Pack your bags – we may have found the first planet outside of our galaxy

Alas, we will have to wait 70 years to confirm the sighting

Astronomers have for the first time discovered what looks like a planet outside the Milky Way, judging by a study published this week in Nature.

Over 4,000 exoplanets have been spotted orbiting stars in our galaxy since the early 1990s when scientists confirmed the Solar System isn’t a unique formation.

Our Sun is just one star among the 100 billion or so stars in the Milky Way. Our galaxy, in turn, is just one among the hundreds of billions of galaxies in the universe. It’s only natural to assume therefore that there must be exoplanets circling stars in other galaxies, too, though astronomers have never managed to find one so far away until now.

Researchers led by the Harvard-Smithsonian Center for Astrophysics have stumbled across tantalizing evidence of what could be the first-ever extragalactic planet.

They found, according to the aforementioned paper, a strange pattern in the radiation emitted from an X-ray binary system in the Messier 51 galaxy located 28 million light-years away. These types of systems are made up of a neutron star or black hole gobbling gas from a neighboring star. The team detected a temporary decrease in the strength of X-ray emission, a pattern similar to what astronomers see when an exoplanet orbits stars closer to Earth.

A periodic dip in the brightness of a star, also known as a transit signal, suggests its light is being blocked by an exoplanet passing in front of it during orbit. The level of dimming allows astronomers to estimate the object’s size and mass, and the frequency of these dips shows how close it lies to its host star. Now a similar effect has been observed with the X-Ray binary system.

“This was the unique event that displayed a clear transit signal,” Rosanne Di Stefano, lead author of the paper and a senior astrophysicist at the Smithsonian Astrophysical Observatory, told The Register.

“The modeling of the signal showed that the size of the object is in the planetary range most likely comparable to the size of Saturn.”

The mysterious object has been dubbed M51-ULS-1.

The X-ray emissions were measured by NASA’s Chandra X-ray Observatory, a space telescope launched in 1999. Although the researchers have ruled out other possibilities behind the dip in brightness, such as a passing cloud of gas and dust, for example, it’s difficult to confirm whether the decrease is from a planet or not. Scientists don’t expect to see another transit event for another 70 years or so due to M51-ULS-1 being quite far away from its star; it orbits at about twice the distance of Saturn from the Sun.

“It is a very strong candidate. But it is in a wide orbit, so we will not see it repeat in the near future, something we generally think of as helping with confirmation. The best idea is to keep searching and to find others,” Di Stefano said.

The team hunted for X-ray transits in hundreds of systems in three galaxies: Messier 51, aka the Whirpool galaxy; Messier 101, aka the Pinwheel galaxy; and Messier 104, aka the Sombrero galaxy to find one promising signal.

They’ll scour through archives of previous measurements taken by the Chandra observatory and the XMM-Newton space telescope next to find similar signals for comparison. It may prove useful to search for X-ray transits closer to home, around stars within the Milky Way.

“We know we are making an exciting and bold claim so we expect that other astronomers will look at it very carefully,” said paper co-author Julia Berndtsson, a physics student at Princeton University. “We think we have a strong argument, and this process is how science works.” ®

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