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Astrochemistry

Exoplanet aerosols are simpler than astronomers thought

A comprehensive analysis of gas giant atmospheres shows that silicates and hydrocarbons dominate obscuring aerosols

by Sam Lemonick
June 4, 2020 | APPEARED IN VOLUME 98, ISSUE 22

09822-scicon5-exoplanet.jpg
Credit: NASA/ESA/G. Bacon/Space Telescope Science Institute (Planet)
Just two species, silicates and hydrocarbons, explain almost all the clouds and haze on gas giant exoplanets.

When astronomers want to study the thousands of planets they’ve discovered beyond our solar system, one of their best tools is analyzing the light that shines through the planets’ atmospheres from stars behind them. The light can reveal the chemical composition of an exoplanet’s atmosphere and provide clues about what the planet is like.

But many exoplanets’ atmospheres contain suspended aerosol particles that form light-blocking hazes and clouds and frustrate scientists’ attempts to get good spectroscopic data. If scientists knew what the chemical composition of the aerosols was, they could correct for this effect.

“Aerosols have been one of the most pervasive puzzles in the study of exoplanets,” says Laura Kreidberg of the Max Planck Institute for Astronomy, who was not involved with the work. Peter Gao of the University of California, Berkeley, and colleagues gathered spectral data about exoplanets that comprised a range of temperatures and gravities, then fed them into a detailed computer model that calculated how aerosols would form. The researchers determined that on hotter exoplanets, silicates are the dominant aerosols. Below 677 °C, methane forms and can polymerize, making hydrocarbons the more pervasive aerosol. Gao says other aerosols like iron and sulfide minerals don’t significantly contribute to obscuring hazes or clouds.

“It’s a new understanding of how actually simple this system is,” and will improve exoplanet atmosphere models, Gao says.

The researchers want to apply their models to other spectroscopic data, including light emitted from exoplanets themselves and data about the atmospheres of the hundreds of brown dwarf stars that have been studied.

Gao says this work on gas giant exoplanet atmospheres not only increases astronomers’ understanding of the universe, but also helps scientists practice for studying smaller, cooler exoplanets that might support extraterrestrial life.

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