Super-Earths and the quest to understand our place in the cosmos

Also in 2022, a team led by the University of Montreal discovered two super-Earths orbiting the red dwarf star Kepler-138, located some 218 light-years away in the constellation Lyra. Both planets in this system are believed to be “water worlds.” Although only slightly larger than Earth, these exoplanets are less dense than a rocky planet but denser than the gas giants orbiting our Sun. The most plausible explanation for this is that these planets contain global oceans at least 500 times deeper than those on Earth.

Can super-Earths support life?

Though astronomers are finding an increasing number of super-Earths that could potentially harbor vast oceans of liquid water, searching for life on these worlds is not as straightforward as finding a planet in the habitable zone.

There are many factors that can influence a planet’s potential for habitability, including the density and composition of its atmosphere, the strength of its magnetic field, and its geological activity. But one of the simplest yet most important considerations is the planet’s mass. Super-Earths are thought to have a greater potential for habitability than smaller planets like Mars.

But if they are too massive, they may be covered in a thick layer of gas, which could make it difficult for life to thrive on their surface.

“So far, it’s been hard to learn a lot about super-Earth atmospheres because they are relatively small planets in the grand scheme of things and their atmospheric signatures are commensurately small,” says Christiansen. “Mostly, they seem to have a flat atmospheric spectrum, which could either mean little to no atmosphere, or a very thick, heavy atmosphere that is so dense it’s not allowing any transparency at any wavelengths that would then create spectral features. These super-Earths could also have a very hazy or cloudy atmosphere that is similarly not allowing any transparency.”

Despite the challenge of studying super-Earths, the discovery of these worlds has given scientists more hope that we may one day find life beyond our own planet. The search for life in the universe is one of the great questions of our time, and super-Earths have opened up a whole new realm of possibilities.

How do super-Earths form?

The study of super-Earths has also shed light on the formation and evolution of planets, including those in our solar system. Astronomers think that many super-Earths form from the accumulation of rock and ice in the early stages of a star’s life. As these planets grow larger, they may attract gas from their surrounding environment, eventually becoming gas giants if enough material is available.

“We are still learning about how planets smaller than Neptune, four times the radius of Earth, form,” says Christiansen. “One thing that seems to stand out about super-Earths is that some of them may have started as larger planets 2 to 2.5 times the radius of Earth and have lost a chunk of their atmospheres in some way, perhaps by being so close to their host stars that the stellar radiation is blasting the upper atmosphere away.”

Another possibility is that the residual interior heat “leftover from the planet’s formation is pushing outward so strongly that it puffs off the outer layers of the atmosphere,” says Christiansen. “In general, we think the initial process is the same — accretion of material in a protoplanetary disk (a rotating circumstellar disk of dense gas surrounding a young newly formed star) that builds up until it is a rocky planet — but that subsequent process sculpts and evolves the planet properties into what we see today.”

The discovery of super-Earths is a momentous achievement for both science and humanity. They offer the possibility of liquid water on their surfaces and the potential for habitability, providing a place where life might be able to take hold.

And even if super-Earths don’t harbor life, their study has given astronomers important insights into the formation and evolution of all planets. Super-Earths challenged our understanding of what types of worlds planetary systems should have. Their discovery reminded us of the vast and diverse nature of the universe, which is always helpful in motivating us to continue investigating our place in the cosmos.