why we need to learn more about super-earths

When anyone mentions a Super-Earth, you probably aren’t imagining Earth in a cape, ready to fight crime, which, surprisingly, in not the subject of an experimental web comic yet. Odds are, you’re thinking of rocky planets orbiting other words that sort of look like a giant Earth or Mars and maybe even imagine them being habitable because that’s a real possibility for at least one of the Super-Earths we’ve discovered. But that might not be the right way to think about these worlds because it turns out that once you exceed two terrestrial masses, some very weird and interesting things start to happen, and some of the largest Super-Earths out there may be more accurately classified as dwarf Neptunes as they become too heavy to lose a thick cloud of gas surrounding them during their formation and develop extremely thick atmospheres.

Right now, Super-Earths are the second most commonly found type of planet orbiting an alien star, with Neptune-sized worlds showing up 1.5 times more frequently in observation data. This isn’t to say that the universe is dominated by smallish gas giants because our sample size is too small and too heavily influenced by our tools’ limitations. More powerful telescopes, more time to observe orbital periods, and better techniques could very well change those statistics. But we do understand that Super-Earths make up a fairly significant share of the planetary population, and while smaller versions could very well be a lot like our world with nearly identical gravity, the heavyweights could be a transitional step between a terrestrial world and a gas giant.

what’s it like on the surface of a super-earth?

If you were to take a grand tour of Super-Earths, you’d find a great deal of variety among them. Smaller ones would very much resemble any other rocky world with a few small differences like thicker atmospheres, possibly slightly stronger magnetic fields and more tectonic activity since their innards would have more volume and churn hotter. All of these are very promising for life. A thicker atmosphere and stronger magnetosphere means more protection from solar wind and radiation. More tectonic activity means more nutrient exchange and energy for any living things trying to find a place to call home. With the right temperature range to allow liquid water on the surface or close to it, these worlds could have stable, warm, welcoming oceans.

But as these planets get more massive, things quickly start to get a little less friendly to complex organisms. While, as we noted, more mass means a thicker atmosphere, it also means higher pressures, possibly hundreds of times greater than on Earth’s surface. Thicker crusts mean that the magma convecting underneath lacks the force to create tectonic plates and geologic activity would be limited to occasional volcanoes and deep, sub-surface quakes. Incidentally, enough mass could change the viscosity of molten layers inside the planet, preventing the interactions between different molten layers and making it difficult to generate a magnetosphere.

However, difficult doesn’t mean impossible. There is some research indicating that magnesium oxide, a mineral usually solid on Earth, can liquefy in the mantles of more massive worlds and produce magnetic fields. Life could still exist on these massive planets given the right chemistry since crushing pressures weren’t a deterrent to life on Earth and resisting high gravity shouldn’t be that difficult for native organisms, especially when we consider they would be dealing with only twice the gravity we consider ideal at the very most. But there would be serious concerns about their ability to get enough energy to start a proper food chain and sustain the large populations necessary for a wide variety of lifeforms.

the line between a super-earth and a gas giant

After a certain point, Super-Earths exist in a kind of transitional boundary between a rocky world and a failed gas giant with the hallmarks of both. While we think that terrestrial planets tipping the scales at eight or more Earth masses become the seeds for gas giants as they attract more and more gas from protoplanetary disks of young stars, there’s a possibility that they hit a gas poor pocket or got muscled out from their orbit and fail to generate a thick enough atmosphere to be officially considered gas giants in their own right. Some may have even been gas giants but lost their atmospheres after being knocked into an orbit too close to their parent sun.

In a way these 8+ Earth mass bruisers, occupy a gray area similar to Pluto in astronomical bureaucracy and there may be an argument for classifying Super-Earths into distinct subtypes based on mass, tectonic activity, and thickness of atmosphere, just like one could also argue that a small enough rocky world is fundamentally different from Earth since it can’t attract or keep an atmosphere and is geologically dead after a violent formation and cooling process. It seems that the next step in discussions of how we define a planet may just be coming up with the equivalent of a Hertzsprung-Russell diagram for planets, with Super-Earths occupying a fairly large part of the resulting infographic.

how can we learn more about super-earths?

We’d obviously know a lot more about how Super-Earths work if we could study one of them, and lucky for us, astronomers found a perfect specimen. It’s designated Barnard b, and as the name implies, it orbits Barnard’s Star, just six light years away. Some of our best tools may be just good enough to analyze its atmosphere, take detailed measurements, and maybe even give us an honest to goodness picture, although that’s going to be a very difficult task and require years of preparation to capture meaningful images due to the significant distance involved and the glare of the star in comparison to the relative dimness of the planet.

We’ll start with a few smudged pixels and keep adjusting to get a better look at Barnard b, and if we’re lucky, any other worlds that may be floating around the star unbeknownst to us. While we don’t expect it to be habitable with a temperature estimated at -168 °C based on its distance from its red dwarf sun, if it has a thick enough atmosphere with enough greenhouse gasses, it might be a lot more hospitable to living things than we think. And at roughly 3.25 Earth masses, it’s an average size for a Super-Earth, so it what we find out could be extrapolated across the entire spectrum of these heavy rocky worlds. Hopefully, we can capitalize on this opportunity and put many of the theories outlined here to the test.