Not that long ago,, astronomers weren’t sure that exoplanets even existed. Now we know that there are thousands of them and that most stars probably harbour exoplanets. There could be hundreds of billions of exoplanets in the Milky Way, by some estimates. So there’s no reason to think that stars in other galaxies don’t host planets.

But to find one of those planets in another galaxy? That is a significant scientific achievement.

Astronomers find most exoplanets in our galaxy with the transit method. When a planet passes between us and its star, the star’s light dips a tiny amount as the planet blocks out some of the light. Measuring that slight dip is very difficult, but that’s what planet-hunters like NASA’s Transiting Exoplanet Survey Satellite (TESS) do. That method won’t work in another galaxy. It’s difficult to even discern individual stars in other galaxies, let alone detect the minuscule light blockage when a potential exoplanet transits in front of its star.

But TESS observes in visible light and near-ultraviolet light. What if observing a different part of the spectrum allowed astronomers to see individual stars in another galaxy and even planets orbiting those stars?

A team of astronomers have used data from the ESA’s XMM-Newton spacecraft, which observes x-rays, to discern individual stars in another galaxy. There are fewer bright objects in x-rays than in visible light, so identifying the sources of x-rays isn’t as challenging as in visible light.

The team has published a paper in the journal Nature Astronomy titled “A possible planet candidate in an external galaxy detected through X-ray transit.” The lead author is Rosanne Di Stefano from the Center for Astrophysics, Harvard & Smithsonian. In their paper, the researchers present evidence of a Saturn-sized planet orbiting a star in the Whirlpool Galaxy.

The team studied a particular type of star that shines brightly in x-rays. They’re called x-ray binaries because they exist in pairs, and their unique relationship makes them extremely x-ray emissive. An x-ray binary (XRB) consists of a donor star and an accretor. The donor star is usually a fairly typical star, and the accretor is either a stellar-mass black hole or a neutron star.

An artist's illustration of an x-ray binary. Mass from the yellow star is flowing to the accretion disk around the black hole. Image Credit: By ESA, NASA, and Felix Mirabel - Hubble Site, Public Domain.
An artist’s illustration of an x-ray binary. Mass from the yellow star is flowing to the accretion disk around the black hole. Image Credit: By ESA, NASA, and Felix Mirabel – Hubble Site, Public Domain.

In an x-ray binary, the more massive accretor draws matter away from the donor star. As that matter falls into the donor, an enormous amount of gravitational potential energy is released and heats the material to millions of degrees. The heated material emits x-rays, and these x-rays are detectable by XMM-Newton.

If a large enough object passes between the x-ray binary and us, we could potentially observe a dip in x-rays, the same way TESS observes dips in visible light.

“X-ray binaries may be ideal places to search for planets because, although they are a million times brighter than our Sun, the X-rays come from a very small region. In fact, the source that we studied is smaller than Jupiter, so a transiting planet could completely block the light from the X-ray binary,” explains first author Rosanne Di Stefano in a press release.

Along with data from XMM-Newton, the team used data from NASA’s Chandra X-ray Observatory. Altogether they examined x-ray data from three galaxies for x-ray transits that might indicate the presence of planets. In the Whirlpool Galaxy, they found a transit that completely blocked the x-ray source for a few hours.

This figure from the study shows the region containing the x-ray binary named M51-ULS-1. On the left is a stacked image from Chandra's Advanced CCD Imaging Spectrometer. On the right is a Hubble image of the area in the white square in the Chandra image. The pink circle is the x-ray source<br /><noscript><img class=