Half-Frozen Half-Scorched Worlds

Red dwarf stars are by far the most abundant stars in the galaxy. A terrestrial planet orbiting in the habitable zone of a red dwarf star is expected to be tidally-locked with one side of the planet permanently facing the star. This is because a red dwarf star is much dimmer than the Sun and a planet has to orbit much nearer to it in order to receive sufficient warmth for liquid water to exist on its surface. Being so close to its parent star, tidal evolution quickly causes the planet to become tidally-locked with one hemisphere of the planet permanently pointed towards the red dwarf star, just like the same hemisphere of the Moon always faces the Earth. The outcome of this is a permanent dayside and permanent nightside on the planet.

Figure 1: Habitable zone of the Sun compared to the habitable zone around Gliese 581 - a red dwarf star known to have planets around it.

Figure 2: A tidally-locked planet with a frozen nightside and a scorched dayside. This planet is located in a star system consisting 2 or more stars since part of the planet’s nightside is faintly illuminated by light from a more distant star in the same star system. An ice layer covers the entire nightside of the planet.

With permanent day-night hemispheres, a terrestrial planet in the habitable zone of a red dwarf star is expected to have an exotic climatic system that is very different from the Earth’s. Such a planet will have a hot dayside and a cold nightside, with atmospheric circulation and possibly oceanic circulation connecting both hemispheres. The habitability of this planet is likely to depend on the amount of water it possesses. This is because the cold nightside hemisphere of the planet can act as a cold trap where water gets deposited and forms an ice layer. If the planet has too little water, all of its water can become trapped within the ice layer on the planet’s nightside, leaving the dayside of the planet very dry.

Depending on the abundance of surface water on the planet and the temperature of the planet’s nightside, it appears that the accumulation of ice on the nightside of a tidally-locked planet can be vaguely classed into 3 possible configurations - ice shelf configuration, ice sheet configuration and water-trapped configuration.

The ice shelf configuration involves a tidally-locked Earth-analogue planet which possesses a very large quantity of surface water such that the whole planet is completely covered by a deep global ocean of water. Warmer nightside temperatures can also contribute to an ice shelf configuration since it results in a thinner ice layer. Here, water precipitates as snow and forms an ice layer on the planet’s nightside. The thickness of the ice layer is limited by basal melting because ice begins to melt at high pressures found at the base of the ice layer. As a result, the ice layer can be no more than a few kilometres thick. The presence of a deep global ocean means that the ice layer on the planet’s night side floats as an ice shelf overlying a sub-surface ocean that is directly connected to the ocean on the planet’s dayside. Furthermore, the limited thickness of the ice shelf and the deep ocean keeps the base of the ice shelf from being “grounded” against the ocean floor. The ice shelf configuration follows a mass balance which depends on the balance between surface accumulation of snow and basal melting.

For the same tidally-locked Earth-analogue planet but this time with less surface water and/or colder nightside temperatures, an ice sheet configuration becomes the likely outcome. Here, the ice layer becomes grounded and is referred to as an ice sheet instead of an ice shelf. This occurs because the ocean depth is too shallow to accommodate the thickness of the ice layer and/or the colder nightside temperatures allow for a thicker ice layer. The mass balance of the ice sheet configuration depends on the balance between ice accumulation from snowfall and the flow of ice away from the boundary of the ice sheet.

Finally, the water trapped configuration is the third configuration and it has the potential to strongly affect the habitability of a tidally-locked planet in the habitable zone around a red dwarf star. The water trapped configuration is basically an extreme version of the ice sheet configuration. This configuration occurs for a tidally-locked Earth-analogue planet with very little surface water and/or very cold nightside temperatures. In this case, almost all of the planet’s surface water is frozen and trapped in an ice sheet on the planet’s nightside. This leaves the dayside of the planet extremely dry. Assuming that the planet receives as much insolation as Earth receives from the Sun, it turns out that if the planet has less than a quarter of Earth’s surface water inventory, it could find itself in a water trapped configuration. In reality, the minimum amount of surface water required on a planet before a water trapped configuration is likely to occur depends also on a number of other factors such as the amount of insolation received by the planet, surface topography, global-scale surface weathering, etc.

Figure 3: Schematic plot of three idealized configurations envisioned for the surface water inventory on a tidally-locked terrestrial planet. A view from the pole is shown, with the nightside up and the dayside down. Exaggerated thicknesses are adopted for the ocean (blue) and ice (white) layers. Planets with very little surface water and/or very cold nightside temperatures are more likely to be in the water-trapped configuration. Credit: Kristen Menou (2013)

A water trapped planet with most of its water frozen within an ice sheet on the planet’s nightside is not all bad news for habitability. Abe et al. (2011) show that a drier planet can remain habitable over a slightly wider range of distances from its parent star than a planet with a greater abundance of surface water. Habitable regions on the surface of a water trapped world are likely to be more strongly localized, straddling the “Goldilocks zone” between the very dry dayside and the frigid nightside. It is reasonable to imagine the existence of life on the day-night terminator of the planet as melt water from the edge of the ice sheet flows towards the planet’s dayside.

References:

- Kristen Menou (2013), “Water-Trapped Worlds”, arXiv:1304.6472 [astro-ph.EP]

- Abe et al., “Habitable Zone Limits for Dry Planets”, Astrobiology 2011 June; 11(5): 443-60.