Hot Jupiter
An artist’s concept of a “Hot Jupiter” extrasolar planet. Credit: NASA/JPL-Caltech

The clearest view of the dark side of an exoplanet that istidally locked to its star has been obtained by MIT. The first detailed view of an exoplanet's global atmosphere is provided by their observations and measurements.

We are now moving beyond taking isolated snapshots of specific regions of exoplanet atmospheres, to study them as the 3D systems they truly are.

WASP-121b is a massive gas giant that is nearly twice the size of Jupiter. The planet is an ultrahot Jupiter and was discovered in 2015. The WASP-121b has one of the shortest stars to circle in 30 hours. Its star-facing side is permanently roasting, while its night side is turned toward space.

The hot Jupiters have bright day sides, but the night side is different. The night side of WASP-121b is 10 times fainter than the day side, according to Tansu Daylan, a TESS postdoc at MIT who co-authored the study.

Astronomers have previously detected water vapor and studied how the atmospheric temperature changes with altitude.

A much more detailed picture has been captured by the new study. The temperature changes from the day to the night side were mapped by the researchers. For the first time, they tracked the presence of water through the atmosphere to show how water moves between the day and night sides of a planet.

The water cycle on WASP-121b is much more intense than on Earth, as the atoms that make up water are ripped apart at temperatures over 3000Kelvin. The night side allows hydrogen and oxygen atoms to recombine into water molecule, which then blow back to the day side, where the cycle starts again.

The team calculates that the planet's water cycle is sustained by winds that whip the atoms around the planet at speeds of up to 5 kilometers per second.

It appears that water is not alone in circulating around the planet. The night side is cold enough to host exotic clouds of iron and corundum, a mineral that makes up rubies and sapphires. The clouds may whip around to the day side, where high temperatures can cause metal to form gas. Liquid gems from the corundum clouds could be produced on the way.

With this observation, we are getting a global view of an exoplanet's meteorology.

The study's co-authors are from MIT, Caltech, and other institutions.

Day and night.

The team observed WASP-121b using a 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 astronomer clues to an atmosphere's temperature and composition.

Scientists have observed atmospheric details on exoplanets. It is not easy to do the same for the night side, as you have to watch for small changes in the planet's entire spectrum as it circles its star.

The team observed WASP-121b throughout two full moons, one in 2018, and the other in 2019. The researchers looked through the light data for a specific line that indicated the presence of water.

We saw the water feature and mapped how it changed at different parts of the planet.

The temperature profile of the day and night side was mapped by the changing water feature. They found the day side ranged from 2,500 to 3,500 kelvin at its deepest observable layer. The night side ranged from the deepest layer to the upper atmosphere. The temperature profiles appeared to flip-flops, rising with altitude on the day side and falling with altitude on the night side.

The researchers used the temperature maps and various models to identify chemicals that are likely to exist in the atmosphere. Iron, corundum, and titanium are metal clouds that can be seen on the night side.

The hottest part of the planet is shifted to the east of the star. They thought that the shift was due to winds.

The gas gets heated up at the substellar point but is blowing eastward before it can reradiate to space.

The team estimates the wind speeds at around 5 kilometers per second from the size of the shift.

The winds are much faster than our jet stream, and can move clouds across the planet in about 20 hours, according to Daylan, who led previous work on the planet.

Scientists suspect that carbon monoxide should reside in the atmosphere, so they hope to map changes in not just water vapor but also carbon monoxide with the help of the James Webb Space Telescope.

It would be the first time we could measure a carbon-bearing molecule in the atmosphere.

More information: Thomas Mikal-Evans, Diurnal variations in the stratosphere of the ultrahot giant exoplanet WASP-121b, Nature Astronomy (2022). DOI: 10.1038/s41550-021-01592-w. Journal information: Nature Astronomy Citation: A "hot Jupiter's" dark side is revealed in detail for first time (2022, February 21) retrieved 21 February 2022 from This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.