Science

How big can a super-Earth get and still be “livable”?

Of the thousands of exoplanets discovered to date, the most common are “super-Earths” – worlds larger than Earth but smaller than Neptune. Some of these planets are even in the habitable zones of their parent stars, meaning conditions there could be right for liquid water to be on their surface.

But could these giant rocky exoplanets support life? Or is life limited to smaller planets like ours? Ultimately, only future observations will give us a definitive answer. But at the same time, this question gives us a great opportunity to explore the limits of our understanding of where life can find a home.

On the subject: What actually makes a planet habitable? Our assumptions may be wrong

Definition of habitability

First, we must be clear about what we mean by “inhabited,” as some of the wild worlds in our own solar system extend far beyond what we consider normal. After all, no other known planet is like Earth.

Other small, rocky planets in our solar system are either barren wastelands (Mercury and Mars) or nightmarish hellholes (Venus). Gas giants with their deep crushing atmospheres are out of the question. Some of their frozen moons, however, offer rich liquid water supplies under their hard crust, and they just might be a second big home to life in our own backyard. But for now, we will confine our discussion to Earth-like worlds.

This means that in our survey of super-Earths, we need to find planets that look and behave (and hopefully smell) very similar to our planet. This includes being in the star’s habitable zone to make sure the temperature is okay, as well as having a relatively thick but not too thick atmosphere. These planets also need to have liquid water on the surface, not trapped under frozen crust or evaporated into steam. And finally, they must have a magnetic field to protect this atmosphere and liquid water from the constant brutal onslaught of the solar wind.

Surely there are many more criteria that must be met in order for life to actually exist in the world. But without these basic conditions, the chances of something growing on an alien world are pretty slim, so this is a good place to start.

Choosing the Right Size Planet

Astronomers usually define a super-Earth as any planet ranging in size from the Earth to 10 times its mass. Astronomers tend to name planets larger than this mini-Neptune, but this apparently clear distinction obscures many nuances important in determining habitability.

Obviously, something close to the size of the Earth has a better chance of being habitable because it appears to be very similar to the Earth. And something close to the size of Neptune would probably not be a very fun place for life to find a foothold, because Neptune as a whole is not all that hospitable, at least according to the above definition.

As the mass of the planets increases, the rocky core clings better and better to the dense gaseous atmosphere due to increased gravity. Eventually, there will be so much atmosphere that the planet would be better described as a gas giant than a rocky world. Unfortunately, we don’t have a clear dividing line between these two extremes, and super-Earths are bridging that gap.

The orbit also matters here. If a planet is too close to its parent star, regardless of its size, it will simply fry. Take 55 Cancri e, a rocky super-Earth about 55 light-years away. Its mass is eight times the mass of the Earth, but it is so close to its parent star that it is just a ball of molten rock.

On the other hand, the planet TOI 270c is smaller, about seven times the mass of the Earth. But it’s so far from its parent star that it’s almost entirely gas, making it look more like a mini Neptune than anything else.

Applicants

Ultimately, a habitable super-Earth should have the right density, indicating that it is neither too rocky nor too gassed. But even then, this is just a guess, as astronomers have little information about any particular exoplanet.

Take, for example, Gliese 581c, which is only 20 light years away. This exoplanet is about 5.5 times the mass of Earth and is located in the habitable zone of its star. But astronomers only know its mass, not its radius, so they can’t determine the planet’s density. With such an orbit and such a mass, the planet could be an ordinary rocky world or consist of solid iron. Or it could be a tiny gas world or even one made of diamond.

As for the magnetic field of any exoplanet, this is a matter of pure conjecture. Scientists believe that planets larger than Earth may have strong magnetic fields, but it’s impossible to know for sure. For example, although Venus and Earth are about the same size, only Earth has a strong magnetic field.

Perhaps the best candidate for a habitable super-Earth is LHS 1140b, which orbits a red dwarf about 49 light-years from Earth. It is about 60% wider than our planet, but 6.48 times more massive. It orbits very close to its parent star – its orbital period is only 25 days – but because the star is a cool red dwarf, it is within the habitable zone.

Atmospheric models of LHS 1140b allow for the possibility of a dense atmosphere surrounding a world with oceans of liquid water. Only detailed observations, perhaps with the help of the James Webb Space Telescope, will tell us for sure whether the planet is indeed habitable. In the meantime, it is the reigning champion of the largest planet on which life can exist.

Paul M. Sutter is an astrophysicist at the State University of New York at Stony Brook and the Flatiron Institute, host of Ask an Astronaut and Space Radio, and author of How to Die in Space. Learn more by listening to the Ask an Astronaut podcast, available on iTunes. (will open in a new tab) and askaspaceman.com. Post your question on Twitter using the hashtag #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.

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