A new U of T study challenges our definition of what makes a planet hospitable, finding that snowball planets can create a living environment.
"When we think of a habitable planet, we tend to imagine a relatively warm place like Earth," says Addiv Paradise, a PhD candidate in the Department of Astronomy and Astrophysics and lead author of the study.
"What our study suggests is that planets with a snowball – that is, those with oceans that are completely covered by sea ice – should not be ruled out as inhospitable to life."
For the Paradise study, colleagues, including Principal Kristen Menou, Assistant Professor in the Department of Physical and Environmental Sciences and the Center for Planetary Sciences, conducted thousands of three-dimensional computer simulations of planets in the Earth's inner habitable zone.
What they found was that if there are dark, bare land that receives enough sunlight, these regions can be warm enough for liquid water and life without causing the ice to recede.
"These planets can receive a similar amount of light as the Earth and the temperature will be above freezing, but unlike Earth, they are trapped in a snowball state where the rest of the planet remains frozen."
According to geological data, the Earth has entered the state of a snowball at least twice, the most recent was about a billion years ago and lasted tens of millions of years.
What probably caused Earth to emerge from the state of the snowball is the large buildup of carbon dioxide (CO2) into the atmosphere, mostly released by volcanoes for millions of years. Once out of the snowball, the heat-induced rain removes the CO2 from the air where it is swallowed up by rocks and soil. This balances the CO2 it is added by volcanoes and helps stabilize warm conditions around the world.
With a constant planet for the snowball, CO2 is removed faster than the speed at which it returns through volcanoes. This may cause the planet to enter a snowball, but it will only lock in this state if the speed at which it is CO2 removed (by precipitation from these spots of warm earth) can balance the speed at which volcanoes return it.
"What we're arguing about here is that we shouldn't write off snowball planets as inhospitable to life," says Paradise.
"Some of the habitable planets we find will be quite different from ours in terms of weather and climate, so we have to put up with this fact."
The study, published in the journal Geophysical Research Letters, received funding from the Natural Sciences and Engineering Research Council of Canada (NSERC).
There is no way to know what percentage of exoplanets (those outside our solar system) are in a snowball state. As Paradise points out, the only way to know right now is to physically observe them, and the telescopes we have are not powerful enough.
He says another lengthy question is how much CO2 is on the middle planet and how volcanically active is the middle planet compared to Earth.
"If most planets are less volcanically active than Earth, you can expect a significant portion of them to be in a snowball. At the same time, if Earth is less volcanically active than most, you would expect snow planets to be rare. "
As for what life on these planets might look like for a snowball, Paradise says it probably would be aquatic. The earth descended into a snowball as soon as life first began to emerge and was completely watery.
"With these snowball planets we're looking at, ocean waters under the ice are unlikely to be radically different from those in the Arctic Ocean," says Paradise.