Chromium hydride, found in the temperature range of 1,200-2,000 degrees Kelvin, has been detected in the hot Jupiter WASP-31b’s atmosphere. This discovery paves the way for the molecule to be used as a “thermometer” for exoplanets.
Chromium hydride (CrH), a molecule that’s relatively rare and particularly sensitive to temperature, is useful as a “thermometer for stars,” according to astronomer Laura Flagg. This is because it’s abundant only in a narrow range between 1,200-2,000 degrees Kelvin.
Flagg, a research associate in astronomy at Cornell University’s College of Arts and Sciences (A&S), has used this and other metal hydrides to determine the temperature of cool stars and brown dwarfs. In theory, she said, chromium hydride could do the same for hot Jupiter exoplanets, which are comparable in temperature to brown dwarfs – if these particular molecules are present in exoplanet atmospheres. Previous research, at low resolution, hinted that they are.
New Discoveries on Hot Jupiter WASP-31b
Now, Flagg and a Cornell-led team of scientists have confirmed, using high-resolution spectral observations, the presence of chromium hydride in an exoplanet atmosphere of the hot Jupiter WASP-31b, opening the door to use this temperature-sensitive molecule species as a “thermometer” to determine the temperature and other characteristics in exoplanets.
Flagg is lead author of “ExoGemS Detection of a Metal Hydride in an Exoplanet Atmosphere at High Spectral Resolution,” published on August 16 in The Astrophysical Journal Letters. Co-authors include: Ray Jayawardhana, the Hans A. Bethe Professor and professor of astronomy (A&S); Jake D. Turner, Hubble Research Fellow in the Cornell Center for Astrophysics & Planetary Science; Ryan J. MacDonald, previously a research associate at the Carl Sagan Institute and now a NASA Sagan Fellow at the University of Michigan; and Adam Langeveld, postdoctoral researcher in astronomy (A&S).
Chromium hydride has no previous confirmed detections in any exoplanet, and this marks the first detection of a metal hydride from a high-resolution exoplanet spectrum, the researchers wrote.
Significance of the Discovery
The definitive detection of metal hydrides in WASP-31b is an important advancement in the understanding of hot giant planet atmospheres, Flagg said, although the discovery doesn’t give new information about the individual planet. Discovered in 2011, WASP-31b orbits an F5 star once every 3.4 days. It has an extremely low density, even for a giant planet, and the new study confirms its equilibrium temperature at 1,400 Kelvin – in range for chromium hydride.
“Chromium hydride molecules are very temperature sensitive,” Flagg said. “At hotter temperatures, you see just chromium alone. And at lower temperatures, it turns into other things. So there’s only a specific temperature range, about 1,200 to 2,200 Kelvin, where chromium hydride is seen in large abundances.”
In our solar system, the only detected occurrence of this molecule is in sunspots, Flagg said: the sun is too hot (around 6,000 K on the surface) and all other objects are too cool.
Methods and Instruments Used
In her research, Flagg uses high-resolution spectroscopy to detect and analyze exoplanet atmospheres, comparing the overall light from the system when the planet is to the side of the star against when the planet is in front of the star, blocking some of the star’s light. Certain elements block more light at certain wavelengths and less light at other wavelengths, revealing what elements are in the planet.
“High spectral resolution means we have very precise wavelength information,” Flagg said. “We can get thousands of different lines. We combine them using various statistical methods, using a template – an approximate idea of what the spectrum looks like – and we compare it to the data and we match it up. If it matches well, there’s a signal. We try all the different templates, and in this case, the chromium hydride template produced a signal.”
Chromium is rare, even at the right temperature, so researchers need sensitive instruments and telescopes, Flagg said.
To analyze WASP-31b, the researchers used high-resolution spectra from one new observation in March 2022 as part of the Exoplanets with Gemini Spectroscopy survey from Hawaii‘s Maunakea, using Gemini Remote Access to CFHT ESPaDOnS Spectrograph (GRACES). They supplemented the GRACES data with archival data taken in 2017, which was not intended to look for metal hydrides.
“Part of our data for this paper was old data that was on the very edge of the data set. You wouldn’t have looked for it,” Flagg said. She is now on the lookout for chromium hydride and other metal hydrides in other exoplanets – and the evidence may already exist.
“I’m hoping that this paper will encourage other researchers to look in their data for chromium hydride and other metal hydrides,” Flagg said. “We think it should be there. Hopefully, we’ll get more data that will be suitable for looking for chromium hydride and eventually build up a sample size to look for trends.”
Reference: “ExoGemS Detection of a Metal Hydride in an Exoplanet Atmosphere at High Spectral Resolution” by Laura Flagg, Jake D. Turner, Emily Deibert, Andrew Ridden-Harper, Ernst de Mooij, Ryan J. MacDonald, Ray Jayawardhana, Neale Gibson, Adam Langeveld and David Sing, 16 August 2023, The Astrophysical Journal Letters.