Found: The first exoplanet outside of our Milky Way

In a galaxy far, far away lies an exoplanet circling a binary system that contains a neutron star or black hole.

Astronomers believe they have discovered the first extragalactic exoplanet beyond our own galaxy. The binary system M51-ULS-1, located 28 million light-years away near the heart of the Whirlpool Galaxy (M51), consists of either a neutron star or a black hole tangoing with a more typical companion star.

Astronomers used X-ray data rather than more traditional visual observations to locate the distant planet hidden in this system.  “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.

exoplanet outside of Milky Way
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

The new study,

which was published in Nature Astronomy, looked at three galaxies: M51, M101, and M104. Using the Chandra X-ray Observatory and the European Space Agency’s XMM-Newton, the team targeted more than 200 total star systems within these galaxies. They discovered only one exoplanet in all of those systems.

So far, researchers have primarily used two methods to identify the over 4,000 confirmed exoplanets. The radial velocity method determines how much a star wobbles when an orbiting planet gently tugs on its stellar host. Even though stars have far more mass than the planets that orbit them, even a small planet can cause its star to move slightly, leaving an imprint in the star’s light.

In contrast, the transit method takes advantage of a planet passing in front of its star. This temporarily dims the starlight by a noticeable amount. Even though planets are much smaller than stars, researchers can detect these delicate but detectable variations in brightness.

Although both the radial velocity and transit methods are clearly effective, they can only find planets up to about 3,000 light-years from Earth. That is still well within the bounds of our Milky Way galaxy, which spans approximately 100,000 light-years.

To find the first extragalactic planet,

scientists decided to look for passing planets within X-ray binaries. A white dwarf, neutron star, or black hole would pull material from a companion star in these systems. When this material collides with the exotic stellar remnant, it becomes superheated and emits X-rays.

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

The black hole or neutron star

in the M51-ULS-1 system is closely orbited by a star 20 times the mass of the Sun. This makes the system one of the most visible X-ray binaries in M51. Using Chandra data, researchers discovered that the X-rays typically emitted by the system dropped to zero for 3 hours. The researchers believe that a Saturn-sized exoplanet is orbiting the compact object at a distance of 19.2 astronomical units (AU; where 1 AU is the average distance between Earth and the Sun). That is roughly twice the distance between Saturn and the Sun.

Of course, an exoplanet isn’t the only possibility for why the X-ray signal was disrupted. A cloud of dust passing in front of an X-ray source can also obscure it. The researchers did consider this explanation, too, but they ultimately concluded it was less likely than an exoplanet.

Unfortunately, it will take a long time to confirm the extragalactic detection. Because of its large orbit, the candidate will not pass in front of the source for another 70 years.

Rough past

If M51-ULS-1 is a planet, the Saturn-sized object has a turbulent past.

The presence of a neutron star or black hole indicates that the system once housed not only the current companion star, but also another dying star. This doomed star would have used up all of its fuel before exploding as a supernova, bathing any nearby planets in intense radiation.

And, because the system’s massive current companion star is still alive and well, it’s entirely possible that this extragalactic exoplanet will be forced to survive another destructive supernova in the future.

READ MORE: Mysterious World Appears to Be The First Exoplanet Ever Found Orbiting 3 Stars

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