphere of a blisteringly hot giant exoplanet that is tidally locked in a tight orbit around its star. That is the conclusion of astronomers who have developed a detailed 3D model of the atmosphere of WASP-121b, which is a “hot Jupiter” that is about 850 light-years from Earth. Their study also reveals how water and metal is transported between the exoplanet’s hot and cold sides.

The team from the Massachusetts Institute of Technology observed WASP-121b using a spectroscopic camera aboard NASA’s Hubble Space Telescope. The exoplanet is slightly more massive than Jupiter and it is so close to its host star that it completes an orbit in just 30 h, one of the shortest orbits ever detected by astronomers.

The team studied both the night side of the exoplanet – which always faces away from the star – and its blistering hot day side, which always faces the star. Their observations allowed them to model the atmosphere of the gas giant exoplanet. What is more, the team is the first to track the water cycle on a planet outside of the solar system.  Their study reveals conditions so extreme that the hot Jupiter’s night side could experience rains of liquid metal, ruby and sapphire.

“Just measuring the day side temperature of an exoplanet yields an incomplete picture of the global climate on the planet. Understanding the night side fills in this knowledge gap,” team member Tansu Daylan tells Physics World. He adds that the team measured the spectrum of the exoplanet at all viewing angles, not just its dark side, and then inferred a temperature map.

Raining metal

The exoplanet’s proximity to its star and the fact that it is tidally-locked results in extreme conditions, with temperatures as high as 3500 K on the day side. This is hot enough to vaporize metals. Daylan adds that previous studies have indicated the presence of metals in the day side atmosphere. This means that metal clouds would be blown across the night-side hemisphere by winds on the planet in excess of 18,000 km/h.

“Our new data gave us direct evidence for these winds because the hottest region of the day side atmosphere was slightly to the east of the ‘noon’ point right underneath the star,” says Thomas Mikal-Evans, who led the research. “This means that the gas must be getting heated up at noon but then getting blown eastwards before it has a chance to re-emit thermal radiation to space.”

Now based at The Max Planck Institute for Astronomy, Mikal-Evans, says that before the team’s study, astronomers had made unusual observations of the transition line between the day and night sides of WASP-121b – a region known as the day-night terminator of the atmosphere.

“Previous observations showed that titanium was missing from the atmosphere, but its chemical cousin vanadium was present in the atmosphere, Mikal-Evans adds. “Since these two atoms are chemically similar, it seemed odd that we’d observe one but not the other”.

Mikal-Evans adds, “Our new data reveal for the first time that the temperatures on the night side hemisphere drop low enough for titanium and aluminium gas to precipitate and rain down to deeper layers of the atmosphere, whereas vanadium precipitates at lower temperatures making it harder for it to rain out”.

The fact that the temperature drops low enough for titanium and aluminium rain on the night side, combined with the absence of these metals in the gas phase at the day-night terminator, allows the team to conclude that titanium and aluminium are indeed raining down on the night side.

Droplets of ruby and sapphire

He adds that aluminium would probably condense in the form of corundum, which is an aluminium oxide. When traces of elements like chromium, iron, and titanium are included in corundum, it becomes the gems ruby and sapphire. “So, it could be raining droplets of ruby and sapphire on the nightside hemisphere.”

The team also found that WASP-121b’s powerful winds sustain a water cycle by moving water from the day side to the night side of the gas giant.

Not surprisingly, the exoplanet’s water cycle is far more dramatic and violent than that of Earth’s. Mikal-Evans describes it as “a giant conveyor belt” shipping molecules between the vastly different hemispheres of WASP-121b.

Molecules ripped apart

“We were able to observe that most of the water molecules get ripped apart on the day side because it’s so hot, while those that survive deeper in the atmosphere are glowing strongly at infrared wavelengths,” says Mikal-Evans. “The hydrogen and oxygen atoms from the disrupted water molecules then get blown around to the night side hemisphere, where the lower temperatures allow them to recombine to form water vapour once more before they are blown back around to the dayside hemisphere to repeat the cycle.”

The team has booked time on the new James Webb Space Telescope to study WASP-121b in even greater detail. They plan to observe changes in not just water vapour but also carbon monoxide, which is believed to be in the exoplanet’s atmosphere.

“As our technology continues to improve, one day we can hope to do something similar for planets that more closely resemble our own Earth,” Mikal-Evans concludes. “We’re still just taking the first steps down this long and challenging path  —  but we’re certainly on our way!”

The research is described in Nature Astronomy.

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