This article was originally published on The conversation. (opens in new tab) The publication contributed the article to Space.com Expert Voices: Op-Ed & Insights.
Astronomers are now routinely discovering planets orbiting stars outside the solar system – called exoplanets. But in the summer of 2022, teams working on NASA’s Transiting Exoplanet Survey Satellite found some particularly interesting planets orbiting the habitable zones of their parent stars.
A planet is 30% larger than Earth (opens in new tab) and orbits its star in less than three days. The other is 70% larger than Earth (opens in new tab) and may 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 a professor of astronomy (opens in new tab) who studies galactic nuclei, distant galaxies, astrobiology (opens in new tab) and exoplanets. I am closely following the search for planets that may harbor 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 terrestrial clones – planets with properties close to those of Earth. But the research showed that the best chance astronomers have of finding life on another planet is likely to be on a super-Earth similar to those recently found.
Related: “We can find life outside the solar system in 25 years,” says the researcher
Common and easy to find
Most super-Earths orbit cool dwarf stars, which are less massive and live much longer than the sun. There are hundreds of cool dwarf stars for every star like the Sun, and scientists have found super-Earths orbiting about 40% of cool dwarfs (opens in new tab) they have looked. Using this number, astronomers estimate that there are tens of billions (opens in new tab) of super-earths in habitable zones where liquid water can exist only in our Galaxy. Since all life on Earth uses water, water is considered critical to habitability.
Based on current predictions, about a third of all exoplanets (opens in new tab) they are super-Earths, making them the most common type of exoplanet in the Galaxy. The nearest one is only 6 light years away (opens in new tab) from the earth. You could even say that our solar system is unusual in that it does not have a planet with a mass between this 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 (opens in new tab) from Earth-sized planets. There are two methods astronomers use to detect exoplanets. One looks for a planet’s gravitational influence 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 detection methods are easier with a larger planet.
Super-Earths are extremely habitable
More than 300 years ago, the German philosopher Gottfried Wilhelm Leibniz argued that Earth was the “best of all possible worlds (opens in new tab)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 is not the best of all possible worlds.
Due to the Earth’s tectonic activity and changes in the brightness of the sun, the climate has changed over time from boiling hot in the ocean to freezing cold across the planet. Earth has been uninhabited by humans and other larger creatures for most of its 4.5 billion year history. The simulations suggested that long-term habitability of Earth was not inevitable (opens in new tab), but it was a matter of luck. People are literally lucky to be alive.
Researchers have come up with a list of characteristics (opens in new tab) which make a planet very favorable for life. Larger planets are more likely to be geologically active, a feature scientists believe would promote biological evolution (opens in new tab). Thus, the most habitable planet would have about twice the mass of Earth and be between 20% and 30% larger in volume. It will also have oceans shallow enough for light to stimulate life down to the sea floor and an average temperature of 77 degrees Fahrenheit (25 degrees Celsius). It would have an atmosphere thicker than Earth’s that would act as an insulating blanket. Finally, such a planet would orbit a star older than the Sun to give it a longer lifetime to grow and have a strong magnetic field that shields it from cosmic radiation (opens in new tab). Scientists believe that these characteristics combined would make a planet highly habitable.
By definition, super-Earths have many of the characteristics of a habitable superplanet. To date, astronomers have discovered two dozen super-Earth exoplanets (opens in new tab) which is, if not the best of all possible worlds, theoretically more habitable than Earth.
Recently, there has been an exciting addition to the list of habitable planets. Astronomers are beginning to discover exoplanets (opens in new tab) that have been ejected from their star systems (opens in new tab)and there may be billions of them (opens in new tab) wandering the Galaxy. If a super-Earth were ejected from its star system and had a dense atmosphere and water surface, it could sustain life for tens of billions of years (opens in new tab)much longer than life on Earth could be sustained before the sun died.
Detection of life on extra-Earth
To detect life on distant exoplanets, astronomers will look for biosignatures, byproducts of biology (opens in new tab) that are 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 able to do some of that science and is scheduled 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 planets discovered this summer, are also compelling targets (opens in new tab)s for James Webb.
But the best chance 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 (opens in new tab)the thirty meter telescope (opens in new tab) and the 25.4 meter long Giant Magellan Telescope (opens in new tab). These telescopes are all under construction and are due 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-off 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
Editor’s note: The story has been updated to correct the size of the Giant Magellan Telescope.
This article is republished from The Conversation (opens in new tab) with a Creative Commons license. Read the original article (opens in new tab).
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