It Might Be Time We Look for Superhabitable Exoplanets

The term “super Earth”—a type of exoplanet—is a little misleading. Our Earth is pretty super, what with its breathable air and a temperate climate, so a super Earth sounds a little like a planet with a nicer atmosphere and an even more temperate climate. It’s not. A super Earth is simply a planet that has a mass higher than Earth’s but lower than the ice giants Neptune and Uranus. But there are exceptions to every rule, like the super Earth astronomers think might be super habitable—i.e. more habitable than Earth.

We tend to think of habitable worlds as those planets that orbit in their parent star’s habitable zone; that is, a planet the right distance from the star such that it’s warm enough for water to exist in a liquid form on its surface. Since Earth is the only habitable planet we know, it’s the planet that has shaped our definition of what a habitable planet is.

But the more we study planets and moons, even those in our own solar system, the more we’re forced to revise that definition. To be habitable, a world doesn’t have to be in a star’s habitable zone. Other factors like tidal heating have to be taken into account, a phenomenon that could make planets far from their stars habitable, or planets close to their stars inhabitable. Astronomers René Heller and John Armstrong have a new term for these potentially habitable worlds that don’t fit the conventional definition: “superhabitable.” Their full paper is available on Arxiv.

When we talk about habitable exoplanets, we talk a lot about the planets’ distance from their parent star because the star’s light is a main source of heat. The habitable zone, loosely, is determined on the inner edge at the point where the moist runaway greenhouse effect destroys a planet through desiccation and on the outer limit by the point where carbon dioxide freezes and turns the planet into a permanent snowball. But it’s not that simple, because stars have life cycles and are of different types. Stars vary in terms of size and brightness, meaning the habitable world around a star can change over the course of its lifetime.

And there are other factors to do with the planet that affect its habitability. Like tidal heating, when a planet’s interior is stressed by the gravitational effects of one or many other bodies. Tidal heating could warm a planet in its star’s habitable zone to the point where the runaway greenhouse effect takes over. Tidal locking, when a planet has one side permanently facing its star, could similarly render a planet inhospitable. The temperature extremes could wreak havoc on whatever atmosphere a planet might have. Changes to a planet’s orbit through some traumatic event, a change in a planet’s axial tilt, or even something simple like the ratio of land to water on the planet’s surface all affect climate and atmosphere, potentially making planets in a star’s habitable zone ill suited for life.

But these phenomena that are detrimental to planets in one orbit could be beneficial in another. Imagine moons orbiting gas giant planets, sort of like Jupiter and its dozens of moons (there are 50 confirmed, another 18 awaiting confirmation). Those moons being so far from their planet’s host star wouldn’t get much light or heart from the star, but they could get light and heat reflected off their host planet. And if a moon is close enough to its giant planet, gravitational forces could lead to tidal heating, and rather that produce a runaway greenhouse effect, in this case it could heat the moon to the point of habitability. This is one of the ways a moon outside a star’s habitable zone could be habitable.

Of course, the life that could exist on any of these super habitable exoplanets or exomoons would almost certainly be very different than anything we see on Earth. Even the extremophiles, organisms that thrive in the Earth’s most extreme environments, might not be up to the conditions on these distant moons. And if there are many factors that change the habitable zone, it might turn out that Earth, our benchmark planet in the search for extra terrestrial life, is actually rare. Earth could be a marginally habitable planet, and we might just be very lucky to be alive at the right time.