Astronomers now routinely discover planets orbiting stars outside of the solar system- they’re called exoplanets. 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. These two exoplanets are super-Earths-more massive than Earth but smaller than ice giants like Uranus and Neptune.
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.
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.
Since all life on Earth uses water, water is thought to be critical for habitability. 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.
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.
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. 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. Larger planets are more likely to be geologically active, a feature that scientists think would promote biological evolution.
So the most habitable planet would have roughly twice the mass of Earth and be between 20 to 30 percent larger by volume.
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. 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 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 that have been ejected from their star systems, and there could be billions of them 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, far longer than life on Earth could persist before the sun dies.
To detect life on distant exoplanets, astronomers will look for biosignatures, byproducts of biology that are detectable in a planet’s atmosphere.
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.