An image showing Earth and Neptune with a medium-sized planet in the center.

Super-Earths are bigger and more habitable than Earth, and astronomers are discovering more of the billions they thought existed

Astronomers now routinely discover planets orbiting stars outside our solar system — they’re called exoplanets. But in the summer of 2022, a team of researchers on NASA’s Transiting Exoplanet Survey satellite discovered some particularly interesting planets orbiting within the habitable zone of their parent star.

A planet is 30% larger than Earth and orbits its star in less than three days. The other is 70% larger than Earth and may have deep oceans. The two 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 pay close attention to the search for planets that could host life.

Earth remains the only place in the universe that scientists know of where life exists. It seems logical to focus the search for life on Earth clones – planets with properties close to Earth. But research suggests that astronomers’ best chance of finding life on another planet is likely to be on a super-Earth similar to the recently discovered one.

A super-Earth is any rocky planet larger than Earth and smaller than Neptune. Image credit: Aldaron, CC BY-SA

Common and easy to find

Most super-Earths orbit cold dwarf stars with lower masses and much longer lifespans than the Sun. Every star like the Sun has hundreds of cool dwarfs, and scientists have found that super-Earths orbit 40 percent of the cool dwarfs they’ve observed. Using this number, astronomers estimate that there are tens of billions of super-Earths in the habitable zone where liquid water could exist in the Milky Way alone. Since all life on Earth uses water, water is considered essential for habitability.

According to current projections, about a third of all 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 could even say that our solar system is unusual because it doesn’t have 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 easier to find and study than Earth-sized planets. Astronomers use two methods to detect exoplanets. One looks for the planet’s gravitational effect on its parent star, and the other looks for the brief dimming of the star’s light as the planet passes in front of it. Both detection methods are easier for larger planets.

Super Earth is super habitable

More than 300 years ago, the German philosopher Gottfried Wilhelm Leibniz believed that Earth was “the best of all possible worlds.” Leibniz’s argument was designed to address the question of why evil exists, but modern astrobiologists explore similar questions by asking what makes a planet suitable for 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 shifted over time from boiling ocean heat to freezing cold on a global scale. Earth has been uninhabitable by humans and other large creatures for most of its 4.5 billion-year history. The simulations suggest that the long-term habitability of Earth is not inevitable, but a matter of opportunity. Humans are really lucky to be alive.

Researchers have come up with a list of attributes that make planets very favorable for life. Larger planets are more likely to be geologically active, a feature that scientists believe promotes biological evolution. Therefore, the most habitable planets are roughly twice the mass of Earth and 20 to 30 percent larger than Earth. It will also have oceans shallow enough to allow light to stimulate life all the way down to the seafloor, with an average temperature of 77 degrees Fahrenheit (25 degrees Celsius). Its atmosphere would be thicker than Earth’s and act as an insulating blanket. In the end, such a planet would orbit stars older than the sun to prolong the development of life, and it would have a strong magnetic field that would protect it from cosmic radiation. Scientists believe these properties combine to make the planet super habitable.

By definition, super-Earths have many of the properties of a super-habitable planet. So far, astronomers have discovered two dozen super-Earth exoplanets that are theoretically more habitable than Earth, if not the best of all possible worlds.

Recently, there has been an exciting addition to the list of habitable planets. Astronomers have begun to discover exoplanets ejected from their star systems, possibly billions of them roaming the Milky Way. If a super-Earth were ejected from its star system and had a dense atmosphere and a watery surface, it could sustain life for tens of billions of years, far longer than life on Earth could have lasted before the sun died.

Detecting life on super-Earths

To detect life on distant exoplanets, astronomers will look for biosignatures, the detectable by-products of biology in planetary atmospheres.

NASA’s James Webb Space Telescope was designed before astronomers discovered exoplanets, so the telescope isn’t optimized for exoplanet research. But it’s able to do some of that science, and plans to target two potentially habitable super-Earths in its first year of operation. Another group of super-Earths with huge oceans discovered in the past few years, as well as planets discovered this summer, are also strong targets for James Webb.

But the best chance of finding signs of life in exoplanet atmospheres will come from 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 will begin collecting data by the end of the century.

Astronomers know the ingredients for life are out there, but habitable doesn’t mean inhabited. Before researchers find evidence of life elsewhere, life on Earth may be a unique accident. While there are many reasons why habitable worlds show no signs of life, if astronomers look at these super-habitable super-Earths in the next few years and find nothing, humanity may be forced to conclude that the universe is a lonely place.conversation

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Image Source: NASA Ames/JPL-CalTech

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