A Planet in Another Galaxy

Humanity spent years wondering if there were planets outside our solar system, and now we know the answer: very much yes. There are thousands of exoplanets in just our little corner of the galaxy, and there’s every reason to expect the same is true of other galaxies. In fact, a researcher from the Harvard-Smithsonian Center for Astrophysics has found strong evidence of a planet orbiting a pair of stars in the distant M51 galaxy. It’s not the first potential extragalactic planet detection, but it’s shaping up to be the most likely candidate.

We lack the technology to image exoplanets directly (usually), even when they’re right next door in Proxima Centauri. Planets are so dim compared with the stars they orbit that we can only infer their presence by the way they affect the star’s gravity (radial velocity) or luminance (transits). Most exoplanets have been detected by the transit method, which involves watching for dips in brightness caused by planets passing in front of their host stars. That’s very similar to what astronomers did to spot the M51 planet candidate, which they’ve dubbed M51-ULS-1b.

Past detections of extragalactic planets have relied on gravitational lensing, but M51-ULS-1b was detected via what appears to be an X-ray transit. However, that’s only possible because it’s orbiting a very strange pair of stars. It’s a perfect storm; M51-ULS-1 is a binary system, and one element of it is a neutron star or black hole that’s devouring a nearby star. That makes M51-ULS-1 a very bright, compact source of X-rays. In 2012, the Chandra X-ray Observatory was scanning M51, also known as the Whirlpool Galaxy, when the X-ray signal from the M51-ULS-1 system dipped. No one was watching for this, so it went unnoticed until just recently when Rosanne Di Stefano at the Harvard-Smithsonian Center took a closer look.

Upon further analysis, the drop in X-ray brightness was symmetrical and lasted about three hours. It was very similar to the brightness changes seen when an exoplanet passes in front of a nearby star. One possible explanation for this is that a Saturn-sized planet in this solar system obscured the signal as it orbited to binary. Di Stefano and her team note that other options like a white dwarf transiting the system don’t match what we know about this part of the Whirlpool Galaxy.

It’s going to be difficult to confirm M51-ULS-1b is an extragalactic planet. After all, the M51 galaxy is 23 million light-years away. Even instruments like the James Webb Space Telescope will struggle to resolve details that far away. However, teams around the world may start developing techniques to detect M51-ULS-1b and similar extragalactic planets. It may just be a matter of time until we have a confirmed planet in another galaxy.

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