Over the last three decades, humanity has discovered thousands of planets in our galaxy beyond the Solar System, with new ones being announced every week. A recurring question in this field is the existence of planets in other galaxies. While some indirect evidence suggests extragalactic exoplanets exist, a new study may have found the first planet candidate to orbit a known host system in another galaxy.
Researchers reports a dip in the luminosity of a bright X-ray source (XRB) known as M51-ULS-1. The source is likely a neutron star or a black hole orbited by a massive star located in the Whirlpool Galaxy (M51). According to their paper in Nature Astronomy, both of these objects are orbited by the potential planet. As it moves around the two objects, it ends up eclipsing the X-ray source, producing a dip in luminosity.
"We found that one of the bright X-ray sources experienced a transit which appears to most likely due to the passage of a planet in front of it," lead author Dr Rosanne Di Stefano, from the Harvard-Smithsonian Center for Astrophysics, told IFLScience.
The techniques used to detect exoplanets in the Milky Way are not powerful enough to spot a planet 23 million light-years away. This is why the team had to use X-ray sources to look for them. These are bright enough to spot planets around them. That said, XRBs are also very variable, so dips in luminosity don't automatically mean a planet is there. But given that we have found planets in so many different environments, the team believed that some of the many dips detected by telescopes might be planets.
"It is a large job so we also have been screening for other interesting effects as well. It is absolutely true though that one of our main goals was to search for evidence of planets," Dr Di Stefano explained to IFLScience.
The object was seen producing this eclipse just once. Confirmation is only given after multiple detections. Based on the signal, the team used models to try and understand more about the source. While the team cannot exclude the possibility the eclipsing body is a substellar object such as a brown dwarf, the best candidate so far is a planet with a radius comparable to Saturn.
A second detection method is needed to confirm exoplanets in the Milky Way, something that is not easy at those distances. So this possible planet won't be confirmed for the time being.
"The detection methods used for local planets won't work at these distances and with the high level of crowding in these distant galaxies. The very best approach is to extend the searches to find additional systems. Some of them are likely to have shorter orbital periods and repeat," Dr Di Stefano told IFLScience.
Many such candidate detections are possible using data already collected, especially within our own galaxy where the potential existence of planets around XRBs can be confirmed with other methods.
