Ponder how our planet got its water. The current view is that objects beyond the ‘snow line,’ where water ice is available in the protoplanetary disk, were eventually pushed into highly eccentric orbits by their encounters with massive young planets like Jupiter. Eventually some of these water-bearing objects would have impacted the Earth. The same analysis works for exoplanetary systems, but the amount of water delivered to a potentially habitable planet depends, in this scenario, on the presence of giant planets and their orbits.

Dorian Abbot (University of Chicago) and colleagues Nicolas Cowan and Fred Ciesla (both at Northwestern University) note the consequences of this theory of water delivery. One is that because low mass stars are thought to have low mass disks, they would have fewer gas giants and would produce less gravitational scattering. In other words, we may find that small planets around M-dwarfs are dry. On the other hand, solar-mass stars and above could easily have habitable planets with amounts of water similar to the Earth.

Waterworlds and their Future

‘Waterworlds’ are planets that may have formed outside the snow line and then migrated to a position in the habitable zone. A planet like this could be completely covered in ocean. In any case, we can expect habitable zone planets could have a wide range of water mass fractions; i.e., the amount of water vs. the amount of land. The Abbot paper studies how variable land surfaces could influence planetary habitability, and the authors attack the question using a computer model for weathering and global climate, assuming an Earth-like planet with silicate rocks, a large reservoir of carbon in carbonate rocks, and at least some surface ocean.

Image: A waterworld may be a planet in transition, moving from all ocean to a mixture of land and sea. Credit: ESA – AOES Medialab.

Interestingly, the researchers found that partially ocean-covered planets like the Earth are not dependent upon a particular fraction of land coverage as long as the land fraction is greater than about 0.01:

We will ?nd that the weathering behavior is fairly insensitive to land fraction when there is partial ocean coverage. For example, we will ?nd that weathering feedbacks function similarly, yielding a habitable zone of similar width, if a planet has a land fraction of 0.3 (like modern Earth) or 0.01 (equivalent to the combined size of Greenland and Mexico). In contrast, we will ?nd that the weathering behavior of a waterworld is drastically different from a planet with partial ocean coverage.

What that means is that planets with some continent and some ocean should have habitable zones of about the same width, no matter what the percentage of land to water. The conclusion is based upon the fact that silicate weathering feedback helped to maintain habitable conditions through Earth’s own history. The weathering of surface silicate rocks is the main removal process for carbon dioxide from the atmosphere, and it is temperature dependent, thus helping to buffer climate changes and expanding the size of the habitable zone around a star.

Seafloor weathering also occurs, but the authors point out that it is thought to be weaker than continental weathering and to depend on ocean chemistry and seawater circulation more than surface climate. That would mean carbon dioxide would be removed less efficiently from the atmosphere of a waterworld, which would produce higher CO2 levels and a warmer climate. A planet like this would be less able to buffer any changes in received solar radiation (insolation) and would thus have a smaller habitable zone.

Planetary Evolution at Work

All this is leading up to an absorbing conclusion about waterworlds. Assuming that seafloor weathering does not depend on surface temperature, planets that are completely covered by water can have no climate-weathering feedback. Thus the conclusion that a water world has a smaller habitable zone than a planet with even a few small continents. But a waterworld may be, depending on its position in its solar system, a planet in a state of transition. Abbot and company posit a mechanism that would put a waterworld through a ‘moist greenhouse’ stage which would turn it into a planet with only partial ocean coverage, much like the Earth. Here what would have been complete loss of water is stopped by the exposure of even a small amount of land:

We ?nd… that weathering could operate quickly enough that a waterworld could “self-arrest” while undergoing a moist greenhouse and the planet would be left with partial ocean coverage and a clement climate. If this result holds up to more detailed kinetic weathering modeling, it would be profound, because it implies that waterworlds that form in the habitable zone have a pathway to evolve into a planet with partial ocean coverage that is more resistant to changes in stellar luminosity.

A waterworld thus becomes an Earth-like planet after going through a ‘moist greenhouse’ phase — this occurs when a planet gets hot enough that large amounts of water are lost by photolysis in the atmosphere and hydrogen escapes into space. As water is lost and land begins to be exposed, the moist greenhouse phase can then be stopped by reducing the carbon dioxide through silicate weathering. This is the process the authors call ‘waterworld self-arrest.’

Although we have not performed a full analysis of the kinetic (non-equilibrium) effects, the order-of-magnitude analysis we have done indicates that a habitable zone waterworld could stop a moist greenhouse through weathering and become a habitable partially ocean-covered planet. We note that this process would not occur if the initial water complement of the planet is so large that continent is not exposed even after billions of years in the moist greenhouse state…

It’s also true that waterworlds at the outer edge of the habitable zone would not be in a moist greenhouse state in the first place. We’re likely to find waterworlds, then, but some of them may be in the process of transformation, becoming planets of continents and oceans. And any Earth-sized planet discovered near the habitable zone would be a good candidate to have a wide habitable zone and a stable climate if it has at least a small area of exposed land. That makes discovering the land fraction of any Earth-class planet we observe through future planet-finder missions a priority. The authors believe that missions of the Terrestrial Planet Finder class should be able to determine the land fraction by measuring reflected visible light.

The paper is Abbot et al., “Indication of insensitivity of planetary weathering behavior and habitable zone to surface land fraction,” accepted at The Astrophysical Journal (preprint). Thanks to Andrew Tribick for the pointer.