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% larger than Earth (opens in new tab) and orbits its star in less than three days. The other is 70% larger than the Earth (opens in new tab) and might host a deep ocean. These two exoplanets are super-Earths — more massive than the Earth but smaller than ice giants like Uranus and Neptune.
I'm a professor of astronomy (opens in new tab) who studies galactic cores, distant galaxies, astrobiology (opens in new tab) 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.
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% of cool dwarfs (opens in new tab) they have looked at. Using that number, astronomers estimate that there are tens of billions (opens in new tab) 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 (opens in new tab) are super-Earths, making them the most common type of exoplanet in the Milky Way. The nearest is only 6 light-years away (opens in new tab) 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 (opens in new tab) 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 (opens in new tab)." 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 planetwide, 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 (opens in new tab), but was a matter of chance. Humans are literally lucky to be alive.
Researchers have come up with a list of the attributes (opens in new tab) that make a planet very conducive to life. Larger planets are more likely to be geologically active, a feature that scientists think would promote biological evolution (opens in new tab). So the most habitable planet would have roughly twice the mass of the Earth and be between 20% and 30% larger by volume. It would also have oceans that are shallow enough for light to stimulate life all the way to the seafloor and an average temperature of 77 degrees Fahrenheit (25 degrees Celsius). It would have an atmosphere thicker than the Earth’s that would act as an insulating blanket. Finally, such a planet would orbit a star older than the Sun to give life longer to develop, and it would have a strong magnetic field that protects against cosmic radiation (opens in new tab). Scientists think that these attributes combined will make a planet super habitable.
By definition, super-Earths have many of the attributes of a super habitable planet. To date, astronomers have discovered two dozen super-Earth exoplanets (opens in new tab) that are, if not the best of all possible worlds, theoretically more habitable than Earth.
Recently, there’s been an exciting addition to the inventory of habitable planets. Astronomers have started discovering exoplanets (opens in new tab) that have been ejected from their star systems (opens in new tab), and there could be billions of them (opens in new tab) roaming the Milky Way. If a super-Earth is ejected from its star system and has a dense atmosphere and watery surface, it could sustain life for tens of billions of years (opens in new tab), far longer than life on Earth could persist before the sun dies.
Detecting life on super-Earths
To detect life on distant exoplanets, astronomers will look for biosignatures, byproducts of biology (opens in new tab) that are detectable in a planet’s atmosphere.
NASA's James Webb Space Telescope was designed before astronomers had discovered exoplanets, so the telescope is not optimized for exoplanet research. But it is able to do some of this science and is scheduled to target two potentially habitable super-Earths in its first year of operations. Another set of super-Earths with massive oceans discovered in the past few years, as well as the planets discovered this summer, are also compelling target (opens in new tab)s for James Webb.
But the best chances for finding signs of life in exoplanet atmospheres will come with the next generation of giant, ground-based telescopes: the 39-meter Extremely Large Telescope (opens in new tab), the Thirty Meter Telescope (opens in new tab) and the 25.4-meter Giant Magellan Telescope (opens in new tab). These telescopes are all under construction and set to start collecting data by the end of the decade.
Astronomers know that the ingredients for life are out there, but habitable does not mean inhabited. Until researchers find evidence of life elsewhere, it’s possible that life on Earth was a unique accident. While there are many reasons why a habitable world would not have signs of life, if, over the coming years, astronomers look at these super habitable super-Earths and find nothing, humanity may be forced to conclude that the universe is a lonely place.
Editor’s Note: The story has been updated to correct the size of the Giant Magellan Telescope.
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