Astronomers have found tempting signs of a planet in a star system outside the Milky Way that, if confirmed, would be the first found in any other galaxy.
Discovery, reported in a study published on Monday. in the journal Nature Astronomy, it shows a new technique for finding distant worlds and could significantly expand the search for so-called extragalactic exoplanets.
“It’s always fun to find something that’s the first of its kind,” said lead researcher on the study, Rosanne Di Stefano, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics. “When we started finding planets locally, it made sense that there were planets in other galaxies, but that’s humbling and really exciting.”
The possible planet was discovered in a spiral galaxy called Messier 51, also known as the Whirlpool Galaxy, which is more than 23 million light years from Earth.
The first exoplanets, or planets outside the solar system, were discovered in the 1990s and required a combination of a number of complicated detection techniques. Since then, however, NASA missions like the Kepler Space Telescope and the Transiting Exoplanet Survey Satellite have discovered an abundance of worlds across the galaxy.
More than 4,000 Exoplanets discovered and confirmed, but so far they were all in the Milky Way. Most were also less than 3,000 light-years from Earth. If confirmed, the planet in the Whirlpool galaxy would be a thousand times farther away than any other identified alien planet.
The possible extraterrestrial world was found in a binary X-ray system, a type of star system that creates and emits X-rays and usually consists of a normal star and a collapsing star, such as a neutron star or a black hole.
Astronomers often use what’s known as the “transit method” to look for planets. Transits occur when a planet orbits in front of its parent star, temporarily blocking part of it and causing an observable break in the star’s light. Di Stefano and his colleagues applied the same basic idea, but instead of using optical light, they monitored the changes in X-ray brightness of the binary star system in the Whirlpool galaxy.
Di Stefano said the region that produces bright X-rays is relatively small, allowing him to see transits that block most or all of the X-ray emissions.
“It’s a very obvious sign,” she said.
Using data from NASA’s Chandra X-ray Observatory, Di Stefano and his colleagues observed that the transit took about three hours, and they were able to roughly estimate the size of the object that completely blocked the X-ray source. They estimate that the potential planet is the size of Saturn and that it is much farther from its star than Earth is from the sun.
Bruce Macintosh, a professor of physics at Stanford University who was not involved in the research, said the finding was exciting because, if confirmed, it would show not only that planets are distributed throughout the cosmos, but that they are also in unlikely places.
“The good thing is that they found a planet orbiting a neutron star that is part of a system that went through a supernova explosion and had a complicated and interesting evolutionary history,” he said. “It’s exciting that a planet can survive when its star explodes.”
Confirmation that there really is a planet in the X-ray binary will likely take some time. The planet’s distant orbit means it will likely be about 70 years before astronomers can see another transit.
“And because of the uncertainties about how long an orbit lasts, we don’t know exactly when to look,” says co-author of the study, Nia Imara, an assistant professor at the University of California at Santa Cruz. said in a statement.
Macintosh said the method of examining X-ray transits is “smart” but unlikely to be used to find hundreds of thousands of planetarium candidates, as it also depends on luck.
“You can only see transits if there are objects right between you and the object you are looking at,” he said. “And you only see him walk past the target object for a few minutes or hours.”
However, according to Di Stefano, it is gratifying that the new method of searching for extragalactic exoplanets, which she and her colleagues first theorized in 2018, has led to such a tempting result.
“We didn’t know if we were going to find something and we were very lucky to have found something,” she said. “Now we expect other groups around the world to study more data and make even more discoveries.”
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