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Snowball Worlds: Why Earth-Like Planets Across the Universe Are Uninhabitable

Astronomers have discovered that up to millions of Earth-like planets are unlivable because of massive climate shifts.
​"Snowball Earth" by Walter Meyers, courtesy of artist.

Complex life took billions of years to evolve, thanks in part to the erratic climate swings of Earth's youth. This included at least one "snowball" episode, wherein the entire planet froze over. Now astronomers think these same sorts of climate swings may be holding back life all over the universe.

We know that there are probably ​millions to billions of potentially habitable planets in the universe, so why aren't more of them teeming with life?

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In ​a forthcoming study, University of Toronto astrophysicist Kristen Menou finds wild climatic fluctuations are probably a common occurrence on otherwise habitable worlds. What's more, it seems that life on Earth didn't just get a lucky break: Vascular plants, critical in stabilizing Earth's climate today, may have played an unexpectedly large role smoothing out our climate in the past. But Earth-like planets that sit a bit too far from their star are apt to get stuck in a miserable loop of snowball cycles, preventing plant life from ever gaining a foothold.

The reason for all this has to do with complex interactions between a planet's surface radiation, silicate weathering rates, and—unexpectedly—life itself.

Over geologic time, the silicate-carbon cycle is the thermostat that controls our planet's temperature. Silicate rocks draw heat-trapping CO2 out of the atmosphere as they weather; until eventually, Earth freezes over. During glacial periods, the Earth slowly warms as atmospheric CO2 is replenished by volcanic activity. In the present study, Menou was interested in learning what controls the efficiency of that planetary thermostat—and, by extension, a planet's climatic stability.

To find out, Menou ran a series of climate models on hypothetical Earth-like planets that orbit sun-like stars, varying two key parameters: The distance between a planet and its star, and the sensitivity of the silicate-carbon pump to the planet's atmosphere. A CO2-insensitive planet pulls carbon from the atmosphere at a constant rate, regardless of how much carbon's there. But when weathering rates are CO2-sensitive, the silicate pump speeds up or slows down depending on the composition of the atmosphere. In essence, the pump becomes more efficient.

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In scenarios where silicate weathering was insensitive to atmospheric CO2 concentrations, Menou's models produced two stable climates: One at 288 Kelvin, Earth's present climate, and another at 229 Kelvin, wherein the entire planet is frozen. As the silicate-carbon pump became more CO2 sensitive, planets spent more of their time frozen over.

Earth, during the ​Ice Age. Artist's impression, Wikimedia

And when a planet with a super efficient silicate-carbon pump is a bit too far from its star—say, twenty percent farther than Earth—all semblance of climatic stability falls to pieces and the poor sucker cycles erratically between frozen wasteland and slush pile.

That sounds a bit like how things used to be on Earth. So, what changed on the home front to allow for an ideal climate for life? Nobody's sure, but Menou's worked out a series of events that could have stabilized Earth's climate. For one, the sun has gotten brighter over Earth's lifetime, effectively pulling our planet into the warm edge of the habitable zone. Extra sunlight probably helped plants spread across the Earth some half a billion years ago.

And from the rise of plants, another change occurred: The efficiency of Earth's silicate-carbon pump plummeted. The reasoning here is that plants themselves pump a lot of CO2 into the soil, causing CO2 concentrations down at bedrock—where silicate weathering is actually taking place—to rise well above atmospheric levels. In essence, plants decouple the silicate-carbon pump from the atmosphere.

The spread of plants across the Earth's surface may explain why our planet hasn't snowballed in over half a billion years.And on planets with just a wee bit less starlight, plant-like life may never catch on, dooming a whole slew of potentially habitable worlds to a never-ending cycle of hellish winters.

The more science reveals about the relationship between the living and nonliving Earth, the more I'm convinced that life on Earth didn't arise by mere happenstance. Life, rather, seems to have terraformed Earth for itself.