Title: Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate
Authors: Jun Yang, Gwenaël Boué, Daniel C. Fabrycky, Dorian S. Abbot
First Author’s Institution: Department of Geophysical Sciences, University of Chicago
Paper Status: Accepted at Astrophysical Journal Letters
Astronomers and non-astronomers alike are eager to find Earth-like planets in the habitable zone, or the orbital distance around a star that allows for liquid water on a planet’s surface. As seen in Figure 1, this is usually calculated by simply looking at the star’s spectral type and the distance of the planet to its star, which tells you roughly how much energy a planet receives, and therefore whether it can maintain a surface temperature between the freezing and boiling points of water. But distance to the star is only the first piece of the puzzle: details about the planet itself play a huge role in setting the habitable zone boundaries. These planetary details can include the composition and structure of the planet’s atmosphere and its cloud cover—and cloud cover is in part determined by the planet’s rotation rate. In this paper, Yang et al. use a circulation model to study the effects of different rotation rates on cloud formation, and by extension its effects on the habitable zone boundaries.
Many of us tend to naively think that planets with very thick atmospheres are bad bets for habitability because of Venus’ famous example in our own solar system. Venus’ well-known runaway greenhouse effect has indeed made the planet very inhospitable, but it turns out that thick cloud cover can be beneficial to close-in planets. A planet with a high albedo will reflect a large amount of starlight off the top of its atmosphere, thereby protecting the underlying layers from ever absorbing much of the stellar flux. This can allow very cloudy planets to have habitable zones much closer to their stars than cloud-free planets. But what makes some planets cloudier than others? It turns out that a big part of the answer is how quickly a planet rotates.
A planet’s rotation rate obviously determines the length of a day on that planet. Planets with long days and nights will have large temperature gradients and atmospheric circulation that is driven by warm air rising on the day side and falling on the night side, while planets with short days and nights will have smaller temperature differences. Additionally, quickly rotating planets will have a strong Coriolis force, which breaks up latitudinal (equator to pole) circulation into narrow bands called Hadley cells. Slowly rotating planets have a weak Coriolis force that results in global Hadley cells. Both of these effects have strong predictive powers for how clouds form. See Figure 2 for reference. Notice in particular that a slowly rotating planet forms more clouds (i.e., is redder in the figure) when stellar flux increases, while a quickly rotating planet forms less clouds. Even better for hot conditions, the clouds on the slower planet form primarily at the sub-stellar point (marked as a black dot on the figure) where incident stellar flux is strongest, cooling the planet right where it needs it most.
The authors use a 3D general circulation model that was built to model Earth’s climate, and study what happens if you change the planet’s rotation rate and/or the kind of star the planet orbits. They compare this to simpler 1D radiative-convective atmospheric models that ignore atmospheric changes with latitude and longitude. Their results are shown in Figure 3.
For planets that rotate slowly enough to be tidally locked (the red line in Figure 3), the authors find that including clouds can allow these planets to receive twice the stellar flux that the 1D models predict and still remain habitable. Figure 3 shows that all the purple planets, which would be considered too hot by conventional metrics, could be considered in the habitable zone if their rotation rates are slow enough. Of course, not all planets that rotate slowly now did so throughout their history, and the authors warn against taking their results at face value; a planet’s history has as many implications for its habitability as its current status. But since it’s much easier for astronomers to find and especially characterize planets closer to their host stars than farther away, this has big implications for our ability to study planets in the habitable zone.