The Whirlpool Galaxy (left) in X-ray and optical light. On the right is an artist’s concept of the M51-ULS-1 system with the neutron star or black hole syphoning material from its companion star. The planet is eclipsing the X-rays generated by the superheated material around the compact object.

X-ray: NASA/CXC/SAO/R. DiStefano, et al.; Optical: NASA/ESA/STScI/Grendler; Illustration: NASA/CXC/M.Weiss
Astronomers believe they’ve spotted the first extragalactic exoplanet beyond our own galaxy. Residing some 28 million light-years away near the heart of the Whirlpool Galaxy (M51), the binary system M51-ULS-1 consists of either a neutron star or a black hole that’s tangoing with a more standard companion star.

To find the distant planet hiding in this system, astronomers relied on X-ray data rather than more standard visual observations. “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,” said study lead Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics in a press release.

The new research, published in Nature Astronomy, examined three galaxies: M51, M101, and M104. The team targeted more than 200 total star systems within these galaxies using the Chandra X-ray Observatory and the European Space Agency’s XMM-Newton. Within all those systems, they found only one exoplanet.

Hunting exoplanets

Researchers have mainly used two methods to spot the over 4,000 confirmed exoplanets so far. The radial velocity method measures how a star slightly wobbles when an orbiting planet around it gently tugs on its stellar host. Even though stars hold considerably more mass than the planets around them, even a petite world can cause its star to move around a bit, leaving an imprint in the star’s light.

The transit method, on the other hand, takes advantage of a planet crossing in front of its star. This briefly dims the starlight by a detectable amount. Even though planets are much smaller than their stars, researchers can measure these small but recognizable fluctuations in brightness.

Although both the radial velocity and transit methods are clearly effective, they are only useful for finding planets out to about 3,000 light-years from Earth. That’s still well within the boundaries of our Milky Way galaxy, which is about 100,000 light-years across.

So, in order to find this first extragalactic planet, scientists opted to search for passing planets within X-ray binaries. These systems would contain either a white dwarf, neutron star, or black hole pulling in material from a companion star. As this material falls onto the exotic stellar remnant, it becomes superheated, producing X-rays.

Unlike with optical light transits — where a relatively small planet only blocks a tiny amount of starlight — in such binary systems, the area where X-rays are produced is tiny enough that even a planet can block a significant portion (if not all) of the X-ray light. That means that searching for X-ray transits are detectable at a much greater distances than visual transits.

In the case of the M51-ULS-1 system, the black hole or neutron star is closely orbited by a star some 20 times the mass of the Sun. This makes the system one of M51’s brightest X-ray binaries. By examining Chandra data, researchers saw that for 3 hours, the X-rays typically emanating from the system dropped to zero. According to the researchers, this suggests that a Saturn-sized exoplanet is orbiting the compact object at some 19.2 astronomical units (AU; where 1 AU is the average distance between Earth and the Sun). That’s about twice as far as Saturn is from the Sun.

Of course, an exoplanet isn’t the only explanation for why the X-ray signal could have been disrupted. X-ray sources can also be obscured by, say, a cloud of dust passing in front of it. The researchers did consider this explanation, too, but they ultimately concluded it was less likely than an exoplanet.

Unfortunately confirming the extragalactic detection will take a long time. With such a wide orbit, the candidate won’t pass in front of the source again for another 70 years.

Rough past

If M51-ULS-1 is a planet, however, the Saturn-sized object has a rather tumultuous history.

The presence of a neutron star or black hole means that once upon a time, the system was home to not just the current companion star, but also another dying star. This doomed star would have burned through all of its fuel before erupting as a supernova, bathing any planets nearby with intense radiation.

And, because the system’s massive current companion star is still kicking, it’s entirely possible that this extragalactic exoplanet might be forced to withstand another nasty supernova in the future.