Astronomers now routinely discover planets orbiting stars outside of the solar system— they’re called exoplanets. But in summer 2022, teams working on NASA’s Transiting Exoplanet Survey Satellite found a few particularly interesting planets orbiting in the habitable zones of their parent stars.

One planet is 30 percent larger than Earth and orbits its star in less than three days. The other is 70 percent larger than Earth and might host a deep ocean. These two exoplanets are super-Earths—more massive than Earth but smaller than ice giants like Uranus and Neptune.

I’m a professor of astronomy who studies galactic cores, distant galaxies, astrobiology, and exoplanets. I closely follow the search for planets that might host life.

Earth is still the only place in the universe scientists know to be home to life. It would seem logical to focus the search for life on Earth clones—planets with properties close to Earth’s. But research has shown that the best chance astronomers have of finding life on another planet is likely to be on a super-Earth similar to the ones found recently.

A super-Earth is any rocky planet that is bigger than Earth and smaller than Neptune. Image Credit: Aldaron, CC BY-SA

Common and Easy to Find

Most super-Earths orbit cool dwarf stars, which are lower in mass and live much longer than the sun. There are hundreds of cool dwarf stars for every star like the sun, and scientists have found super-Earths orbiting 40 percent of cool dwarfs they have looked at. Using that number, astronomers estimate that there are tens of billions of super-Earths in habitable zones where liquid water can exist in the Milky Way alone. Since all life on Earth uses water, water is thought to be critical for habitability.

Based on current projections, about a third of all exoplanets are super-Earths, making them the most common type of exoplanet in the Milky Way. The nearest is only six light-years away from Earth. You might even say that our solar system is unusual since it does not have a planet with a mass between that of Earth and Neptune.

Another reason super-Earths are ideal targets in the search for life is that they’re much easier to detect and study than Earth-sized planets. There are two methods astronomers use to detect exoplanets. One looks for the gravitational effect of a planet on its parent star and the other looks for brief dimming of a star’s light as the planet passes in front of it. Both of these detection methods are easier with a bigger planet.

Super-Earths Are Super Habitable

Over 300 years ago, German philosopher Gottfried Wilhelm Leibniz argued that Earth was the “best of all possible worlds.” Leibniz’s argument was meant to address the question of why evil exists, but modern astrobiologists have explored a similar question by asking what makes a planet hospitable to life. It turns out that Earth is not the best of all possible worlds.

Due to Earth’s tectonic activity and changes in the brightness of the sun, the climate has veered over time from ocean-boiling hot to planet-wide, deep-freeze cold. Earth has been uninhabitable for humans and other larger creatures for most of its 4.5-billion-year history. Simulations suggest the long-term habitability of Earth was not inevitable, but was a matter of chance. Humans are literally lucky to be alive.

Researchers have compiled a list of characteristics that make a planet ideal for life. Larger planets are more likely to be geologically active, which experts believe will aid biological evolution. As a result, the most habitable planet would be nearly twice the mass of Earth and 20 to 30 percent larger in volume. It would also have shallow enough oceans for light to promote life all the way to the seafloor, as well as an average temperature of 77 degrees Fahrenheit (25 degrees Celsius). It would have a thicker atmosphere than Earth, which would act as an insulator. Finally, such a planet would orbit a star older than the sun, allowing life to flourish for a longer period of time, and it would have a strong magnetic field that shields against cosmic radiation. Scientists believe that combining these characteristics will result in a planet that is extremely habitable.

Super-Earths, by definition, exhibit many of the characteristics of a super livable planet. Astronomers have discovered over a dozen super-Earth exoplanets that, while not the best of all imaginable worlds, are theoretically more habitable than Earth.

There has recently been an interesting addition to the list of livable planets. Exoplanets that have been expelled from their star systems have been discovered by astronomers, and there could be billions of them roaming the Milky Way. If a super-Earth with a dense atmosphere and wet surface is evicted from its solar system, it may maintain life for tens of billions of years, considerably longer than life on Earth could survive before the sun dies.

Discovering Life on Super-Earths

Astronomers will seek for biosignatures, which are byproducts of biology that can be detected in a planet’s atmosphere, to identify life on distant exoplanets.

Because NASA’s James Webb Space Telescope was developed before astronomers discovered exoplanets, it is not well suited for exoplanet study. However, it is capable of doing some of this science and plans to target two potentially habitable super-Earths in its first year of operation. Another group of super-Earths with vast oceans identified in recent years, as well as the planets discovered this summer, are also intriguing candidates for James Webb.

The next generation of big ground-based telescopes, however, will have the highest prospects of finding indications of life in exoplanet atmospheres: the 39-meter Extremely Large Telescope, the Thirty Meter Telescope, and the 24.5-meter Giant Magellan Telescope. These telescopes are all being built and will begin gathering data by the end of the decade.

Astronomers know that the building blocks for life exist elsewhere, but livable does not imply inhabited. Until scientists discover proof of life elsewhere, it’s plausible that life on Earth was a one-of-a-kind accident. While there are various reasons why a livable world would not contain signs of life, astronomers may be forced to conclude that the universe is a lonely place if they look at these super habitable super-Earths and find nothing.

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