Snowball Earths around F stars and M stars


An outline of the paper: The Effect of Host Star Spectral Energy Distribution and Ice-Albedo Feedback on the Climate of Extrasolar Planets by Shields et al.

The climate of Earth is stable in two states: the present “warm” state, and the snowball state, with ice sheets and glaciers covering the whole planet. Our planet seems to have switched to the snowball state and back during its history.

The transition to global glaciation is triggered by the “ice-albedo feedback”: ice and snow are very reflective. If the clime cools, ice spreads, more solar heat is sent back to space, and the climate gets even cooler.

The authors of the paper use the modern arsenal of exo-climate studies, a 1-D radiative transfer and a general circulation model, to study the transition to a snowball planet for an Earth-like planet orbiting an F-type or M-type star. They keep the flux from the star fixed, i.e. the planet’s orbital distance is adapted so that the amount of star light it receives remains the same.

With the M star,  more of the flux will be in the near infrared, because an M star is cooler than the Sun. The greenhouse gases water and CO2 absorb efficiently in the infrared, so such a planet will remain much warmer, 72 K hotter according to the calculation. Ice and snow absorb in the near infrared, so the ice-albedo feedback will also be weaker.

Around an F star, the effect is opposite: ice and snow are very reflective in the visible and blue, where most of the flux of an F star is located. The mean temperature will be cooler, and the ice-albedo feedback stronger.

Global temperature as a function of the flux from the star. For a Sun-like star, the transition to an ice world occurs at 93% of the present solar luminosity. For an F star, 98%, and for an M star, only 73%. The climate remains mild at much lower fluxes with an M star thanks to the absorption at starlight by the greenhouse gases. [detail of Figure 8 from the paper]

One caveat is that in this paper the authors consider only a planet with a 24-hour rotation rate, but habitable planets around M dwarfs are expected to be tidally synchronised, so these results cannot be directly translated into implications for the habitable zone around M-dwarfs.

The paper outlines several other more subtle consequences of the change in the stellar spectrum. For instance, there is less rain in the M-star models, because there is less shortwave light reaching the surface and driving evaporation. The Hadley cells (the vertical equator-to-tropic rolls) are weakened, which increases the temperatures near the ground.

This study adds two entries to the growing catalogue of realistic exo-climates:

A “snowball’’ world around an F star. The planet receives almost as much light from its Sun as Earth does, but most of it is sent back to space by the intensely bright snow cover, so that the ice sheets extend all the way to the equator.

A dim, warm-greenhouse world around an M star. The planet receives 25% less light from its star than Earth does, most of it in the infrared, but it is kept warm by the powerful greenhouse of its thick CO2-rich atmosphere. The air is humid but it rarely rains, because the atmosphere is warmer than the ground and the air can’t rise to form clouds.

Exo-giraffe on the equatorial ice sheet of an F-star Earth

Exo-giraffe on the equatorial ice sheet of an F-star Earth


About Author

I am a professor of planetary science at the University of Exeter. My specialty is the study of exoplanets, in particular the observation and modelling of exoplanet atmospheres. I have done my PhD a the University of Geneva and worked in Chile, France and Switzerland.