Massive versions of Earth have been found. And billions more could be out there

This article first appeared on The Conversation.

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 found some particularly interesting planets orbiting in the habitable zones of their parent stars.

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A 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 could harbor a deep ocean.

Both of these exoplanets are super-Earths – more massive than Earth but smaller than ice giants like Uranus and Neptune.

I’m a professor of astronomy studying galactic cores, distant galaxies, astrobiology and exoplanets. I am closely following the search for planets that could harbor life.

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 Earth clones – planets with Earth-like properties.

But research has shown that astronomers’ best chance of finding life on another planet is probably on a super-Earth similar to those recently discovered.

Common and easy to find

Most super-Earths orbit cool dwarf stars, which are lighter in mass and live much longer than the Sun.

For every star like the Sun, there are hundreds of cool dwarf stars, and scientists have found super-Earths orbiting 40 percent of the cool dwarfs they’ve studied.

Using this number, astronomers estimate that there are tens of billions of super-Earths in the Milky Way alone in habitable zones where liquid water can exist.

Since all life on Earth consumes water, water is considered critical to habitability.

According to current projections, about a third of all exoplanets are super-Earths, making them the most common type of exoplanets in the Milky Way. The nearest is only six light-years from Earth.

It could even be said that our solar system is unusual in not having a planet with a mass between Earth and Neptune.

Another reason super-Earths are ideal targets in the search for life is that they are much easier to discover and study than Earth-sized planets.

There are two methods astronomers use to discover exoplanets. One is looking for a planet’s gravitational pull on its parent star, and the other is looking for a brief eclipse in a star’s light as the planet passes in front of it.

Both detection methods are easier with a larger planet.

Super Earths are 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 address the question of why evil exists, but modern astrobiologists have pursued a similar question by asking what makes a planet livable. 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 shifted over time from searing heat in the ocean to freezing cold around the world.

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 rather a matter of chance. People are literally lucky to be alive.

Researchers have compiled a list of properties that make a planet very hospitable to life.

Larger planets are more likely to be geologically active, a trait scientists believe would encourage biological evolution.

So the most habitable planet would be about twice the mass of Earth and between 20 and 30 percent larger in volume.

It would also have oceans shallow enough for light to stimulate life down to the seafloor and an average temperature of 25°C.

It would have an atmosphere thicker than Earth’s, acting as an insulating blanket.

After all, 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 believe these properties together will make a planet super habitable.

By definition, super-Earths have many of the characteristics of a super-habitable planet.

To date, astronomers have discovered two dozen super-Earth 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 to detect exoplanets ejected from their star systems, and billions of them could be roaming the Milky Way.

If a super-Earth is ejected from its star system and has a dense atmosphere and a watery surface, it could sustain life for tens of billions of years, much longer than life on Earth could exist before the Sun died.

Discover life on super-earths

To detect life on distant exoplanets, astronomers 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 is not optimized for exoplanet research.

But it’s capable of doing some of that science, and is set to target two potentially habitable super-Earths in its first year of operation.

Another group of super-Earths with vast oceans discovered in recent years, as well as the planets discovered this summer, are also compelling targets for James Webb.

But the best chances of finding signs of life in exoplanet atmospheres will come with the next generation of giant, ground-based telescopes: the 39-meter Extremely Large Telescope, the 39-meter Telescope, and the 24.5-meter Giant Magellan telescope.

These telescopes are all under construction and are expected to begin collecting data by the end of the decade.

Astronomers know the ingredients for life are out there, but habitable doesn’t mean inhabited.

Until researchers find evidence of life elsewhere, it’s possible that life on Earth was a one-of-a-kind accident.

While there are many reasons why a habitable world would show no signs of life, if astronomers look at these super-habitable super-Earths and find nothing in the years to come, humanity may be forced to conclude that the universe is a lonely place.

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First full color image from NASA’s new space telescope.