This Exoplanet Is So Hot, It Apparently Tears Apart Water Molecules

Computer simulation of WASP 121b
Computer simulation of WASP 121b
Image: NASA/JPL-Caltech/Vivien Parmentier/Aix-Marseille University (AMU)

There’s an exoplanet whose surface is so hot, it rips apart water molecules. It’s almost a star, but not quite; it’s an ultra-hot, Jupiter-like world located around 880 light-years from Earth.


Researchers took a closer look at data on WASP 121b, where daytime temperatures can exceed 2,200 Kelvin (3,500 degrees Fahrenheit). Comparing it to data from similar planets, including one in a different star system called WASP 103b that was observed by the Hubble Space Telescope, they demonstrated what kind of wild planet we’re dealing with here.

“The biggest surprise was that in a lot of ways, it looks more like a star than a planet,” one of the study’s authors, junior fellow Laura Kreidberg from the Harvard-Smithsonian Center for Astrophysics, told Gizmodo. “We looked at it because we wanted to measure how much water it has in its atmosphere, but when we looked at the data, we didn’t see any water.” It turns out that the planet was simply too hot.

WASP 121b is too close to its parent star to image directly, so researchers study it by watching how the planet’s presence affected the light coming from the star. When the planet is on either side of or almost behind the star, Hubble sees the most light from it. When the planet is in front of the star, the system appears dimmest. It also appears to dim a bit when the planet passes behind the star. The color of the light also changes slightly, as the researchers see the added spectral lines, or spectra, from the planet. Astronomers use spectra to identify the chemical elements and molecules present on celestial bodies like stars and planets.

Normally, these systems are treated as single points of light. But in this case, scientists used the data to get a sense of the planet’s atmospheric composition in three dimensions. “Over the course of the orbit, we see every longitude of the planet,” said Kreidberg, whose measurements of WASP 103b added to the understanding of WASP 121b. “We’re going from 1D to 3D.” She imaged longitudinal sections of that planet’s atmosphere and the elements it contains, like looking at the sections of a beach ball.

But the planet was missing water vapor, which the astronomers expected it would have given what we know about other, similar planets. The researchers used computer modeling based on observations of other ultra-hot Jupiters and realized that the planet might get so hot on its day side that the water vapor breaks apart into its components, oxygen and hydrogen.

These ideas explain only some of the story, according to the paper published recently in Astronomy and Astrophysics. The spectra of this planet and of other ultra-hot Jupiters seems betray even more unknown quirks. A bigger telescope like the forthcoming James Webb Space Telescope could observe these spectra at better resolutions and different wavelengths, in order to get a better view of what’s going on, the authors write.


This planet is far different from any of the worlds in our own Solar System. It will take more advanced technology to fully flush out the nature of these exoplanets and their mysterious atmospheres.


This article has been updated to clarify that measurements of a different exoplanet, WASP103b, contributed to the understanding of WASP 121b.


Former Gizmodo physics writer and founder of Birdmodo, now a science communicator specializing in quantum computing and birds


How close do one of these planets need to be in order to experience a significant amount of drag from the star’s atmosphere, and/or other factors?