What really makes a planet habitable? Our assumptions may be wrong

Artist's illustration of potentially habitable exoplanet Proxima b, which orbits the red dwarf star Proxima Centauri.
Artist's illustration of potentially habitable exoplanet Proxima b, which orbits the red dwarf star Proxima Centauri. (Image credit: ESO/M. Kornmesser)

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of "Ask a Spaceman" and "Space Radio," and author of "How to Die in Space." Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights.

Remember Hoth, that ice-covered world from "The Empire Strikes Back"? Though some creatures eked out an existence on the planet's surface, it was a pretty miserable place to live — and generally considered uninhabitable, because all the water content of that world had frozen. As we continue to uncover thousands of planets orbiting other stars, and especially as we narrow down the searches for Earth-like planets, we might want to ask this: How common are these ice-covered planets, and might they be capable of hosting life?

As usual, the answer is, it depends. The amount of water on a planet greatly influences how easily it can turn into an ice ball, according to new simulations performed by an international collaboration of astrophysicists. For a planet like Earth, a mere 8% reduction in sunlight is enough for it to freeze. But drier planets are more robust, pushing the boundaries of habitability beyond our current limits and expanding the options for finding life on another world.

Related: How habitable zones for alien planets and stars work 

We have no idea how common Earth-like planets are, especially ones with about the same percentage of water covering the surface. Is an Earthy 70% more or less common? How special is our planet? We'll have to wait a few decades, and do many more exoplanet surveys, to get firm answers to those kinds of questions. In the meantime, we can use computer simulations to explore how various kinds of planets might behave and evolve in their home systems.

But planets are complex, and the temperatures of those planets depend on many factors. The amount of sunlight they receive is pretty important, obviously. But so is the reflectivity of the planet, because radiation that just bounces off the surface and escapes into space doesn't contribute to warming. And so is the amount of moisture in the atmosphere, which enables a greenhouse effect that can warm a planet considerably (as we are currently experiencing on Earth due to human activities). 

Take, for instance, land planets, which have only small amounts of liquid water on their surfaces. If you took a land planet the exact same size as Earth and placed it in Earth's orbit around the sun, it would be colder than our planet, because there would be a lot less water vapor in the atmosphere and so its greenhouse abilities would be reduced.

However, at reduced levels of sunlight, the land planet would actually be warmer, because it would have fewer clouds and less snow on the surface. This would make the planet less reflective and better able to capture that juicy sunlight to keep itself warm.

From Earth to Hoth

Taking this thought process to the extreme, an international group of astronomers studied the evolution of land planets while changing the amount of sunlight those planets received. Unsurprisingly, they found that when you cool a planet too much, it freezes. But they also found that land planets can far outlast their Earth-like aqua cousins, the scientists reported in a paper published to the preprint database arXiv in November.

The problem is water: When a planet cools a little, some of its liquid water turns into ice. Because ice is much shinier than water, that little bit of additional ice reflects a little more sunlight, preventing that sunlight from continuing to warm the planet. So the planet cools off a bit more, a little more ice forms and the reflectivity climbs a smidge. Repeat the process, and you end up with a reverse runaway greenhouse effect called runaway glaciation — essentially, the planet turns itself into a giant snowball, the scientists explained.

Previously work has already shown that in Earth's case, if the sunlight we received dropped by only 8% and we maintained the current level of carbon dioxide in the atmosphere, it would be enough to set up this disastrous cycle. In fact, this "snowball Earth" phenomenon may have already happened once or twice in our planet's geologic history.

But land planets can avoid this scenario longer than water planets, simply because land planets lack enough water to cover large parts of their surfaces. Land planets with the same amount of carbon dioxide can withstand a star only 77% the brightness of the sun without freezing over completely, the researchers found in their simulation.

Related: 10 exoplanets that could host alien life

The edges of habitability

This logic works in the opposite direction, too. Water vapor is a key greenhouse gas, so if you turn up the heat of the sun, a planet like Earth would turn itself into something like Venus: It would heat up, releasing more water into the atmosphere, which would trap more heat and, in turn, release more water — and so on, until there's a runaway greenhouse effect. Indeed, our planet is ultimately doomed to that fate: In a few hundred million years, the sun will be bright and hot enough to trigger this scenario.

Because land planets lack significant amounts of moisture, they'll always have less water vapor in their atmospheres. Crank up the heat, and … nothing much happens. Indeed, you could put a land planet around a star that is pumping out 80% more heat than the sun, and it would do just fine, the new simulations found.

This result greatly changes our assumptions of what makes a planet habitable. The habitable zone around a star is the estimated region where liquid water can exist on the surface, meaning it's neither too cold to freeze nor too hot to evaporate. But earlier estimates of the habitable zone assume Earth-like compositions, with the same amount of water on their surfaces as Earth.

Land planets are much hardier than Earth, however. They maintain liquid water both closer too and farther from their star than conventional habitability calculations suggest. This means that if we were to find an Earth-size planet that sits outside the traditional habitable zone, we shouldn't write it off just yet.

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Paul Sutter
Space.com Contributor

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.