Though there have been thousands of exoplanets—planets that orbit a star outside the solar system—discovered so far, all of these have remained within the Milky Way galaxy.
Nasa's Chandra X-Ray telescope
For the first time ever, astronomers have come into contact with a planet that is orbiting in another galaxy. The planet in question is as big as Saturn and was discovered by Nasa's Chandra X-Ray telescope in the Messier 51 galaxy—located roughly 28 million light years away from the Milky Way.
The planet was discovered by a technique in which the passage of the planet in front of a star dims some of its light giving way to a characteristic dip in brightness detected by telescopes. This same technique has been used many times before to find exoplanets in our own galaxy. Another method to further detect extra-galactic planets has also shown great promise—microlensing. Dr. Di Stefano, from the Harvard-Smithsonian Center for Astrophysics in Cambridge, US, told BBC News:
The method we developed and employed is the only presently implementable method to discover planetary systems in other galaxies. It is a unique method, uniquely well-suited to finding planets around X-ray binaries at any distance from which we can measure a light curve.
More research needed
Although initial sightings have been spotted by the team of scientists, they admit much more research will be needed in order to fully understand this new discovery. Co-author of the research and professor at Princeton University, Julia Berndtsson explained:
We know we are making an exciting and bold claim so we expect that other astronomers will look at it very carefully. We think we have a strong argument, and this process is how science works.