Astronomers Find Evidence of a Planet Outside Our Galaxy for the First Time

Astronomers have found signs of a planet orbiting a star outside our galaxy, the Milky Way, for the first time.

The team found evidence of a possible planet in the spiral galaxy Messier 51 (M51), which is around 28 million light-years from Earth. The galaxy is also known as the Whirlpool Galaxy because of its distinctive appearance.

They calculated that the exoplanet could be approximately the size of Saturn, the solar system's second-largest planet after Jupiter, and believe it is orbiting a star and a partner that is either a neutron star or a black hole.

Until now, all the planets found outside the solar system, or exoplanets, have still been located within the confines of the Milky Way. Most of these planets have also been located within 3,000 light-years of the Earth.

"Of the thousands of exoplanets that have been discovered up to this point, all are located in our galaxy, the Milky Way, and most of these are within a few thousand light-years of our solar system," Nia Imara, one of the team members and a researcher at the University of California, told Newsweek.

"Now, our work has uncovered good evidence of a planet in a galaxy some 25 million light-years away.

If this is indeed a planet in M51, it is thousands of times more distant than even the furthest previously detected exoplanet.

Extra Galactic Planet
The location of the first detected extra galactic planet in the galaxy M51. Astronomers made the discovery by examining X-ray emissions from a binary system containing a neutron star or black hole. CXC/SAO/R. DiStefano, et al.; Optical: NASA/ESA/STScI/Grendler/NASA

If confirmed, the find will join the catalog of the 4,000-plus exoplanets detected so far, but will also become the first object in a new list of extragalactic planets.

The astronomers who made the discovery used NASA's Chandra X-ray Observatory, searching nearby galaxies in the X-ray region of the electromagnetic spectrum. It is the reliance on X-rays that makes the detection of more distant worlds possible.

While even stars in neighboring galaxies can be difficult to distinguish in visible light and other regions of the electromagnetic spectrum, because stars shine less brightly in the X-rays, using these emissions can make distinguishing individual objects much easier.

"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 Rosanne Di Stefano, a researcher from the Center for Astrophysics at Harvard & Smithsonian, in a NASA press release.

Di Stefano was the lead researcher on the study, published in the journal Nature Astronomy.

In order to spot this planet, Di Stefano and the team adopted a technique that has been used with great success to detect exoplanets.

As exoplanets pass the face of their parent star, they block the light from that star and cause tiny, but detectable, dips in the star's light output. This happens for light across the entire electromagnetic spectrum, including X-rays.

This method has been used by astronomers with both ground-based and space-based telescopes—such as those carried by NASA's Kepler and Transiting Exoplanet Survey Satellite missions—to discover thousands of exoplanets.

To use the technique,Di Stefano's team had to use stars that are associated with strong output in the X-ray region of the electromagnetic spectrum.

Places to Search for Planets

For this, the team used X-ray binaries, systems of two orbiting stars one of which is a compact stellar remnant, a neutron star or a black hole, that is feeding on material from its companion. When this material falls to the compact object surface, the intense gravitational force heats it tremendously and causes it to release high-energy X-ray emissions.

"X-ray binaries may be ideal places to search for planets, because, although they are a million times brighter than our sun, the X-rays come from a very small region," Di Stefano said. "In fact, the source that we studied is smaller than Jupiter, so a transiting planet could completely block the light from the X-ray binary."

Searching for dips in these X-ray emissions that could be caused by an orbiting star, Di Stefano and the team spotted such a signature around the X-ray binary – M51-ULS-1 – in the galaxy M51.

Studying the signal in detail they discovered the emission of X-rays dropped to zero as something transited the binary. This lasted for three hours, then the X-ray transmission resumed.

"This really highlights the power of the technique we developed, which takes advantage of the extremely bright x-ray emission that can be generated by compact binaries," said Imara, who co-authored the study.

A planet in another galaxy
An ESA diagram explains how the discovery of a Saturn-sized planet outside the Milky Way was made. ESA

After eliminating other possibilities for the dip in X-ray emissions, such as dust or another object, the team concluded that they were caused by a transiting dark planet about the size of gas giant Saturn. The extragalactic planet is orbiting the X-ray binary data at a distance of twice the orbit of Saturn around the sun.

The researchers believe that given the orbit of this possible planet, it won't transit its parent star and compact stellar companion again, blocking X-rays, for another 70 years.

"Unfortunately, to confirm that we're seeing a planet we would likely have to wait decades to see another transit," said Imara. "And because of the uncertainties about how long it takes to orbit, we wouldn't know exactly when to look.''

If this is a planet in this system, the authors point out that it must have survived the tremendous supernova explosion that created the compact stellar object in the X-ray binary.

"There are so many fascinating questions and implications raised by this work. For instance, we'd like to understand the evolution of planets in high-energy environments," Imara added. "As for the exoplanet candidate in the M51-ULS-1 system, we can infer from the data that its orbit is wide enough to have ensured its survival thus far."

Update 10/27/21, 4:15 a.m. ET: This article was updated to add comments from Nia Imara.

Whirlpool Galaxy (M51)
An image of Whirlpool Galaxy (M51) in X-rays. The galaxy hosts the first extra-galactic planet detected by astronomers. XMM-Newton/ESA