In a faraway galaxy, you take a breath and look up to the sky. Instead of the Sun, you see two celestial bodies rising. One is pretty similar to our Sun, but is distorted as a stream of stellar material falls from it and is drawn into a ring around the other. This is what a “sunrise” might look like from M51-ULS-1b, possibly the first planet found in a galaxy beyond our own!

5640 exoplanets have been discovered already in our own Milky Way Galaxy. Based on our current understanding of physics and science, there’s no reason to doubt the existence of planets in other galaxies across the universe, but detecting them is really difficult. Milky Way exoplanets are often detected through transit light curves – the observed slight dimming of a star as one of its planets passes in front of it. Because distant galaxies are so far away, this transit method isn’t really applicable to (potential) planets there. The light of an individual star is indistinguishable from the light of stars around it.

Luckily for Di Stefano et al. (the paper's access is currently restricted), the transit method appears to be successful for detecting planets in other galaxies but when applied only to X-rays from X-ray binaries (XRBs). XRBs are made up of a “dead” remnant of a massive star – either a black hole or neutron star – which is gravitationally tied to a still “living” star. As the two objects orbit one another, the much denser remnant pulls and heats material from the star such that the system blasts out X-rays. XRBs are much less common than stars, so it’s easier to detangle these signals.

With this technique in mind, the authors turned to archived Chandra X-ray light curves to search for transits. From over 2500 light curves examined, one showed clear evidence of a transiting planet. Observed on 20/09/12 in a spiral arm of the Whirlpool galaxy, XRB M51-ULS-1 exhibits a telltale dip in X-ray emission for around 3 hours before returning to its regular emission levels. From this, the authors estimate that the planet’s orbit is at around the same radius of Saturn’s orbit around the Sun.

But how do we actually know that this is a planet and not some other object or phenomenon? After all, XRB luminosities can naturally fluctuate, accompanying changes in X-ray colours. However, the X-ray colours are constant throughout this transit, ruling out this intrinsic variation. The authors of the paper also explored the possibility that the eclipser was another object (i.e., a star). They found that, using the object’s radius, it could either be a brown dwarf star or a planet. Statistically, such a small object is far more likely to be a planet than a brown dwarf. Additionally, a planet is more likely to exist in orbit around an XRB as planets form simultaneously with stellar systems and could’ve plausibly survived the death of the remnant star due to its wide orbit.

If this is indeed a planet, then we should be very very excited! We could use the same techniques to discover more planets in distant galaxies and expand our understanding of planetary environments.

published: 07/03/24 by kaan evcimen