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The Impact of Land on an Ocean World’s Habitability

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water world
Artist's illustration of the view from a water-covered exoplanet. [David A. Aguilar/CfA]

Which habitable-zone planets can actually support life? A recent study uses a nearby planet — Proxima Centauri b — to examine how the presence and size of a land mass impacts the habitability of an ocean world.

A Target for Potential Life

In our galaxy, roughly 80% of stars are cool, dim M dwarfs — and one in six of these is thought to host an Earth-sized planet in its habitable zone. But being in a star’s habitable zone doesn’t guarantee a planet’s habitability! M-dwarf habitable-zone planets present valuable targets for observations and models to better understand which of these worlds can support life.

eyeball planet

Artist’s impression of a cold, tidally-locked planet. Ice covers much of the planet’s surface, but the point directly facing the planet’s host star remains ice-free. [NASA/JPL-Caltech]

Most habitable-zone planets around M dwarfs are likely tidally locked: one side of the planet experiences constant day; the other, constant night. Nominally, this would cause only one region of the planet to be heated — the point closest to the star — and the rest of the planet would be locked in darkness and ice. But if the planet is covered in a dynamic ocean, heat can be transported around the planet via ocean currents, affecting the potential habitability of the world. 

Do continents get in the way of this heat transport? And how do land masses affect the circulation of nutrients in the ocean, critical for sustaining ocean-based photosynthetic life? A new study explores the particular case of a tidally locked ocean planet with a continent — and it uses the nearby Proxima Centauri b as a model to do so.

Modeling a Nearby World

At just 4.2 light-years away, Proxima b is the closest known exoplanet and presents an excellent target for future follow-up observations. This habitable-zone M-dwarf planet is probably tidally locked, and estimates of its density have led to speculation that the planet is covered in a large ocean.

ocean transport

Ocean heat transport in the authors’ models for varying continent size; from top to bottom, continents (noted as the white rectangle in the figure) cover 0%, 4%, 22%, and 39% of the planet surface. Continents at the substellar point inhibit ocean heat transport. [Adapted from Salazar et al. 2020]

In a recent publication led by Andrea Salazar, a team of scientists from the University of Chicago has used a general circulation model to explore how heat and nutrients are transported on an ocean-covered, tidally locked Proxima b — both with and without the presence of a land mass in the ocean.

Salazar and collaborators placed a continent at the point on the planet closest to the star — because land masses are thought to migrate to the planet–star axis over time — and tested a range of continent sizes, covering from 0 to 40% of the total planet surface.

Promising Outcomes

The authors find that the presence of a continent decreases how efficiently heat and nutrients are transported from the dayside to the nightside of the planet — the larger the continent, the less efficient the transport. Nonetheless, in all cases, an ice-free ocean is maintained on the planetary dayside, and nutrients are circulated and delivered to the layer of the ocean where photosynthesis is viable, providing ideal conditions for photosynthetic marine life.

This work suggests that the presence of both a dynamic ocean and continents won’t decrease the habitability prospects of tidally locked planets like Proxima b. This is good news as we prepare for future observations with the James Webb Space Telescope, which may provide further insight into this nearby, potentially habitable world and others like it.

Citation

“The Effect of Substellar Continent Size on Ocean Dynamics of Proxima Centauri b,” Andrea M. Salazar et al 2020 ApJL 896 L16. doi:10.3847/2041-8213/ab94c1