Of the thousands of exoplanets discovered so far, the most common are “super-Earths” – worlds larger than Earth but smaller than Neptune. Some of these planets are even within the habitable zones of their parent stars, which means conditions could be right for hosting liquid water on their surfaces.
But could these giant, rocky exoplanets actually sustain the conditions for life? Or is life limited to smaller planets like ours? Ultimately, only future observations will give us a conclusive answer. But until then, the question gives us a perfect opportunity to explore the limits of our understanding of where life might find a home.
Related: What really makes a planet habitable? Our assumptions may be wrong
Defining Habitability
First, we need to be clear about what we mean by “habitable,” because some wild worlds in our own solar system extend far beyond what we would consider normal. After all, no other known planet looks like Earth.
The other little one rocky planets of our solar system are either arid wastelands (Mercury and March) or nightmarish hells (Venus). The gas giants – with their deep and crushing atmospheres – are excluded. Some of their frozen moons, however, offer rich abundances of liquid water beneath their rigid crusts, and they just might be a great second home for life in our own backyard. But for now, we’ll limit our discussion to worlds that resemble Earth.
This means that, in our investigation of super-Earths, we need to find planets that look and act (and hopefully smell) very much like our planet. This includes sitting in the habitable zone of a star, to ensure temperatures are just right, as well as having relatively thick but not too thick atmospheres. These planets also need to have liquid water on the surface, not locked under a frozen crust or boiled in vapor. And finally, they must have a magnetic fieldto protect this atmosphere and this liquid water from the brutal and constant assault of the solar wind.
Of course, there are many more criteria that must be in place for a world to truly welcome life. But without those basic conditions, the chances of anything growing on an alien world are pretty slim, so it’s a good place to start.
Choose the right planet size
Astronomers generally define a super-Earth as any planet between earth size and 10 times more massive. Astronomers tend to call planets larger than this mini Neptunesbut this seemingly sharp distinction hides many nuances that are important in determining habitability.
Obviously, something closer to the size of Earth has a better chance of being habitable, since it’s presumably very similar to Earth. And something closer to the size of Neptune probably wouldn’t be a very fun place for life to find a foothold, since Neptune in general isn’t very hospitable, at least by the definition above.
As planets increase in mass, a rocky core clings better and better to a thick, gaseous atmosphere, due to its gravity. Eventually, there will be so much atmosphere that the planet would be better characterized as a gas giant than a rocky world. Unfortunately, we lack a clear dividing line between these two extremes, and super-Earths bridge that gap.
This is where orbit matters too. If a planet is too close to its parent star, regardless of size, it will just get fried. Take 55 Cancer and, a rocky super-Earth about 55 light-years away. It’s eight times the mass of Earth, but it’s so close to its parent star that it’s just a ball of molten rock.
On the other hand, the planet TOI 270c is smaller, at about seven times the mass of Earth. But it’s so distant from its parent star that it’s almost entirely gas, making it more of a mini-Neptune than anything.
The pretenders
Ultimately, a habitable super-Earth must have the right density, indicating that it is neither too rocky nor too gaseous. Even then, it’s just a guess, as astronomers have little information about any particular element. exoplanet.
Take for example, Gliese 581c, which is only about 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. At this orbit and mass, the planet could be a typical rocky world, or made of solid iron. Or it could be a small world of gas, or even made of diamonds.
As for the magnetic field of any exoplanet, that is pure speculation. Scientists believe that planets larger than Earth are likely to harbor strong magnetic fields, but it’s impossible to know for sure. For example, while Venus and Earth are roughly the same size, only Earth has a substantial magnetic field.
Maybe the The best candidate for a habitable super-Earth is LHS 1140bwhich orbits around a red dwarf star about 49 light years from Earth. It is about 60% larger than our planet but 6.48 times more massive. It orbits extremely close to its parent star – its orbital period is only 25 days – but since the star is a cool red dwarf, it sits comfortably within the habitable zone.
Atmospheric models of LHS 1140b allow for the possibility of a thick atmosphere hugging a world with oceans of liquid water. Only detailed observations, perhaps with the James Webb Space Telescope, will tell us with certainty whether the planet is truly habitable. In the meantime, it’s the current reigning champion of the largest planet that could possibly support life.
Paul M. Sutter is an astrophysicist at SUNY 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 a Spaceman” podcast, available on itunes (opens in a new tab) and askaspaceman.com. Ask your own question on Twitter using #AskASpaceman or following Paul @PaulMattSutter and facebook.com/PaulMattSutter.