Astronomers now routinely discover planets that orbit stars outside the solar system – they are called exoplanets. But in the summer of 2022, teams working on NASA’s Transiting Exoplanet Survey Satellite found a few intriguing planets orbiting in the habitable zones of their parent stars.
One planet is 30% larger than Earth and orbits its star in less than three days. The other is 70% larger than Earth and may host a deep ocean. These two outer planets are super-Earths – a mass larger than Earth but smaller than ice giants like Uranus and Neptune.
I am a professor of astronomy who studies galactic nuclei, distant galaxies, astrobiology, and exoplanets. I am closely following the search for planets that might host life.
Earth is still the only place in the universe that scientists know is home to life. It would seem logical to focus the search for life on Earth clones – planets with near-Earth characteristics. But research has shown that astronomers’ best chance of finding life on another planet is likely to be on a super-Earth similar to the one found recently.
Shared and easy to find
Most giant planets orbit cold dwarf stars, which are less massive and live much longer than the Sun. There are hundreds of gorgeous dwarf stars for every star like the Sun, and scientists have found super-Earths orbiting around 40% of the cooler dwarfs they looked at. Using that number, astronomers estimate that there are tens of billions of super-Earths in habitable zones where liquid water could exist in the Milky Way alone. Since all life on Earth uses water, water is believed to be essential for life.
Based on current projections, about a third of exoplanets are super-Earths, making them the most common type of exoplanet in the Milky Way. The closest is only six light years from Earth. You might even say that our solar system is unusual because it does not have a planet with a mass between Earth and Neptune.
Another reason that super-Earths are ideal targets in the search for life is that they are much easier to detect and study than Earth-sized planets. There are two methods astronomers use to discover exoplanets. One looks for a planet’s gravitational effect on its parent star, while the other looks for a brief dimming of the star’s light as the planet passes in front of it. Both detection methods are easier with a larger planet.
More than 300 years ago, German philosopher Gottfried Wilhelm Leibniz argued that the Earth was “the best of all possible worlds.” Leibniz’s argument was intended to address the question of why evil exists, but modern astrobiologists have explored a similar question by asking the question of what makes a planet hospitable to life. It turns out that Earth isn’t the best of all possible worlds.
Due to Earth’s tectonic activity and changes in the brightness of the Sun, the climate has shifted over time from the boiling temperature of the ocean to the extreme cold of the planet. Earth has been uninhabitable for humans and larger creatures for most of its 4.5 billion year history. Simulations indicate that the Earth’s long-term habitability was not deterministic, but rather a matter of chance. Humans are literally lucky to be alive.
Researchers have come up with a list of traits that make a planet so conducive to life. Larger planets are more likely to be geologically active, a feature that scientists believe will boost biological evolution. So the most habitable planet would have roughly twice the mass of Earth and be between 20% and 30% larger in size. It would also have oceans shallow enough for light to stimulate life all the way to the sea floor and an average temperature of 77 degrees Fahrenheit (25 degrees Celsius). It would have a thicker atmosphere than the Earth’s, which would act as an insulating cover. Finally, such a planet would orbit a star older than the Sun to give life longer to evolve, and would have a strong magnetic field protecting against cosmic radiation. Scientists believe that these traits combined will make the planet habitable.
By definition, a super-Earth possesses many of the attributes of a super-habitable planet. So far, astronomers have discovered twenty super-Earths that are, if not the best of all possible worlds, more habitable than Earth in theory.
Recently, there has been an exciting addition to the inventory of habitable planets. Astronomers are beginning to discover exoplanets that have been ejected from their star systems, and there could be billions of them wandering around the Milky Way. If the super-Earth was expelled from its star system and had a dense atmosphere and watery surface, it could sustain life for tens of billions of years, much longer than life on Earth could last before the Sun died.
Detecting life on super-Earth
To discover life on distant exoplanets, astronomers will look for biosignatures, which are byproducts of biology that can be detected in a planet’s atmosphere.
NASA’s James Webb Space Telescope was designed before astronomers discovered exoplanets, so the telescope is not optimized for exoplanet research. But it is capable of doing some of that science and is set to target two potentially habitable super-Earths in its first year of operations. Another group of super-Earths with huge oceans that have been discovered in the past few years, in addition to the ones discovered this summer, are also compelling targets for James Webb.
But the best chances of finding signs of life in the atmospheres of exoplanets will come with the next generation of giant ground-based telescopes: the 39-meter Very Large Telescope, the 30-meter Telescope, and the 24.5-meter Giant Magellan Telescope. These telescopes are all under construction and scheduled to begin collecting data by the end of the decade.
Astronomers know the ingredients for life are there, but habitable doesn’t mean it’s inhabited. Until researchers find evidence of life elsewhere, it is possible that life on Earth was a unique accident. While there are many reasons why the habitable world does not bear signs of life, if, over the coming years, astronomers look at this super-habitable Earth and find nothing, humanity may be forced to conclude that the universe is a lonely place.
Chris Impey, Distinguished University Professor of Astronomy, University of Arizona
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