In “The Little Prince,” the classic novella by Antoine de Saint-Exupéry, the titular prince lives on a house-sized asteroid so small that he can watch the sunset any time of day by moving his chair a few steps.
Of course, in real life, celestial objects that small can’t support life because they don’t have enough gravity to maintain an atmosphere. But how small is too small for habitability?
In a recent paper, Harvard University researchers described a new, lower size limit for planets to maintain surface liquid water for long periods of time, extending the so-called habitable zone or “Goldilocks zone” for small, low-gravity planets. This research expands the search area for life in the universe and sheds light on the important process of atmospheric evolution on small planets.
The research was published in The Astrophysical Journal.
“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,” said Constantin Arnscheidt ’18, first author of the paper. “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.”
Generally, planets are considered habitable if they can maintain surface liquid water (as opposed to frozen water) long enough to allow for the evolution of life, conservatively about 1 billion years. Astronomers hunt for these habitable planets within specific distances of certain types of stars — stars that are smaller, cooler and lower mass than our sun have a habitable zone much closer than larger, hotter stars.
