What Are Some Clues to the Climates of Exoplanets?

In the past few decades, the number of planets discovered beyond our Solar System has grown exponentially. To date, a total of 4,158 exoplanets have been confirmed in 3,081 systems, with an additional 5,144 candidates awaiting confirmation. Thanks to the abundance of discoveries, astronomers have been transitioning in recent years from the process of discovery to the process of characterization.

In particular, astronomers are developing tools to assess which of these planets could harbor life. Recently, a team of astronomers from the Carl Sagan Institute (CSI) at Cornell University designed an environmental “decoder” based on the color of exoplanet surfaces and their hosts stars. In the future, this tool could be used by astronomers to determine which exoplanets are potentially-habitable and worthy of follow-up studies.

This new method is described in a study that recently appeared in the Monthly Notices of the Royal Astronomical Society, titled “How surfaces shape the climate of habitable exoplanets.” The study was conducted by Jack Madden, a Ph.D. student with the Carl Sagan Institute (CSI), and Lisa Kaltenegger – an associate professor of astronomy at Cornell and the director of the CSI.

Artist’s impression of of Kepler-1649c orbiting around its host star. Credit: NASA’s Ames Research Center/Daniel Rutter

As they indicate in their study, astronomers will be able to study exoplanets in detail in the coming years. Thanks to advances in adaptive optics, coronographs, spectrometers, and interferometry, next-generation space and ground-based telescopes will able to directly image exoplanets that are smaller and orbit closer to their stars – which is typically where rocky planets orbit within stars’ habitable zones.

Unfortunately, it remains a challenge to book time with an advanced telescope and the resources of exoplanet hunters are still limited. As such, astronomers will still need a method for narrowing down the long list of candidates so they know which planets are more likely to yield positive results. As Madden explained in a recent Cornell Chronicle:

“We looked at how different planetary surfaces in the habitable zones of distant solar systems could affect the climate on exoplanets… Reflected light on the surface of planets plays a significant role not only on the overall climate, but also on the detectable spectra of Earth-like planets.”

For the sake of their study, they considered how different planetary surfaces can influence the climate, atmospheric composition, and remotely-detectable spectra of a rocky planet, depending on the nature of the host star. Overall, they considered stars that range from the F0V to K7V spectral classes, which is everything from main sequence Yellow-White Dwarfs to main sequence Orange Dwarfs.

Artist’s depiction of the Morgan-Keenan spectral diagram, showing how stars differ in colors as well as size. Credit: Wikipedia Commons

Madden and Kaltenegger then combined this information to see how the interplay of surface features and different types of stars would affect habitability. For instance, a terrestrial planet with lots of basaltic rock would absorb light from even a cooler K-type star and become very hot. But the presence of sand (the result of wind and water erosion), clouds, and foliage would have a cooling effect.

From this, Madden and Kaltenegger were able to create an updated version of the time-tested 1D climate-photochemistry model. This new version will allow astronomers to characterize the habitability of rocky exoplanets based on their wavelength-dependent surface albedo. As Kaltenegger explained:

“Depending on the kind of star and the exoplanet’s primary color – or the reflecting albedo – the planet’s color can mitigate some of the energy given off by the star. What makes up the surface of an exoplanet, how many clouds surround the planet, and the color of the sun can change an exoplanet’s climate significantly.

Madden likens it to a person wearing a dark or light-colored shirt. On a hot day, a black shirt will absorb heat and make a person feel hotter, whereas a white shirt deflects it. The same holds true for stars and planets orbiting them. “There’s an important interaction between the color of a surface and the light hitting it,” he added. “The effects we found based on a planet’s surface properties can help in the search for life.”

An artist’s concept of a rocky world orbiting a red dwarf star. (Credit: NASA/D. Aguilar/Harvard-Smithsonian center for Astrophysics).

Madden, Kaltenegger, and other exoplanet researchers looki forward to next-generation telescopes becoming operational available. This includes the ESO’s Extremely Large Telescope (ELT) and the Giant Magellan Telescope (GMT) in Chile and the Thirty-Meter Telescope (TMT) on Manua Kea, Hawaii.

Space-based telescopes like the Nancy Grace Roman Space Telescope (which is scheduled to launch by the mid-2020s) will also be a major asset to exoplanet-hunters. Between their advanced instruments and optical sensitivity, these telescopes will be able to observe exoplanet atmospheres directly and obtain spectra from them.

With this updated model, astronomers will be able to not only learn the chemical compositions of exoplanet atmospheres (which will allow them to identify potential biosignatures), but also place constraints on these planets’ habitability by simply observing light reflected from the surface.

Further Reading: Cornell Chronicle