The potential planet is thought to be around the size of Saturn
Astronomers may have spotted the first planet in another galaxy. Observations of a stellar system in the Whirlpool Galaxy, M51, have shown what looks to be a planet transiting a star.
As even the nearest galaxies are millions of lightyears from the Milky Way, it’s often impossible to distinguish individual stars at visible wavelengths, meaning most techniques currently used to find exoplanets are useless. But at X-ray wavelengths, stars are much dimmer and therefore easier to distinguish. Taking advantage of that, astronomers used ESA’s XMM-Newton and NASA’s Chandra X-ray Observatory space telescopes to look at a special type of stellar system known as an X-ray binary, in which a star is being consumed by a nearby neutron star or black hole, creating a hot disc of dust that glows brightly in X-rays.
“X-ray binaries may be ideal places to search for planets, because although they are a million times brighter than our Sun, the X-rays come from a very small region,” says Rosanne Di Stefano from the Harvard–Smithsonian Center for Astrophysics, who led the study. “The source that we studied is smaller than Jupiter, so a transiting planet could completely block the X-ray binary’s light.”
The X-ray binary blinked out of sight for a few hours before coming back again, looking unchanged from before the transit. The team considered other possible explanations – such as a passing cloud of gas, another star, or that it might be a variable X-ray binary – but a transiting planet was the best fit for the pattern of fading light seen.
Usually the next step would be to look for at least two more transits, as this is considered necessary to confirm it’s a planet. But the dimming pattern suggests the world is orbiting its star at a minimum of 10 times the Earth–Sun distance, so astronomers will have to wait another 70 years for it to transit again. But they are optimistic, nevertheless.
“Now that we have this new method for finding possible planet candidates in other galaxies, our hope is that by looking at all the available X-ray data in the archives, we find many more. In the future, we might even be able to confirm their existence,” says Di Stefano. www.esa.int