Astronomers find signs of first intergalactic exoplanet

Astronomers think they’ve found a Saturn-sized planet in the Whirlpool Galaxy, around 30 million light-years from Earth. If confirmed, this would be the first intergalactic exoplanet, beyond our own Milky Way, and would extend the search for new worlds to greater distances than ever before.

“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,” says Rosanne Di Stefano, an astronomer from the Harvard Center for Astrophysics and lead author of the study describing the new planet, published today in Nature Astronomy.

Di Stefano and the team found the planet, dubbed M51-ULS-1b, using the transit method, which is currently the most successful exoplanet-hunting technique – astronomers have used it to discover more than 2600 of these planets orbiting stars outside the solar system. Essentially, it searches for shadows: when a planet crosses in front of its star, it causes a dip in the star’s brightness by a tiny but measurable amount.

In this case, the team used NASA’s orbiting Chandra X-ray Observatory to spot a slight dip in the brightness of an X-ray binary system, which typically has at its core a neutron star or a black hole sucking in gas from a nearby companion star. This causes the material around the neutron star (or black hole) to heat up, emitting powerful X-rays.

Because the X-ray-producing region is small, a planet orbiting in front of it could block out nearly all the radiation, making the planet easier to spot at greater distances.

The dip in brightness from the M51-ULS-1 system could also have been caused by a cloud of gas or dust, but the researchers think a planet best fits the data.

“Unfortunately, to confirm that we’re seeing a planet we would likely have to wait decades to see another transit,” says co-author Nia Imara from the University of California at Santa Cruz. “And because of the uncertainties about how long it takes to orbit, we wouldn’t know exactly when to look.”

So stay tuned for 70 years or so.

A grainy image of a black dot of a planet passing on front of a star
A direct image of the planet 2M0437. Credit: Subaru Telescope

In bonus exoplanet news, astronomers have directly imaged a baby exoplanet more than 400 light-years away. This is an astonishing feat, because it’s difficult to separate the light of an exoplanet from the overwhelming brightness of its parent star – it’s akin to trying to spot a moth flying around a lighthouse from tens of kilometres away.

Named 2M0437b, the exoplanet formed just a few million years ago, making it one of youngest that astronomers have ever directly imaged.

“This serendipitous discovery adds to an elite list of planets that we can directly observe with our telescopes,” says Eric Gaidos, an astronomer at the University of Hawai’i at Mānoa and lead author of the study, available as a preprint on

“By analysing the light from this planet, we can say something about its composition, and perhaps where and how it formed in a long-vanished disk of gas and dust around its host star.”

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