There are so many super-Earths in the vast universe. Which one will be our second home?

How big should a super-Earth be to remain habitable?

An artist's sketch of a super-Earth. Credit: NASA/JPL-Caltech

Of the thousands of exoplanets discovered so far, "super-Earths" are the most common. These planets are larger than Earth and smaller than Neptune, and some are in the habitable zone of their parent stars, which means that some conditions on these planets may be suitable for the existence of liquid water on their surfaces.

But could these giant rocky planets actually have the conditions to support life? Or would life only arise on smaller planets like Earth? We’ll ultimately have to rely on future observations to get a definitive answer, but until then, this question gives us a perfect opportunity to explore and expand our understanding of where life could find a home.

Defining habitability What is habitability?

First, we need to be clear about what we mean by "habitable," because some of the wild worlds in our solar system are far beyond what we consider normal. After all, no other known planet is quite like Earth.

The other small, rocky worlds in our solar system are either barren wastelands (like Mercury and Mars) or nightmarish hellscapes (like Venus). The gas giants, with their thick, dense atmospheres, are out of the question. However, some of their icy moons store large amounts of liquid water beneath their hard shells, so they could be other places in the solar system that could support life. But for now, we're going to limit our discussion to planets that look similar to Earth.

This means that when we look for super-Earths, we need to find planets that look and behave (and preferably smell) very similar to our own. This includes being in the star's habitable zone, so that the temperature is just right and the atmosphere is neither thin nor too thick. Liquid water must also exist on the surface of such a planet, rather than being locked up under a frozen crust or boiling into water vapor. Finally, they must have a magnetic field to protect the atmosphere and liquid water from the constant and brutal onslaught of solar storms.

Of course, there are many more conditions that need to be met to make a place truly habitable. But without these basic conditions, it is basically impossible for anything to survive on this alien world, so it is a good decision to start with these basic conditions.

Picking the right size planet How big is the planet?

Astronomers generally classify any planet between the same size as Earth and 10 times larger as a super-Earth. They tend to call planets larger than these criteria mini-Neptunes, but this sharp distinction actually obscures many important details for judging habitability.

Obviously, a planet closer to Earth's size is more likely to be habitable, because we assume that such a planet is very similar to Earth. Similarly, if the planet is more similar to Neptune's size, then it may not be a place where life can find a place to settle. This is because Neptune is not very habitable overall according to the definition above.

As a planet's mass increases, its core becomes more capable of holding a thick atmosphere due to the pull of gravity. Eventually, the planet will have an atmosphere so thick that "gas giant" is a better description than "rocky planet." Unfortunately, we lack a sharp boundary between these two extremes, and the existence of super-Earths connects the two extremes.

So the orbit of a planet is also very important. If a planet is too close to its parent star, then no matter how big the planet is, it will get scorched. Take Xuanyuan Zeng 19, for example. It is a rocky super-Earth 55 light-years away from Earth and is eight times more massive than Earth. But because it is so close to its parent star, it is just a molten ball of magma.

Meanwhile, planet TOI 270c is smaller, about seven times more massive than Earth. But it's so far from its parent star that it's made almost entirely of gas. That makes it more like a mini-Neptune than a super-Earth.

The contenders The champion of the "Most Habitable Planet" competition?

Ultimately, a habitable super-Earth would need to have just the right density, since it would mean it's not too rocky or too gaseous. Even so, it's just a guess -- astronomers don't know much about any particular exoplanet, after all.

Take Gliese 581c, for example. It's only about 20 light-years away. This exoplanet is about 5.5 times the mass of Earth and is in the habitable zone of this planet. But astronomers only know its mass, not its radius, so they can't tell how dense the planet is. At that orbit and mass, the planet could be a typical rocky world, or made entirely of solid iron. Or it could be a small gaseous planet, perhaps even made of diamond.

As for the magnetic field of any exoplanet, it can only be determined by observation. Scientists think that planets larger than Earth may have a strong magnetic field, but it is impossible to determine this magnetic field. For example, Venus is about the same size as Earth, but only Earth has a strong magnetic field.

We don't understand the habitable zones of alien planets.

Perhaps the most likely candidate for a habitable super-Earth is LHS 1140b, which is 49 light-years away. It is about 1.6 times wider than Earth, but weighs 6.48 times more. It orbits very close to its star, even taking only 25 days to complete one orbit. But because the star is a red giant, the planet is still within the habitable zone of its star.

Atmospheric models of LHS 1140b support the possibility that the planet has a thick atmosphere surrounding an ocean of liquid water. But perhaps only careful observations by a telescope like the James Webb Space Telescope will tell us if the planet is truly habitable. Until then, this planet is the largest possible world that could host life.

BY:Paul Sutter

FY:Sumyee

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