The most common stars in our galaxy are dwarfs—smaller, reddish stars that emit far less light than our Sun. Because of this reduced output, the habitable zones of these stars are quite close-in. This means that any planets in the habitable zone will be close enough to become tidally locked, only showing a single face to the star, just as the Moon keeps just one side pointed at the Earth.

This led to the proposal that any watery worlds in the vicinity could form what's called an "Eyeball Earth." Directly under the local star, the light would be intense enough to melt a circular patch of water, while the rest of the planet would remain locked in a deep freeze.

Exoplanet climates go 3D

Now, a full model of the ocean and climate of a tidally locked planet suggests that the ice and oceans of these planets would be dynamic, distorting the dark pupil of the eyeball into something resembling a lobster. There's also good news and bad news for life on these watery planets. Although the analysis suggests a narrower habitable zone, more of the planet's surface would have the potential to support life.

The new paper, written by two researchers at Peking University, represents the next step in our analysis of exoplanets. Initially, estimates of habitability—defined as temperatures that could allow liquid water on the planet surface—were based on a single slice through the atmosphere, with things like scattering and greenhouse gasses setting the amount of light that reaches the planet's surface. But, in the real world, atmospheres form clouds, distribute heat through winds and convection, and exhibit other sorts of complex behavior.

These are the sorts of things that are handled in the full, three-dimensional climate models built to study the Earth, so effort was put into adapting these models to handle exoplanets that differed significantly from Earth. It was the result of one of these models that first predicted the existence of Eyeball Earths.

But these models didn't capture a critical part of the distribution of heat on the Earth: the ocean circulation. Instead, it treated the entire ocean as a two-dimensional slab. The new study corrects for that by using a coupled ocean-atmosphere climate model, the Community Climate System Model version 3.

For their planet, they used Gliese 581g, a potentially Earth-like planet orbiting in the habitable zone of an M dwarf star 20 light years away. The exoplanet is only about 1.5 times the size of Earth, and its orbit takes about 36 days. Critically for the model, it's close enough to its host star to receive 866 Watts/square meter at the top of its atmosphere (the Earth receives 1,366). Since we don't know what Gliese 581g's atmosphere looks like, the authors assumed an Earth-like composition, but varied the amount of CO 2 to change the intensity of the greenhouse effect. The planet was assumed to be covered in a deep ocean.

A melted lobster

After giving the model 1,100 years to come to equilibrium, the authors sampled a century of its climate. With carbon dioxide concentrations similar to the Earth's (330 parts per million in the model), the eyeball vanished. That's because ocean currents formed along the equator and brought in ice from the west that split the eyeball into two lobes that flanked the equator—the claws of the lobster. The currents then transferred heat to the east, which melted the ice to form the lobster's tail. Critically, the coldest spot on the planet ended up being -60°C—too warm for carbon dioxide to freeze out as dry ice, which would kill the greenhouse warming and turn the planet into a snowball.

A smaller but otherwise identical pattern formed when the CO 2 concentrations were dropped to 3ppm. Raise them to 200,000ppm, and the entire planet melted.

In addition to the ocean current that altered ice distribution, a thermohaline circulation (like the ocean conveyor belt on Earth) formed, which sent warmer water toward the poles. However, most of the action took place above 400m from the ocean's surface; below that, temperatures were pretty homogeneous. In the atmosphere, a jet stream also formed over the equator, which also distributed some heat to the unlit side of the planet.

To test how habitability changed with distance from the star, the researchers fixed the amount of CO 2 to 330ppm, but varied the amount of energy reaching the top of the atmosphere. They found that as the amount of incoming light dropped, the outer edge of the habitable zone—where all the water froze and a snowball planet formed—was actually closer in than a model without ocean circulation put it. On the inner side, the planet also melted at the point where a planet with a simplified ocean still had 70 percent of its surface covered in ice. This rapid melting increases the water vapor in the air, making it easier for the planet to enter a runaway greenhouse state.

So the new model suggests the habitable zone of watery planets near M dwarfs is a bit more narrow than previous studies had suggested. That's the bad news for life. The good news is that, in this model, the ice never got very thick on the dayside of the planet. At 330ppm, most of the day side only had ice that was 3m thick. That's thin enough to allow light to reach the water underneath, meaning photosynthesis is a possibility over the entire dayside of the planet. That's in sharp contrast to earlier models.

Although this model is a major improvement, it still lacks a key feature that's likely to exist on planets: continents, or at least features on the seafloor that differ greatly in height. These will radically alter the currents on the planet, and thus radically alter the distribution of heat within the ocean. Unfortunately, we're even less likely to know anything about continents on exoplanets than we are about the composition of their atmospheres. Still, a few model runs with different configurations might provide some interesting information.

PNAS, 2013. DOI: 10.1073/pnas.1315215111 (About DOIs).