MIT astronomers have obtained the clearest view of the perpetual dark side of an exoplanet that is “tidal blocked” to its star. Their observations, combined with measurements of the planet’s permanent day side, 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,” says Thomas Mikal-Evans, who led the study as a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research.
The planet at the center of the new study, published February 21, 2022 in Nature Astronomy, is WASP-121b, a huge 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 blocked by the tides, so that its side facing the star is always the same, permanently at very high temperatures, while its “nocturnal” side always faces space.
“Hot Jupiters are famous for having very bright sides of the day, but the night side is very different. The night side of WASP-121b is about 10 times weaker than its day side, ”says Tansu Daylan, an MIT postdoc working on NASA’s MIT-led mission TESS, co-author of the study. Astronomers had previously detected water vapor and studied how atmospheric temperature changes with altitude on the day side of the planet.
The new study captures a much more detailed picture. The researchers were able to map the dramatic changes in temperature from day to night and see how these temperatures change with altitude. They also monitored the presence of water across the atmosphere to show, for the first time, how water circulates between the day and night sides of a planet.
While on Earth, cyclic water first evaporates, then condenses into clouds, then rains from the sky, on WASP-121b the water cycle is much more intense: on the daytime side, the atoms that make up the water are torn apart. at temperatures above 3,000 Kelvin. These atoms are blown around the night side, where cooler temperatures allow the hydrogen and oxygen atoms to recombine into water molecules, which then blow back to the day side, where the cycle begins again.
The team calculates that the planet’s water cycle is supported by winds that whip atoms around the planet at speeds of up to 5 kilometers per second.
It also seems that water isn’t the only one circulating around the planet. Astronomers have found that the night side is cold enough to host exotic clouds of iron and corundum, a mineral that makes up rubies and sapphires. These clouds, like water vapor, can whisk around the daytime side, where high temperatures vaporize metals into gas form. Along the way, exotic rains could be produced, such as liquid corundum gems.
The team observed WASP-121b using a spectroscopic camera aboard NASA’s Hubble Space Telescope. The instrument observes light from a planet and its star and breaks that light down into its constituent wavelengths, the intensities of which provide astronomers with clues about the temperature and composition of an atmosphere.
Through spectroscopic studies, scientists have observed atmospheric details on the sides of the day of many exoplanets. But doing the same for the night side is much more complicated, as it requires observing small changes in the entire spectrum of the planet as it surrounds its star.
For the new study, the team observed WASP-121b in two complete orbits, one in 2018 and the other in 2019. For both observations, the researchers looked at light data for a specific line, or spectral characteristic, that indicated the presence of water vapor.
The changing water feature helped the team map the temperature profile of both the day and night sides. They found that the diurnal side ranges from 2,500 kelvins at its deepest observable layer, to 3,500 K at its topmost layers. The night side ranged from 1,800 K at its deepest layer, to 1,500 K in its upper atmosphere. Interestingly, the temperature profiles appeared to increase with altitude on the daytime side, a “thermal inversion” in meteorological terms, and decrease with altitude on the night side.
The researchers then ran the temperature maps through various models to identify chemicals that may exist in the planet’s atmosphere, given the 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 on the planet is displaced east of the ‘substellar’ region directly below the star. They deduced that this shift is due to extreme winds.
“The gas is heated at the substellar point but is blown east before it can re-radiate into space,” explained Mikal-Evans. Astronomers later this year hope to map changes in carbon monoxide as well, which scientists suspect should reside in the atmosphere. “It would be the first time that we could measure a carbon carrier molecule in this planet’s atmosphere,” said Mikal-Evans. “The amount of carbon and oxygen in the atmosphere provides clues as to where these types of planets are formed.”
