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

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of “ask an astronautand “space radio” and author of “how to die in space.” Sutter contributed this article to Space.com Expert Voices: Op-Ed & Insights.

Remember Hoth, that ice-covered world from “The Empire Strikes Back”? Although some creatures survived on the planet’s surface, it was a pretty miserable place to live – and generally considered uninhabitable, as all of the water content of this world had frozen. As we continue to discover thousands of planets orbiting other stars, and especially as we refine the search for Earth-like planets, we might want to ask this question: how common are these ice-covered planets and might they be able to host life?

As usual, the answer is, it depends. The amount of water on a planet greatly influences how easily it can turn into a ball of ice, according to new simulations 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 limits of habitability beyond our current limits and expanding the options for find life in another world.

Related: How Habitable Zones Work for Alien Planets and Stars

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

But the planets are complex and the temperatures of these planets depend on many factors. The amount of sunlight they receive is quite significant, obviously. But so does the reflectivity of the planet, because radiation that simply bounces off the surface and escapes into space does not contribute to warming. The same goes for the amount of humidity in the atmosphere, which allows a Greenhouse effect which can significantly warm a planet (as we are currently experiencing on Earth due to human activities).

Take, for example, the terrestrial planets, which have only small amounts of liquid water on their surfaces. If you took a terrestrial planet exactly the 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 much less water vapor in the atmosphere and therefore its greenhouse effect capacities would be reduced.

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

From Earth to Hoth

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

The problem is water: when a planet cools down a bit, some of its liquid water turns to ice. Because ice is much brighter than water, that extra bit of ice reflects a little more sunlight, preventing that sunlight from continuing to warm the planet. Thus, the planet cools a little more, a little more ice forms and the reflectivity climbs a little. Repeat the process and you end up with an inverted greenhouse effect called galloping glaciation – essentially the planet turns into a giant snowball, explain the scientists.

Previous work has already shown that in the case of the Earth, if the sunlight we receive drops by only 8% and we maintain the current level of carbon dioxide in the atmosphere, that would be enough to set up this disastrous cycle. In fact, this “earth snowball“This phenomenon may have happened once or twice in the geological history of our planet.

But terrestrial planets can avoid this scenario longer than watery planets, simply because terrestrial planets lack enough water to cover much of their surface. Terrestrial planets with the same amount of carbon dioxide can support a star at just 77% of the sun’s luminosity without freezing completely, the researchers found in their simulation.

Related: 10 exoplanets that could harbor extraterrestrial life

The edges of habitability

This logic also works in the opposite direction. Water vapor is a key greenhouse gas, so if you increase the heat from the sun, a planet like Earth would turn 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 was a runaway greenhouse effect. Indeed, our planet is ultimately doomed to this fate: in a few hundred million years, the sun will be bright and hot enough to trigger this scenario.

As the terrestrial planets lack humidity, they will always have less water vapor in their atmosphere. Turn up the heat, and… not much happens. Indeed, you could put a terrestrial planet around a star that pumps 80% more heat than the sun, and it would do just fine, according to the new simulations.

This result dramatically changes our assumptions about 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 is neither too cold to freeze nor too hot to evaporate. But previous estimates of the habitable zone assume Earth-like compositions, with the same amount of water on their surface as Earth.

Terrestrial planets, however, are much more resilient than Earth. They keep liquid water both closer and farther from their star than conventional habitability calculations suggest. This means that if we were to find an Earth-sized planet outside the traditional habitable zone, we shouldn’t rule it out just yet.

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