Snowball planets may still have habitable hotspots
When looking for other planets that could be hosting life, astronomers tend to focus on those that are very Earth-like. But doing so might be limiting our scope too much and underselling life's hardiness. A new study has used simulations to show that even Earth-like planets completely encased in ice could still have areas warm enough for life, muddying up the already-murky definition of habitable worlds.
One key aspect to Earth's liveability is its location. Our home planet orbits the Sun at exactly the right distance in the Habitable Zone (HZ), where surface temperatures are not too hot and not too cold for life to thrive. But distance isn't the only factor – previous studies have suggested that planets in the HZ could effectively freeze over if their orbits are too elliptical, they're too tilted on their axis, or other atmospheric and geological processes intervene. In these cases, the oceans would freeze all the way to the equator, creating a "snowball planet" effect and ruining the chances for whatever life may be there.
But maybe this isn't the death sentence we thought it was. After all, we do know of one planet that went through a snowball phase and didn't destroy all life – Earth. On the contrary, it's been thought that the Snowball Earth phase played an important role in the evolution of more complex lifeforms.
So the researchers on the new study ran simulations to see how habitable these snowball planets might be. The team ran a few thousand versions of the simulation, tweaking variables like their orbits, what gases are in the atmosphere, how much sunlight the planets get, and how their continents are configured.
The team found that under some conditions, these snowballs could remain habitable. Right in the middle of the continents, far away from the frozen seas, land temperatures could reach a balmy 10° C (50° F). That's plenty warm enough for liquid water – and life – to exist.
Interestingly, the simulations also revealed new mechanisms for how planets enter and exit these snowball phases. Carbon dioxide is a big player in that respect, and in a reversal of our current climate problems too little of the greenhouse gas can plummet a planet into a snowball state.
Since carbon dioxide is often lost by being absorbed into oceans, it's long been thought that once a snowball planet's oceans have frozen over, it would stop losing carbon dioxide. But the new simulations suggested that that wasn't always the case, with some configurations of snowball planets still losing CO2.
This could also mean that not all snowball planets will be as lucky as Earth in escaping the icy grip. It's thought that Earth gradually warmed up as volcanic activity pumped more CO2 into the atmosphere. But, the simulations suggest, weathering of rocks on the surface, which also traps the gas, could balance that out under some circumstances.
Altogether, the study adds further evidence to the idea that defining a planet as habitable or not is less clear-cut than previously thought. After all, lifeforms have turned up in some pretty extreme environments here on Earth, and could evolve along different lines on bizarre alien worlds like Saturn's moon Titan.
The research was published in the Journal of Geophysical Research.