Over its history, a planet might host oceans long enough to support life.
What would allow a planet to host life? We often define the habitable zone as the distance from a star where water can remain liquid on a planet’s surface. Starward of the inner edge we imagine planets in a Venus-like state — broiling hot and dry. Beyond the outer edge, we picture Mars-like worlds, freeze-dried and oceanically deprived.
Since stars become brighter as they evolve, the inner and outer edges of the zone will both move outward, away from the heat, as hot worlds boil and frozen worlds melt. So we often talk about the “continuously habitable zone” hosting planets that remain in this zone over a star’s lifetime.
But when we study planetary evolution we learn this simple guideline is inadequate, because without knowing a planet’s history, its position doesn’t really tell us if it’s habitable. What if a planet can be conducive for life, even if its oceans are not stable?
Two recent results illustrate this ambiguity. Some colleagues and I have modeled the history of Venus with an eye toward understanding the inner edge of habitability. We used a 3D climate model to simulate billions of years of evolution under the warming Sun and, when we included the complex interplay of clouds, topography, planetary rotation, and atmospheric motions, we learned something unexpected: As the Sun warms and the oceans evaporate, the clouds arrange themselves to keep the planet cool and slow down the loss of oceans. Thus, although the oceans of Venus became unstable early in its history, the process of actually losing them to space likely took billions of years. Venus, while outside the habitable zone, might have been habitable for much of its lifetime.
More recently, a group of researchers at the University of Washington has considered the evolution of icy moons, like those of Jupiter and Saturn. As planets migrate inward toward a star — as we’ve learned young giant planets are wont to do — their icy moons will melt. How long will their oceans last? That depends on size. Oceans on a larger moon, such as Ganymede, might become stable and last as long as its host planet is in the habitable zone. For smaller moons like Europa, surface oceans would not be stable but would instead fully evaporate, though this process could take well over a billion years.
These results made me realize that a planet does not have to be a stable resident of the habitable zone to host life.
We describe a chemical as being “metastable” if in its current conditions it will eventually decompose but the rate of this decomposition is quite slow. Similarly, I think we have to consider worlds that are “metahabitable.” If oceans can persist for hundreds of millions or even billions of years, this might be plenty of time for life to flourish on worlds that are not in a stable habitable state.
We don’t yet know how long an ocean takes to come alive, but Earth’s history hints at less than 100 million years. Since we suspect that microbial life can travel from planet to planet, hitchhiking on meteorites, then a few worlds with metastable oceans may suffice to cultivate and maintain life in a planetary system. Especially now that we know of systems like Trappist-1, around which many planets orbit very close together (S&T: June 2017, p. 12), it’s not hard to imagine life following oases of temporary habitability, surviving by hopping from one promising world to the next.
This article originally appeared in print in Sky & Telescope's July 2017 issue.