They detect signs of what would be the first planet discovered outside the Milky Way

Computer simulations match that a drop in light intensity from an x-ray binary in the whirlpool galaxy was caused by the transit of a planet.

The finding has been reported in ‘Nature Astronomy’ by a team of astronomers using NASA’s Chandra and ESA’s XMM Newton telescopes.

Some of the brightest objects that can be studied in outer galaxies are the so-called X-ray binaries. They consist of a very compact object, a neutron star or a black hole, which is eating material from a companion or “donor” star. that orbits around it. The falling material is accelerated by the intense gravitational field of the neutron star or black hole and heats up to millions of degrees, producing a large amount of bright X-rays. Astronomers hope that, in theory, this would cause a drop in the observed X-ray light curve.

Rosanne Di Stefano of the Center for Astrophysics, Harvard & Smithsonian in the United States, and her colleagues searched the Chandra and XMM-Newton data of three galaxies for such X-ray transits, drops in light that could be explained by planets. And they found a very special signal in the Whirlpool Galaxy (M51) that they decided to study in more detail. L and completely blocked the signal for a few hours, before he returned.

Then the game of carefully crossing out possible explanations began, before researchers could even consider the option of an extragalactic planet. “We first had to make sure the signal wasn’t caused by anything else.” says in a statement Di DStefano, whose team argues against a number of possibilities in his new publication. “We did this by doing an in-depth analysis of the X-ray drop in the Chandra data, analyzing other drops and signals in the XMM data, and also modeling the including a planet ”.

Could the X-ray fallout be caused by tiny stars like a brown or red dwarf? No, they argue, the system is too young for that and the object in transit too large.

Could it be a cloud of gas and dust? Not likely, the team says, because the drop indicates a transiting object with a well-defined surface, which would not be the same for a passing cloud. Even if the planet had an atmosphere,

Could the fall be explained by variations in the brightness of the source itself? The authors of the article are confident that this is not the case, because although the light from the source disappeared completely for a few hours before returning, the temperature and colors of the light remained the same.

Finally, the team also compared the fall to another light blockage caused by the “donor” star passing in front of the compact star. This was observed in part by XMM-Newton and caused a much longer blackout, which was

“We did computer simulations to see if the fall has the characteristics of a planet in transit, and we found it to be a perfect fit. We are pretty sure that this is nothing else and that we have found ”, Agrega Di Stefano.

The size of Saturn

The team also speculates on the planet’s characteristics based on their observations: it would be the size of Saturn, orbiting the binary star system from dozens of times the Earth-Sun distance. It would make a full orbit roughly every 70 years and s, not making it habitable for life as we know it on Earth.

This long orbit of the candidate planet is also a limitation for the study, because the event cannot be repeated in the short term. This is why the team is careful to say that, for which the wider community might find other explanations, although they have not been found after careful investigation by the team. “We can only say with confidence that it does not fit with any of our other explanations,” the author clarifies.

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