Harvard and MIT Researchers Find Limitations in Current Models of Exoplanet Atmospheres

UPDATED: November 5, 2022, at 6:45 a.m.

A team of Harvard and MIT researchers has discovered accuracy limitations in climate models used to describe the properties of exoplanets — planets outside the solar system — given an influx of cosmic data from the newly launched James Webb Space Telescope.

The team advises that the models, which researchers use to probe exoplanets’ atmospheres and learn more about their history and potential to host life, be revised in light of the extraordinarily precise data of the Webb Telescope.

The study — published last month in Nature Astronomy — details how existing methods, which are used to characterize the atmospheres of nearby planetary bodies, can misinterpret data collected by Webb. This accuracy pitfall, which the astronomers refer to as the “opacity challenge,” chiefly concerns models that explore how atmospheres emit light markers depending on their chemical properties.

Robert J. Hargreaves, a researcher at the Harvard-Smithsonian Center for Astrophysics and a co-author of the paper, said that opacity models make use of the way matter in an exoplanet’s atmosphere interacts with light from its nearby star.

“You essentially trace where light would travel through this atmosphere, and depending on what molecules are present, the temperature, or how big the atmosphere is, you get different absorptions or different strengths of absorption,” Hargreaves said. “That allows you to understand which molecules are present and all the parameters of the atmosphere.”

The researchers discovered that current opacity models had “good fits” with Webb data but could be subject to highly diverging interpretations of an exoplanet’s atmosphere, though the interpretations all emerged from the same spectral data.

Webb’s high resolution and sensitivity rendered current opacity models inadequate interpreters, and with only these insufficient models, the team was unable to determine the temperatures, atmosphere’s elemental composition, and pressure properties of the exoplanets.

Julien de Wit, an MIT planetary science professor who co-led the study, compared the opacity challenge to an out-of-date Rosetta Stone, a translation tool used to decipher ancient texts.

“We have to upgrade these ‘Rosetta Stones,’ which are these opacity models, so that we can really get the subtleties and extract all of the information in these spectra,” he said.

Iouli E. Gordon, a CFA researcher and a co-author of the study, said that opacity models begin with information derived from the CFA’s High-Resolution Transmission and High-Temperature databases, which compile spectral data from a slew of experimental observations and theoretical calculations.

The data then inform the code to predict the properties of exoplanetary atmospheres. To increase accuracy, existing databases should be expanded by adding more data points, according to Gordon.

De Wit likened the shortcomings of having a limited set of planetary data points to attempts at predicting the outcome of a political election where all candidates are from the same family. The planets of the solar system likewise emerged from the same primordial cloud of gas and dust and thus do not offer a diverse sample of possible planets.

"There are different human beings but they share the same histories and values, so your sample size will be biased — same thing with the planet of a solar system,” de Wit said. “But the thing is, even though they are different, they’re formed around the same star around the same time, and so we are going to have a limited understanding of all the complex processes that can be happening there."

Gordon said the remote-sensing quality of opacity modeling can apply to a diverse range of fields beyond exoplanets because molecular spectroscopy can be employed “anywhere you have light being absorbed by gaseous media.”

One example cited by Gordon is that researchers in France used the HITRAN and HITEMP spectroscopic databases to maximize the taste of champagne, which they related to the amount of carbon dioxide emitted from the champagne glass, while Hargreaves discussed possible medical applications to monitor patient breathing and make diagnoses.

“It’s very rewarding for us quantum chemists that what we calculate or measure and then compile in this reference data — it can be used in so many different fields,” Gordon said.

Gordon also cited the far-reaching nature of the team’s work on opacity models to urge scientists to embrace collaborative work across disciplines.

“The message to everybody is: don’t sit only in your laboratory and think that your problem is very narrow,” Gordon said. “It could be so much more rewarding if it is broader and there is interaction with other scientists.”