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Super-Earth is more habitable than Earth astronomers think there are billions

Astronomers now routinely discover planets orbiting stars outside the solar system — they’re called exoplanets.

Astronomers now routinely discover planets orbiting stars outside the solar system — they’re called exoplanets. But in the summer of 2022, teams working on NASA’s Transiting Exoplanet Survey Satellite discovered some particularly interesting planets orbiting 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 can accommodate a deep ocean. These two exoplanets are super-Earth – more massive than Earth, but smaller than ice giants like Uranus and Neptune.

I am an astronomy professor who studies galactic nuclei, distant galaxies, astrobiology and exoplanets. I closely follow the search for planets where life is possible.

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

General and easy to find

Most super-Earths revolve around cool dwarf stars, which have a lower 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 that super-Earth revolves around 40% of the cool dwarfs they’ve looked at.

Using that number, astronomers estimate that there are tens of billions of super-Earths in habitable zones where liquid water can exist only in the Milky Way. Since all life on Earth uses water, water is considered crucial to habitability.

Based on current projections, about a third of all exoplanets are super-Earths, making them the most abundant type of exoplanet in the Milky Way. The closest is just six light-years from Earth. You could even say that our solar system is unusual in that 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 are much easier to detect and study than Earth-sized planets. There are two methods astronomers use to detect exoplanets. One looks for a planet’s gravitational effect on its parent star, and the other looks for a brief dimming of a star’s light as the planet passes in front of it. Both detection methods are easier with a larger planet.

Super-Earth is super habitable

Over 300 years ago, the German philosopher Gottfried Wilhelm Leibniz argued that the Earth was the “best of all possible worlds.” Leibniz’s argument was intended to answer 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 isn’t the best of all possible worlds.

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

Researchers have made a list of the attributes that make a planet highly conducive to life.

Larger planets are more likely to be geologically active, a feature scientists believe would aid biological evolution. So the most habitable planet would be about twice the mass of Earth and between 20% and 30% larger in volume.

It would also have oceans shallow enough for light to stimulate life right down to the seafloor and an average temperature of 77 degrees Fahrenheit (25 degrees Celsius).

It would have an atmosphere thicker than Earth’s and act as an insulating blanket. Finally, such a planet would orbit a star older than the sun to allow life to develop longer, and it would have a strong magnetic field that protects against cosmic rays. Scientists think these qualities combined will make a planet super habitable.

Super-Earth, by definition, has many of the features of a super-habitable planet. To date, astronomers have discovered two dozen superterrestrial exoplanets that, if not the best of all possible worlds, are theoretically more habitable than Earth.

Recently there has been an exciting addition to the inventory of habitable planets. Astronomers have begun discovering exoplanets flung out of their galaxies, and there could be billions of them roaming the Milky Way.

If a super-Earth is ejected from its galaxy and has a dense atmosphere and a watery surface, it can live for tens of billions of years, much longer than life on Earth could exist before the sun dies.

Detect life on super-Earth

To detect life on distant exoplanets, astronomers will look for biosignatures, byproducts of biology detectable in a planet’s atmosphere.

NASA’s James Webb Space Telescope was designed before astronomers discovered exoplanets, so the telescope isn’t optimized for exoplanet research. But it’s capable of doing some of this science, and is set to target two potentially habitable super-Earths in its first year of operation.

Another set of super-Earths with huge oceans discovered in recent years, as well as those discovered this summer, are also attractive 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 Extremely Large Telescope, the Thirty Meter Telescope and the 24.5-meter Giant Magellan Telescope. These telescopes are all under construction and will begin collecting data by the end of the decade.

Astronomers know the ingredients for life are there, but habitable means uninhabited. 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 have no sign of life, if astronomers look at this super-habitable super-Earth over the next few years and find nothing, humanity may be forced to conclude that the universe is a lonely place.

By Chris Impey: University Distinguished Professor of Astronomy, University of Arizona

(The conversation)

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