The first exoplanet outside of our Milky Way

In a galaxy very, very far away, there is an exoplanet that orbits a binary system with a neutron star or black hole.

On the left is the Whirlpool Galaxy in both X-ray and regular light. On the right is an artist’s idea of the M51-ULS-1 system, with the neutron star or black hole sucking material from its companion star. The planet is blocking the X-rays that are coming from the superheated matter around the small object.

NASA/CXC/SAO/R. DiStefano, et al. did the X-rays. NASA/ESA/STScI/Grendler did the optical work. NASA/CXC/M.Weiss did the illustration.

Astronomers think they have found the first exoplanet that is not in our galaxy. M51-ULS-1 is a binary system that is 28 million light years away and near the center of the Whirlpool Galaxy (M51). It is made up of either a neutron star or a black hole that is dancing with a normal star.

Astronomers used X-ray data instead of the more common visual observations to find the faraway planet hiding in this system. In a press release, Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics said, “We are trying to open up a whole new area for finding other worlds by looking for planet candidates at X-ray wavelengths. This makes it possible to find them in other galaxies.”

The new study, which was published in Nature Astronomy, looked at M51, M101, and M104. The team used the Chandra X-ray Observatory and the XMM-Newton from the European Space Agency to look at more than 200 star systems in these galaxies. Only one exoplanet was found in all of those systems.


Looking for planets outside of our solar system

So far, most of the over 4,000 confirmed exoplanets have been found with two main methods. The radial velocity method looks at how much a star wobbles when a planet in orbit around it gently pulls on it. Even though stars have a lot more mass than the planets around them, even a small world can cause its star to move around a bit, leaving a mark in the star’s light.



On the other hand, the transit method uses the fact that a planet passes in front of its star. This briefly makes the stars look a little less bright. Researchers can measure these small but noticeable changes in brightness even though planets are much smaller than their stars.

Even though both the radial velocity method and the transit method work, they can only be used to find planets up to about 3,000 light-years away from Earth. That is still well within the Milky Way galaxy’s borders, which are about 100,000 light-years wide.

So, scientists decided to look for passing planets in X-ray binaries as a way to find this first planet outside of our galaxy. In these systems, there would be a white dwarf, neutron star, or black hole that pulls matter from a nearby star. When this material hits the exotic stellar remnant, it gets so hot that it gives off X-rays.

In optical light transits, even a small planet can only block a small amount of starlight. But in these binary systems, where X-rays are made, the area where they are made is so small that even a planet can block most or all of the X-ray light. That means you can look for X-ray transits much farther away than you can look for visual transits.

In the M51-ULS-1 system, a star with about 20 times the mass of the Sun goes around a black hole or neutron star. This makes it one of the brightest X-ray binary systems in M51. Researchers looked at data from Chandra and found that for 3 hours, the system usually gave off no X-rays. Researchers think this means that an exoplanet about the size of Saturn is orbiting the small object at a distance of 19.2 astronomical units (AU; where 1 AU is the average distance between Earth and the Sun). That’s about twice as far away from the Sun as Saturn is.

Of course, an exoplanet isn’t the only thing that could have messed up the X-ray signal. Sources of X-rays can also be hidden by things like a cloud of dust moving in front of them. Even though the researchers thought about this possibility, they decided in the end that it was less likely than an exoplanet.

Confirming the discovery of something from outside our galaxy will take a long time. With such a large orbit, the candidate won’t pass in front of the source again for another 70 years.

Hard times

But if M51-ULS-1 is a planet, the Saturn-sized object has been through a lot in its past.

If there is a neutron star or black hole in the system, that means that not only the current companion star but also another dying star once lived there. This dying star would have used up all of its fuel before exploding as a supernova and bathing any nearby planets in strong radiation.

And since the system’s massive companion star is still going strong, it’s possible that this extrasolar exoplanet will have to deal with another supernova in the future.

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