MIT astronomers have obtained the clearest view yet of the perpetual dark side of an exoplanet that is “tidally locked” to its star. Their observations, combined with measurements of the planet’s permanent dayside, provide the first detailed view of an exoplanet’s global atmosphere.

“We are now moving beyond taking isolated snapshots of specific regions of exoplanet atmospheres, to studying them as the 3D systems they really are,” says Thomas Mikal-Evans, who led the study. as a postdoctoral fellow at MIT’s Kavli Institute for Astrophysics and Space. Research.

The planet at the center of the new study, which appears today in natural astronomy, is WASP-121b, a massive gas giant nearly twice the size of Jupiter. The planet is an ultra-hot Jupiter and was discovered in 2015 orbiting a star about 850 light-years from Earth. WASP-121b has one of the shortest orbits detected to date, circling its star in just 30 hours. It is also tidally locked, so its star-facing “day” side is forever roasting, while its “night” side is forever facing space.

“Hot Jupiters are famous for having very bright daytime faces, but the nighttime side is a different beast. WASP-121b’s night side is about 10 times weaker than its day side,” says Tansu Daylan, an MIT postdoc working on NASA’s MIT-led mission TESS, who co-authored the study.

Astronomers had previously detected water vapor and studied how atmospheric temperature changes with altitude on the dayside of the planet.

The new study paints a much more detailed picture. The researchers were able to map the dramatic temperature changes from day to night side and see how these temperatures change with altitude. They also tracked the presence of water in the atmosphere to show, for the first time, how water moves between the day and night sides of a planet.

Whereas on Earth water cycles by evaporating, then condensing into clouds, then raining, on WASP-121b the water cycle is much more intense: on the day side, the atoms that make up the water are torn apart at temperatures above 3,000 Kelvin. These atoms are blown to the night side, where cooler temperatures allow the hydrogen and oxygen atoms to recombine into water molecules, which then return to the day side, where the cycle begins again.

The team calculates that the planet’s water cycle is sustained by winds that whip atoms around the planet at speeds of up to 5 kilometers per second, or more than 11,000 miles per hour.

It also appears that water is not the only one circulating around the planet. Astronomers have discovered that the night side is cold enough to harbor exotic clouds of iron and corundum – a mineral that makes up rubies and sapphires. These clouds, like water vapour, can move to the daytime side, where high temperatures vaporize metals into gaseous form. Along the way, exotic rains might be produced, such as liquid gems from corundum clouds.

“With this observation, we really get a holistic view of an exoplanet’s weather,” says Mikal-Evans.

The study’s co-authors include collaborators from MIT, Johns Hopkins University, Caltech and other institutions.

Day and night

The team observed WASP-121b using a spectroscopic camera aboard NASA’s Hubble Space Telescope. The instrument observes the light from a planet and its star, and breaks that light down into its constituent wavelengths, the intensities of which give astronomers clues about the temperature and composition of an atmosphere.

Through spectroscopic studies, scientists have observed atmospheric detail on the dayside of many exoplanets. But doing the same for the night side is much trickier, as it requires monitoring the tiny changes in the planet’s entire spectrum as it orbits its star.

For the new study, the team observed WASP-121b in two full orbits – one in 2018 and the other in 2019. For both observations, the researchers looked at light data for a specific line, or spectral feature. , which indicated the presence of water vapour.

“We saw this water feature and mapped how it changed at different parts of the planet’s orbit,” says Mikal-Evans. “It encodes information about what the temperature of the planet’s atmosphere does as a function of altitude.”

The changing water feature helped the team map the temperature profile of the dayside and nightside. They found that the dayside ranges from 2,500 kelvins in its deepest observable layer to 3,500 K in its upper layers. The nightside ranged from 1,800 K in its deepest layer to 1,500 K in its upper atmosphere. Interestingly, the temperature profiles appeared to switch, rising with altitude on the dayside – a “thermal inversion”, in meteorological terms – and falling with altitude on the nightside.

The researchers then ran the temperature maps through various models to identify chemicals likely to exist in the planet’s atmosphere, given specific altitudes and temperatures. This modeling revealed the potential for metallic clouds, such as iron, corundum, and titanium on the night side.

From their temperature mapping, the team also observed that the hottest region of the planet is shifted east from the “substellar” region directly below the star. They deduced that this displacement is due to extreme winds.

“The gas is heated at the substellar point, but is blown east before it can reroute to space,” says Mikal-Evans.

Based on the magnitude of the change, the team estimates the wind speed to be around 5 kilometers per second.

“These winds are much faster than our jet stream and can probably move clouds across the planet in about 20 hours,” says Daylan, who has led previous work on the planet using TESS. .

Astronomers have reserved time on the James Webb Space Telescope to observe WASP-121b later this year, and hope to map changes not just in water vapor but also in carbon monoxide, which scientists suspect resides in the atmosphere.

“It would be the first time we could measure a carbon-containing molecule in the atmosphere of this planet,” says Mikal-Evans. “The amount of carbon and oxygen in the atmosphere provides clues to where these types of planets form.”

This research was supported, in part, by NASA through a grant from the Space Telescope Science Institute.