How Small Is Too Small For A Planet To Be Habitable?

Artist's impression of an exoplanet. NASA/JPL-Caltech

There are many challenges to work out where life can evolve elsewhere in the Universe, including the fact that we only have one example of where such a circumstance has happened before: Earth. Now a team of researchers from Harvard University has investigated the size of possibly habitable worlds, in particular taking a closer look at what is the smallest planet possible that can be habitable. The findings are reported in The Astrophysical Journal.

The team worked with the assumption that for life to evolve on a planet within the habitable zone of its star, it must have liquid water on its surface for at least 1 billion years. For this to happen, the planet needs to hold on to its atmosphere – below a certain threshold, planets can't do that. The team put that lower limit at 2.68 percent of Earth-mass, or slightly more than twice the mass of the Moon.

“When people think about the inner and outer edges of the habitable zone, they tend to only think about it spatially, meaning how close the planet is to the star,” lead author Constantin Arnscheidt said in a statement. “But actually, there are many other variables to habitability, including mass. Setting a lower bound for habitability in terms of planet size gives us an important constraint in our ongoing hunt for habitable exoplanets and exomoons.”

The habitable zone of a star is the region where the radiation emitted by the star is enough to allow a planet to have water in all three states. It is also known as the Goldilocks zone because a planet cannot be too hot or too cold, but needs to be just right. Depending on the type of star, these can be much closer to it than in the Solar System. But while we think this is a necessary condition, it is not sufficient, which is why it's important to take atmospheres into account.

"Our study focused on the definition of habitability as the ability to maintain surface liquid water on timescales relevant to the evolution of life. If a planet were sufficiently small, it would be unable to maintain surface liquid water regardless of the distance from its star. The critical mass beyond which this occurs, therefore, sets a fundamental limit on this kind of habitability," Arnscheidt told IFLScience.

Atmospheres have a Goldilocks range as well. If an atmosphere absorbs more heat than it can radiate, it might lead to a runaway greenhouse effect and evaporate all the water. Or the atmosphere might be slowly eroded away from its star, which could lead to a frigid, dry world like Mars.

Small planets orbiting in the closer edge of the habitable zone can escape the runaway greenhouse effect, but only within the range described in this paper. Once they get too small, they can no longer hold on to their atmospheres. This work doesn't consider the possibility of life in subsurface ocean like the one on Saturn's moon Enceladus. 

"In this study, we modeled conditions for surface liquid water only. There is a whole other community of scientists working on the possibility of life in subsurface oceans. However, biospheres on exoplanets or exomoons would likely require surface liquid water to be detectable remotely, because of the low productivity of subsurface ocean biospheres," Arnscheidt explained to us. 

Telescope technology is not yet able to find these limit worlds, but in the coming decades, we are likely to find plenty of them and check whether this hypothesis is indeed correct.


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