As Different As Day and Night

Telescopes are getting better and better at detecting the components of exoplanet atmospheres. But what can those components tell us about a planet’s climate? It turns out that water vapor may be especially useful in this regard.

Atmospheres on Tidally-Locked Planets

The Moon orbiting the Earth, with a yellow arrow showing the direction of the Moon’s rotation. The Moon’s rotational period matches its orbital period so that the same face of the Moon faces Earth at all times. Click to play. [NASA’s Scientific Visualization Studio]

As we find more and more exoplanets, we’re realizing that our solar system may be the exception to the rule! The menagerie of exoplanets we’ve discovered so far includes Jupiter-sized planets that are close to their suns, planets with two suns, and planets that take one orbit about their sun to complete one rotation on their axis — these planets are said to be tidally locked.

Just like our tidally locked Moon always shows the same face to the Earth, tidally locked planets always show the same face to their sun. So, a tidally locked planet will have a consistent dayside and nightside. This has fascinating implications for their climate, and even moreso when we consider that there could be tidally locked Earth-like planets!

A cross section of the planet’s atmosphere with a specific model pressure showing humidity with the colored contours and mass flow with the white contours. The nightside is on the left while the dayside is on the right. [Adapted from Ding & Pierrehumbert 2020]

Models of the water vapor runaway greenhouse effect — when radiation is prevented from efficiently leaving a planet — on tidally locked planets show that the nightside emits more thermal radiation than the dayside as the planet approaches the runaway greenhouse state. Since this reversal of thermal emission requires the emergence of clouds and the buildup of water vapor on the nightside of the planet, spotting it in an exoplanet’s atmosphere could be a useful indicator that the atmosphere is not dry.

To achieve nightside buildup of water vapor, the vapor must avoid being caught on the dayside in a “cold trap”, where it would be cooled, condense, and remain on the dayside. On a planet with inefficient cold trapping, the water vapor can be swept to the nightside to contribute to the thermal emission there.

This weak cold trap effect has mostly been modeled for planets with warm, thick atmospheres, but it is feasible for this effect to also occur on planets with thin, temperate atmospheres. A recent study done by Feng Ding (Harvard University) and Raymond Pierrehumbert (University of Oxford, UK) explores the second scenario for slowly rotating tidally locked planets.

Simulating Two Sides of a Planet

The brightness of the modeled planet as seen at a wavelength of 1,000 cm as it rotates. The different lines indicate different pressures and atmospheric conditions. The black vertical dashed line marks the superior conjunction, where the star is between the observer and the planet. Click to enlarge. [Adapted from Ding & Pierrehumbert 2020]

Ding and Pierrehumbert specifically looked at atmospheres rich in water vapor, which would allow for the necessary clouds and weak cold trap to exist. Their model planet had a period of 40 days and accounted for a variety of interactions that could occur on an Earth-like planet with an atmosphere and oceans. They were also able to vary atmospheric pressure and surface temperature and so modeled several different conditions for their planet.

It turns out that thin, temperate atmospheres with weak cold traps do show the same nightside–dayside emission difference as warm, thick atmospheres as they approach the runaway greenhouse state! Interestingly, the difference between the nightside and dayside emissions can point to the relative amount of water vapor in a planet’s atmosphere as well as the atmospheric pressure — insight we can’t gain from the planet’s transmission spectrum. Further properties of a planet’s atmosphere can be determined by observing how the brightness of the planet changes as it rotates.

The subtleties from this study can’t be picked up by our telescopes yet, but possible future missions like the Origins Space Telescope may be able to. There’s no need to rush though: there are still lots more planets to simulate!


“The Phase-curve Signature of Condensible Water-rich Atmospheres on Slowly Rotating Tidally Locked Exoplanets,” Feng Ding and Raymond T. Pierrehumbert 2020 ApJL 901 L33. doi:10.3847/2041-8213/abb941