It's incredible enough that 4,863 extrasolar planets have been discovered since the first two were detected in January 1992. Now, astronomers have found evidence for a planet candidate in one of the most beautiful galaxies in the sky called the Whirlpool, located 28 million light-years from Earth.

This composite photo of the Whirlpool is a combination of an X-ray image from Chandra (purple and blue) and optical light from the Hubble Space Telescope (red, green and blue). The box marks the location of the possible planet candidate, dubbed M51-ULS-1b, orbiting the X-ray binary star M51-ULS-1. 
Contributed / X-ray: NASA, CXC, SAO, R. DiStefano, et al.; Optical: NASA, ESA, STScI-Grendler
This composite photo of the Whirlpool is a combination of an X-ray image from Chandra (purple and blue) and optical light from the Hubble Space Telescope (red, green and blue). The box marks the location of the possible planet candidate, dubbed M51-ULS-1b, orbiting the X-ray binary star M51-ULS-1. Contributed / X-ray: NASA, CXC, SAO, R. DiStefano, et al.; Optical: NASA, ESA, STScI-Grendler

From our face-on perspective the galaxy reveals a striking spiral structure even in modest-sized amateur telescopes. It's also known as Messier 51 (M51) because it's the 51st entry in a famous compendium of clusters, nebulae and galaxies called the Messier catalog.

Exoplanets are planets outside of our solar system that orbit other stars. Until now, astronomers have only detected them inside our own galaxy within about 3,000 light-years of the sun. If confirmed, the new object, called M51-ULS-1b, would be the most remote ever and the first to be found outside the Milky Way.

One of the most common ways to track down these exceedingly faint objects is the transit method. When a planet passes, or transits, in front of its host sun, it produces a characteristic dip in its light. Detecting transits is exacting work because a planet is typically much smaller than the star it passes in front of, so only a tiny fraction of the light is blocked. Most exoplanets, like those found with NASA's Kepler and TESS missions, look for dips in optical light, the same waves our eyes use to see the world.

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This illustration shows possible orbit(s) of the M51 exoplanet candidate from the perspective of hovering directly over the system and looking down. From Earth, we see the planet and binary edge-on — along the same line of sight. 
Contributed / NASA, CXC, M. Weiss
This illustration shows possible orbit(s) of the M51 exoplanet candidate from the perspective of hovering directly over the system and looking down. From Earth, we see the planet and binary edge-on — along the same line of sight. Contributed / NASA, CXC, M. Weiss

But a team of astronomers led by Rosanne Di Stefano of the Center for Astrophysics in Cambridge, Mass. took a different take. They used NASA's Chandra X-ray Observatory to detect the dimming of X-rays from an "X-ray binary," a system where a sun-like star orbits around a super-dense, city-sized neutron star or black hole.

X-rays are as much light as the colors of the rainbow are, only they're invisible to us without the right instrument to detect them. That's Chandra's job. The group interpreted the fading and rebrightening of the X-rays as being a planet passing in front of either the star or black hole.

Unlike transits visible in normal light, where we learned that the planet is puny in relation to its sun, a planet passing in front of an X-ray binary is similar in size to the X-ray source, be it a neutron star, typically around 12 miles (20 kilometers) across, or a black hole with a 12- to 40-mile-wide (19- to 64-kilometer) event horizon. That means a potentially transiting planet can block most or all of the X-rays, making it feasible to detect dips in light across much greater distances than what's possible optically. In other words, the drop is deep enough to not get lost in the noise. Pretty cool.

Of course, there are caveats. Putative planet M51-ULS-1b won't pass in front of its partner again for about 70 years, so it will be a long wait for a confirming observation. It's also possible that a passing gas cloud caused the dimming, although the data strongly favors a planet. If M51-ULS-1b is for real, it survived a tumultuous life that included a supernova explosion that created the neutron star or black hole. And since the companion star may also erupt as a supernova, its future looks bleak . . . for habitability anyway.

For now, I'm going with planet, and hope that the discovery inspires other astronomers to find even more distant objects using the same method. You can read more in the published (free) paper here.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.