Cloudy Challenges to Exploring Exoplanet Atmospheres

6

One of our goals with the soon-to-launch James Webb Space Telescope (JWST) is to better characterize the atmospheres of exoplanets. But will clouds get in the way of our chances?

The Hunt for Water

transmission spectroscopy

As a star’s light filters through a planet’s atmosphere on its way to Earth, the atmosphere absorbs certain wavelengths depending on its composition. [European Southern Observatory]

When an exoplanet transits across the face of its host star, it presents us with a golden opportunity: with a sensitive enough telescope — like the upcoming JWST, scheduled to launch a year from now — we can explore the atmosphere of the planet as it filters the light from its host. Through this transmission spectroscopy, we can look for spectral features that indicate the presence of specific atoms and molecules in the planet’s atmospheric gas.

In the search for life beyond our solar system, surface liquid water is generally considered a necessary ingredient for a habitable world — so signatures of water vapor in planet atmospheres are a prime target for transmission spectroscopy. But any planet with abundant surface water is likely to have something else, too: clouds of liquid and ice condensing in its atmosphere.

A new study led by Thaddeus Komacek (The University of Chicago) explores whether these clouds will foil our chances of detecting water vapor in the atmospheres of terrestrial exoplanets.

transmission spectra

Transmission spectra for planets with clouds (open markers) and without clouds (closed markers), for rotation periods of 8 (orange) and 16 (blue) days. Clouds significantly mute the spectral features, especially for long-period planets. JWST’s expected noise floor is ~20 ppm. [Komacek et al. 2020]

An Obstructed View

Komacek and collaborators examine the results of three-dimensional general circulation models of tidally locked planets orbiting M-dwarf stars. The authors generate simulated transit spectra for planets with different rotation rates, incoming starlight, surface pressure, radius, and more. They then explore whether the presence of clouds in the atmospheres of these planets will impede JWST’s ability to detect the water vapor features that arise from lower in the atmosphere.

The result? Bad news. The authors find that the presence of clouds significantly mutes spectral features; when clouds are present, JWST would typically need to observe 10–100 times more transits of the planet to be able to detect the water vapor features in its atmosphere.

This impact is especially strong for slower-rotating planets. The climate models show that planets with periods longer than about 12 days form significantly more cloud cover on their daysides, due to more water vapor being carried to high altitudes. This leads to even stronger muting of these planets’ spectral features.

Hope for the Future

JWST

An artist’s illustration of the James Webb Space Telescope, set to launch in March 2021. [NASA/JWST]

The authors summarize with the disappointing conclusion that, with JWST, we’re going to have a tough time using atmospheric transmission spectroscopy to spot signs of surface water from the tantalizing targets of terrestrial planets orbiting in M-dwarf habitable zones.

There is still some hope, however. An extended mission lifetime for JWST, lowered signal-to-noise threshold for detection, or the discovery of a habitable-zone planet that JWST can monitor continuously could all push the spectral features above the telescope’s noise limit.

What’s more, since only the features from the atmosphere below the clouds will be affected, species that are well-mixed above the cloud deck may still be detectable. Even on cloudy worlds, we’re sure to have plenty to learn with JWST!

Citation

“Clouds will Likely Prevent the Detection of Water Vapor in JWST Transmission Spectra of Terrestrial Exoplanets,” Thaddeus D. Komacek et al 2020 ApJL 888 L20. doi:10.3847/2041-8213/ab6200