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“We did computer simulations to see whether the dip has the characteristics of a planet transiting, and we find that it fits perfectly. We are pretty confident that this is not anything else and that we have found our first planet candidate outside of the Milky Way,” adds Rosanne.
The team also speculates about the characteristics of the planet based on their observations: it would be the size of Saturn, orbiting the binary star system from tens of times the Earth-Sun distance. It would make one full orbit roughly every 70 years, and be bombarded with extreme amounts of radiation, making it inhabitable by life as we know it on Earth.
This long orbit of the planet candidate is also a limitation for the study, because the event can’t be repeated any time soon. That’s why the team remains careful to say that they found a possible planet candidate, for which the broader community might find other explanations, although they have not been found after careful research by the team. “We can only say with confidence that it doesn’t fit any of our other explanations,” Rosanne clarifies.
Still, this is an exciting step forward in the quest to find a planet outside of the Milky Way. This is the first planet candidate that would orbit a known host system, as compared to candidates found with gravitational lenses. This would also be the first time that a planet is found orbiting an X-ray binary. The existence of those planets is consistent with the fact that planets are found around pulsars (rapidly rotating neutron stars), and some of these pulsars have been part of an X-ray binary in the past.
“The first confirmed planet outside of our Solar System was found around a pulsar, an object typically observed in X-rays. I am excited that X-rays now also play important step in the search for planets beyond the border of our galaxy,” says Norbert Schartel, XMM-Newton Project Scientist for ESA.
“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 of those. In the future we might even be able to confirm their existence,” says Rosanne.