by Chris Impey, University of Arizona [This article first appeared in The Conversation, republished with permission]
Astronomers now regularly discover planets orbiting 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 some particularly interesting planets orbiting in the habitable zones of their parent stars.
A planet is 30% larger than Earth and orbits its star in less than three days. The other is 70% larger than Earth and could host a deep ocean. These two exoplanets are super-Earths: 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 that could host life.
Earth is still the only place in the universe that scientists know is the home of life. It would seem logical to focus the search for life on clones of the Earth, planets with properties close to those of the Earth. But research has shown that astronomers’ best chance of finding life on another planet is likely on a super-Earth similar to those found recently.
Common and easy to find
Most super-Earths orbit cold dwarf stars, which have a lower mass and live much longer than the Sun. There are hundreds of cold dwarf stars for each star like the Sun, and scientists have found that super- Earths orbit around 40% of the cold dwarfs they have observed. Using that number, astronomers estimate that there are tens of billions of super-Earths in habitable areas where liquid water can only exist in the Milky Way. Since all life on Earth uses water, water is believed to be critical for habitability.
Based on 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. It could also be said that our solar system is unusual in that it does not have a planet with a mass between that of Earth and that of 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 used by astronomers 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 of these surveying methods are easier with a larger planet.
The Super Earths are super habitable
About 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 explored a similar question by asking what makes a planet hospitable for life. It turns out that the Earth is not 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 boiling oceanic to freezing cold across the planet. The Earth has been uninhabitable to humans and other larger creatures for most of its 4.5 billion year history. The simulations suggest that the Earth’s long-term habitability was not inevitable, but it was a matter of luck. Human beings are literally lucky to be alive.
Researchers have come up with a list of attributes that make a planet very conductive for life. Larger planets are more likely to be geologically active, a feature that scientists believe may promote biological evolution. So the most habitable planet would have about twice the mass of the Earth and would be between 20% and 30% larger in volume. It would also have oceans low enough to allow light to stimulate life to the bottom of the sea and an average temperature of 77 degrees Fahrenheit (25 degrees Celsius). It would have a denser atmosphere than Earth’s which would act as an insulating blanket. Finally, such a planet would orbit a star older than the Sun to give the development longer life and would have a strong magnetic field that protects against cosmic radiation. Scientists think 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 has been an exciting addition to the inventory of habitable planets. Astronomers have begun to discover exoplanets that have been ejected from their star systems and there may be billions of them 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 persist before the Sun died. .
Detect life on the super Earths
To detect life on distant exoplanets, astronomers will look for biosignatures, by-products 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 is capable of being part of this science and is programmed 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 the planets discovered this summer, are also compelling targets for James Webb.
But the best chance of finding signs of life in exoplanet atmospheres will come with the next generation of gigantic ground-based telescopes: the extremely large 39-meter 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 decade.
Astronomers know that the ingredients for life are out there, but habitable doesn’t mean inhabited. Until researchers find evidence of life elsewhere, it is possible that life on Earth was a one-time accident. While there are many reasons why a habitable world would have no signs of life, if, in 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.
Chris Impey, distinguished university professor of astronomy, University of Arizona
This article was republished by The Conversation under a Creative Commons license. Read the original article.