In a huge leap, 'thermometer' molecule found on an exoplanet

A team of astronomers harness the power of a temperature-sensitive metal hydride to better understand the atmosphere of hot Jupiter exoplanet, WASP-31b. 
Sade Agard
Artist's impression of a hot Jupiter exoplanet.
Artist's impression of a hot Jupiter exoplanet.


A team of astronomers has unlocked a new tool to better understand the complex atmospheres of exoplanets, according to a recent study published in The Astrophysical Journal Letters.

Using high-resolution spectral observations, they successfully confirmed the presence of chromium hydride (CrH) in the atmosphere of the hot Jupiter exoplanet known as WASP-31b. 

Exoplanets and temperatures

Chromium hydride, a molecule that exhibits remarkable sensitivity to temperature variations, has earned its reputation as a "thermometer for stars," according to co-author astronomer Laura Flagg, a research associate in astronomy at the College of Arts and Sciences at Cornell University, U.S.

"Chromium hydride molecules are very temperature sensitive. At hotter temperatures, you see just chromium alone. And at lower temperatures, it turns into other substances," explained Flagg.

This molecule becomes abundant only within a specific temperature range of 1,200 to 2,000 degrees Kelvin. Flagg has previously harnessed the power of metal hydrides, including chromium hydride, to gauge the temperature of cool stars and brown dwarfs.

The potential of chromium hydride as a temperature indicator for hot Jupiter exoplanets had been hinted at by earlier low-resolution research. However, this recent study marks the first definitive detection of a metal hydride, specifically chromium hydride, in an exoplanet's atmosphere at high resolution.

This achievement is being heralded as a significant leap forward in our understanding of the atmospheric conditions of massive planets beyond our solar system. 

While the discovery doesn't yield new insights into the individual exoplanet WASP-31b, it establishes a novel method for investigating similar celestial bodies.

WASP-31b, discovered in 2011, orbits an F5 star with a rapid period of 3.4 days. The study not only confirmed the planet's equilibrium temperature of 1,400 Kelvin but also verified that this temperature range aligns with the conditions conducive to the presence of chromium hydride.

Mass spectroscopy on WASP-31b

In her research, Flagg employs high-resolution spectroscopy to examine the atmospheres of exoplanets. By analyzing the light emitted when a planet is positioned to the side of its star versus when it passes in front of it, Flagg can deduce the presence of certain elements in the planet's atmosphere. 

The technique relies on the fact that different elements absorb and transmit light at specific wavelengths.

"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," said Flagg in a press release.

"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 hydride's scarcity, even at the ideal temperature range, necessitates using sophisticated instruments and telescopes for its detection. 

The researchers relied on high-resolution spectra from observations made in 2022 as part of the Exoplanets with Gemini Spectroscopy survey using the GRACES spectrograph.

As this discovery paves the way for more precise temperature measurements and deeper insights into exoplanet atmospheres, Flagg hopes that other researchers will also be inspired to sift through their data in search of chromium hydride and other metal hydrides. 

By building a larger sample size, scientists could uncover significant trends in the atmospheres of distant planets, expanding our understanding of the universe's diversity.

The complete study was published in The Astrophysical Journal Letters on August 16 and can be found here

Study abstract:

Exoplanet atmosphere studies are often enriched by synergies with brown dwarf analogs. However, many key molecules commonly seen in brown dwarfs have yet to be confirmed in exoplanet atmospheres. An important example is chromium hydride (CrH), which is often used to probe atmospheric temperatures and classify brown dwarfs into spectral types. Recently, tentative evidence for CrH was reported in the low-resolution transmission spectrum of the hot Jupiter WASP-31b. Here, we presenthigh spectral resolution observations of WASP-31b's transmission spectrum from GRACES/Gemini North and UVES/Very Large Telescope. We detect CrH at 5.6σ confidence, representing the first metal hydride detection in an exoplanet atmosphere at high spectral resolution. Our findings constitute a critical step in understanding the role of metal hydrides in exoplanet atmospheres. 

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