Astronomers using a NASA space telescope spotted the first signs of a planet beyond the Milky Way

A planet seemed to pass in front of that object, completely blocking its X-rays. Astronomers have discovered what may be the first planet ever observed in another galaxy.

Until now, astronomers had only found planets that are beyond our solar system yet still within the Milky Way – these are called exoplanets. But recently, researchers pointed NASA’s Chandra X-ray space telescope towards the distant spiral galaxy Messier 51 (M51 for short). There, they saw signs that a planet is orbiting a massive object: either a black hole or the dense, collapsed core of a giant star, which is known as a neutron star. With X-rays, “there may be only several dozen sources spread out over the entire galaxy, so we can resolve them,” Rosanne Di Stefano, an astronomer at the Harvard-Smithsonian Center for Astrophysics who led the research, told the BBC.

But from millions of light-years away, visible light from hundreds of stars can blend together, making it nearly impossible to pick out planets transiting individual stars. So instead of using visible light, the team behind the new finding looked for dips in the brightness of X-rays coming from the M51 galaxy. Black holes and neutron stars aren’t as common as stars. They are also incredibly dense, with lots of matter packed into a relatively small space, so a transiting planet could easily block all of the X-rays they emit, whereas a planet orbiting a sun-like star blocks only a small fraction of its visible light.

To make the discovery, a global team of astronomers used the same technique that’s often used to find exoplanets in the Milky Way. Researchers doing that work essentially look for exoplanet shadows: They watch stars for tiny dips in brightness caused when planets pass in front of them. These are known as transits. Whereas almost all exoplanets found so far have been within 3,000 light-years of Earth, this potential planet is about 28 million light-years away.

Di Stefano’s study was set to publish Monday in the journal Nature Astronomy, though the paper was delayed (Nature still plans to publish it). It offers early evidence that this X-ray technique could be used to find other planets also orbiting neutron stars or black holes in other galaxies. Story continues

If this planet does exist, it probably survived a violent explosion that created the neutron star or black hole that it’s orbiting. That would have washed the planet in waves of intense radiation. So this is not the kind of planet where scientists would look for alien life. “We are trying to open up a whole new arena for finding other worlds by searching for planet candidates at X-ray wavelengths, a strategy that makes it possible to discover them in other galaxies,” Di Stefano said in a press release. X-rays gave this potential planet awayAn animated illustration shows how the potential planet would pass the neutron star or black hole, blocking its X-ray light. A massive star (right) feeds the object. NASA/CXC/A.Jubett

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