'Thermometer' molecule found on exoplanet could help determine similar exoplanets' temperatures

The presence of a rare, temperature-sensitive molecule, detected in an exoplanet's atmosphere, has raised researchers' hopes of using it to determine temperatures on and other characteristics of similar exoplanets.

The molecule, chromium hydride (CrH) and sometimes referred to as a 'thermometer for stars', has previously proved its usefulness in determining temperatures of certain types of stars such as cool stars and brown dwarfs.

This is because the hydride molecule is abundant only between 1,200-2,000 Kelvin (925-1725 degrees Celsius), according to astronomer Laura Flagg, a research associate in the College of Arts and Sciences (A&S) at Cornell University, US.

Thus, theoretically, the presence of the metal hydride could help establish temperatures of exoplanets, which are planets existing outside of our solar system, said Flagg, also the lead researcher of the study published in The Astrophysical Journal Letters.

The exoplanet here, called WASP-31b, is a gas giant planet located 1,252 light-years from Earth and orbits an F-type star, which is hotter and more massive than the Sun. The gas giant planet, one of the 'hot Jupiters', takes 3.4 days to go around its star once, according to the NASA's, the US space agency's, website.

Marking the first detection of a metal hydride on an exoplanet, the study confirmed the exoplanet's equilibrium temperature at 1,400 Kelvin, or 1,125 degrees Celsius, even as the researchers said they didn't learn anything new about the gas giant planet.

The chromium hydride molecule has so far been known to exist in sunspots, which are cooler and darker spots appearing periodically on the Sun's intensely radiant surface, its photosphere, said Flagg. The Sun's surface measures around 6,000 Kelvin or 5,725 degrees Celsius.

She researches exoplanet atmospheres by using high-resolution spectroscopy. The planet's elements, that include metals and non-metals, are detected by the amount of star's light they block and at what wavelengths.

"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," said Flagg.

"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," said Flagg, who is now on the lookout for chromium hydride and other metal hydrides in other 'hot Jupiter' exoplanets.

"I'm hoping that this paper will encourage other researchers to look in their data for chromium hydride and other metal hydrides," she said, adding that the data will be helpful in building up a sample size to look for trends.