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Astronomers imagine they’ve discovered an exoplanet in a galaxy 23 million light-years away

Using a variety of techniques, astronomers have successfully identified thousands of exoplanets, which are planets orbiting stars outside of our own solar system. However, a new research paper introduces a breakthrough: the first discovery of an exoplanet not just in another solar system, but in an entirely different galaxy, millions of light years away.

Finding exoplanets is a relatively simple, yet tedious task. You stare at a star for a long time. When planets orbit this star and their orbits are aligned just right so that they cross the star's face out of our line of sight, you will see a noticeable drop in the brightness of that star. The meticulous part is you have to stare at thousands of stars for extended periods of time to catch one of those happy breaks.

Using this technique, instruments like the Kepler Space Telescope and TESS have captured thousands of candidate planets of all kinds of shapes and sizes orbiting all kinds of stars, revealing a galaxy full of diversity.

So far, however, we have only examined tiny disks in our own Milky Way. Astronomers have good reason to believe that other galaxies are also home to exoplanets. If they can form here, why can't they form anywhere?

However, this simple question has not yet been answered.

The challenge is the incredible distances to other galaxies. The technique described above, known as the transit method, works within a few hundred light years, up to a few thousand light years, depending on how bright the target star is. Galaxies are millions of light years away, making this technique nearly impossible to use. At these distances it is difficult to see even a single star.

Artist's impression of an Accreting X-Ray Pulsar drawing material from its companion star. – NASA

But if a galaxy is home to a super-super-bright star and we are lucky enough to see an exoplanet cross over the face of that star. And that's exactly what we did. According to a new article in the pre-print journal arXiv, a team of astronomers examined an extremely bright X-ray source in M51, the Whirlpool galaxy, located 28 million light-years from Earth.

The X-ray source is so intense that we can observe it and examine changes in its brightness. Astronomers have noted the significant darkening over the course of many observations due to a crossing exoplanet, which they suspect is roughly the size of Saturn.

It is a strange world indeed. This planet orbits a binary pair, with one of the pairs being the remnants of a giant star – either a neutron star or a black hole – while the other is a giant companion that feeds material on it and creates the intense X-ray light.

It's not exactly a pretty place to live, but it shows us that there are many planets throughout the universe.

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