Ancient rocky planets offer further evidence that extraterrestrial life may have existed long before humans.
Scientists may have better success looking for signs of long-gone alien life than for active civilizations.
A rocky, molten planet orbiting one of the galaxy's oldest stars could be scientists' best evidence yet that alien life may have arisen in the distant past.
The planet, called TOI-561 b, is a "super-Earth" 280 light-years away. It's about 50% larger than our planet and three times its mass, but it's unlikely to host life. It orbits so close to its star that the researchers who discovered it calculated that its surface temperature is more than 3,000 degrees Fahrenheit, turning the top layer of rock into molten magma.
But this super-Earth is far older than scientists previously expected for rocky planets, suggesting that other stars could have ancient Earth-like worlds with temperatures more suitable for life. Such planets may have existed for twice as long as Earth, giving them plenty of time to support complex life and even intelligent civilizations.
The star that TOI-561 b orbits lies in the galaxy's "thick disk," the outer region above and below the flat plane that holds most of the Milky Way's material. Stars in the thick disk are about 10 billion years old, and researchers think that this planet is just as ancient.
"TOI-561 b is the first planet with a confirmed rocky composition around such an old star, demonstrating that rocky planets have been forming for most of the history of the universe," Lauren Weiss, a postdoctoral fellow at the University of Hawaii and the lead researcher in this discovery, said at a meeting of the American Astronomical Society. "I just wonder if any of them have anyone we'd like to talk to."
The finding was published in The Astronomical Journal on Monday.
Earth-like planets almost as ancient as our galaxy
The astronomers could tell how old TOI-561 b is because the planet's density is about the same as Earth's, even though its mass is three times more. That means it probably doesn't contain many heavy elements, like iron or magnesium.
It took billions of years for the galaxy to fill with heavy elements, since they have to be forged deep inside stars. When the stars age, die, and explode, these elements disperse and eventually coalesce into new planets. So 10 billion years ago, heavy elements were sparse and planets weren't very dense. That seems to be when TOI-561 b formed.
"Gosh, if we've only been around for 5 billion years, imagine what could have happened on a rocky world that's been around for 10 billion years," Weiss said.
Her team discovered TOI-561 b using the W. M. Keck Observatory on Hawaii's Maunakea. There are two other planets orbiting the star, but their large size and low mass indicates that they're gaseous, like Jupiter.
The hunt for aliens could include fossils
This discovery adds to a growing body of research that suggests life on other planets could have evolved, developed technological civilizations, and gone extinct long before life arose on Earth.
This could open an entirely new path in the search for extraterrestrial life. Instead of listening for messages from aliens seeking out other intelligent life, scientists may have more luck scanning the skies for fossils.
"If you look for evidence that things existed in the past in addition to the present, you have a bigger chance of finding something," Harvard astrophysicist Avi Loeb told Insider's Aylin Woodward.
Loeb's new book "Extraterrestrial: The First Sign of Intelligent Life Beyond Earth" argues that the first interstellar visitor seen passing Earth - a disc-shaped object called 'Oumuamua - was a defunct piece of alien technology.
NASA's next rover to Mars, Perseverance, is set to search for its own remnants of past life. If its landing goes according to plan on February 18, the robot explorer will scan an ancient lakebed for signs of long-gone Martian microbes. It's set to take samples of interesting rocks and dirt and set them aside for a later mission to bring back to Earth.
As for the rest of the galaxy, current efforts are mostly focused on finding more rocky planets that could have habitable temperatures, then examining their atmospheres for signs of life.
NASA's $10 billion James Webb Space Telescope is scheduled to launch into Earth's orbit on Halloween. The project aims to study every phase of the universe's history, helping reveal how the first stars and galaxies formed. By peering at other stars and planets in infrared light, JWST should be able to see the atmospheres of alien planets.
Other telescopes aimed at examining Earth-sized planets like TOI-561 b have been proposed but not yet built. The Habitable Exoplanet Observatory (HabEx) would directly image Earth-like planets for the first time. Another telescope concept, called Large UV/Optical/IR Surveyor (LUVOIR), would also search for signs of habitability, or of life itself, on distant planets.
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