Experts have long known that planets would not be confined to our galaxy, but this is the first time that a celestial body has been discovered outside of the Milky Way.
Researchers from the University of Oklahoma used microlensing – an astronomical phenomenon that allows scientists to use gravity from huge objects such as stars to peer hundreds of billions of lightyears into the universe – to detect the planets.
The scientists say they have detected up to 2,000 planets beyond the Milky Way, in a galaxy around 3.8 billion light years away from Earth and ranging in mass sizes from the moon to Jupiter.
University of Oklahoma researchers used NASA’s Chandra X-ray observatory and were even able to see a quasar – a large celestial object – up to six billion lightyears away.
Xinyu Dai, professor in the Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma College of Arts and Sciences, said: "We are very excited about this discovery. This is the first time anyone has discovered planets outside our galaxy.
"These small planets are the best candidate for the signature we observed in this study using the microlensing technique.
“We analysed the high frequency of the signature by modelling the data to determine the mass.”
The team, which published its research in The Astrophysical Journal were not able to observe the planets directly, but confirmed their presence thanks to the way that gravity bends light around them.
University of Oklahoma postdoctoral researcher Eduardo Guerras said: "This is an example of how powerful the techniques of analysis of extragalactic microlensing can be.
“This galaxy is located 3.8 billion light years away, and there is not the slightest chance of observing these planets directly, not even with the best telescope one can imagine in a science fiction scenario.
"However, we are able to study them, unveil their presence and even have an idea of their masses. This is very cool science."
The gravitational microlensing technique was first predicted by Einstein's theory of general relativity.
The technique has already been used to 53 exoplanets within the Milky Way.