The Whirlpool Galaxy, some 28 million light-years from Earth, looks to our telescopes like a cosmic hurricane littered with sparkling gemstones. Huge, lean arms spiral out from the center of Whirlpool, also known as M51. Cradled within them are young stars flaring to life and old stars expanding, expiring and exploding.
In 2012, NASA’s Chandra Observatory, which sees the sky in X-rays, spotted a curious flicker coming from the galaxy. An X-ray source in one of Whirlpool’s arms switched off for about two hours before suddenly flaring back to life. This isn’t particularly unusual for X-ray sources in the cosmos. Some flare, others periodically dim.
This particular source emanated from an “X-ray binary,” known as M51-ULS-1, which is actually two objects: Cosmic dance partners that have been two-stepping around each other for potentially billions of years. One of these objects is either a black hole or a neutron star, and the other may be a large, very bright type of star known as a “blue supergiant.”
As astronomers looked a little more closely at the X-ray signal from the pair, they began to suspect the cause for the dimming may have been something we’ve never seen before: A world outside of the Milky Way, had briefly prevented X-rays from reaching our telescopes. The team dubbed it an “extroplanet.”
A research team led by astronomer Rosanne Di Stefano, of the Harvard-Smithsonian Center for Astrophysics, published details of their hypothesis in the journal Nature Astronomy on Oct. 25. Their study lays out evidence that the X-ray wink detected by Chandra was potentially caused by a planet, about the size of Saturn, passing in front of M51-ULS-1.
The extroplanet candidate goes by the name “M51-1” and is believed to orbit its host binary at about the same distance Uranus orbits our sun.
While many news sources have championed the detection as the “first planet discovered outside of the Milky Way,” there’s no way of confirming the find. At least, not for another few decades, when the proposed planet is supposed to make another transit of the binary. Di Stefano says the team modeled other objects that could potentially produce the dip in X-rays but came up short. Still, she stresses this is not a confirmed detection.
“We cannot claim that this is definitely a planet,” says Di Stefano, “but we do claim that the only model that fits all of the data … is the planet candidate model.”
While other astronomers are excited by the use of X-rays as a way of discovering distant worlds, they aren’t as…