A few decades ago, scientists weren’t sure there were any planets outside of the nine (or eight) orbiting the sun. Fast forward to today, and they have now catalogued almost 5,000 planets residing in other star systems.
They may have one more to add to the list, but it’s a big one: In a new study that was published in Nature Astronomy on Monday, researchers said they may have found the first planet detected in another galaxy.
“We know we are making an exciting and bold claim so we expect that other astronomers will look at it very carefully,” study co-author Julia Berndtsson of Princeton University said in a statement. “We think we have a strong argument, and this process is how science works.”
The planet looks to be the size of Saturn, and is located in the spiral galaxy Messier 51 (also called the Whirlpool Galaxy), 28 million light-years away. Called M51-ULS-1 for now, it seems to be orbiting a star 20 times more massive than the sun, and is in close proximity to a black hole or neutron star found in the same system.
It’s impossible to actually see a planet that far away. For this discovery, the researchers turned to NASA’s Chandra X-ray Observatory, a space telescope that can find dips and peaks in X-rays emitted by distant objects. X-rays glow off of material that’s been superheated by objects like black holes or neutron stars. But when a planet passes in front of that glow like M51-ULS-1 did, it basically blocks off all the X-rays from being detected by Chandra.
More observations are needed to confirm the planet truly exists, but that presents a huge problem itself, because current calculations predict the planet has a 70-year orbit around its star. That means scientists have to potentially wait decades before they can see M51-ULS-1 blocking that X-ray glow again, thus proving it’s the real deal.
On the plus side, these new findings mean there’s a potentially new method to search for other planets outside the Milky Way galaxy. Berndtsson and her team already have their sights set on scouring Chandra data for 20 other galaxies to hunt for new planets. They’re not wasting any time.