Scientists from the Smithsonian Astrophysical Observatory and Harvard College Observatory may have discovered a new planet outside our galaxy. This discovery marks the first time that astronomers have detected a planet outside the Milky Way. NASA’s Chandra X-ray Observatory uncovered the mysterious celestial body, encouraging scientists to explore space at greater distances.

The possible exoplanet transits a star outside our galaxy, located in Messier 51(M51) spiral galaxy. It’s also known as the Whirlpool Galaxy due to its appearance. Exoplanets, or planets outside our solar system, orbit around a star other than the Sun. Until this latest discovery, astronomers have found all other exoplanets within the Milky Way galaxy. Nearly all of them have been less than about 3,000 light-years away from Earth.

Located about 28 million light-years away, the potential exoplanet in M51 would be thousands of times farther away than those in the Milky Way.

“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 of the Center for Astrophysics, a Harvard & Smithsonian (CfA) in Cambridge, Massachusetts, who led the study.

The journal Nature Astronomy published these findings on October 25, 2021.

Astronomers based their latest discovery on transits, where a planet passes in front of a star and blocks a portion of its light. This event results in a distinctive dip in the host star’s brightness, uncovering the exoplanet’s location. Previously, astronomers have used both ground and space-based telescopes to locate thousands of exoplanets. Telescopes such as those on NASA’s Kepler and TESS missions have found exoplanets by searching for transits, electromagnetic radiation visible to the human eye.

Astronomers May Have Just Found a New Planet Outside the Milky Way

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Di Stefano and colleagues have employed a different method to search for these transits or dips in optical light for their research. They analyzed the brightness of X-rays received from X-ray bright binaries to find transits. These luminous systems usually contain a black hole or neutron star, pulling gas in from an orbiting star. Then, the gasses heat materials near the neutron star or black hole exponentially, creating a visible glow.

The bright X-rays take up a small surface area, so a planet passing in front of it could easily block most of the light. This would make the transit easier to locate since the X-rays might disappear. Astronomers could then detect new planets much farther in space than existing optical light transit studies. These studies rely on tiny decreases in light to locate exoplanets since they block only part of the star.

The team employed this method to detect the new planet in a binary system called M51-ULS-1. The binary system contains a black hole or neutron star orbiting a companion star, weighing about 20 times more than the Sun. The X-ray transit they discovered with the Chandra data lasted nearly three hours, during which time the light completely disappeared.

Based on this and previous research, the team believes the exoplanet candidate in M51-ULS-1 resembles the size of Saturn. They also estimate it orbits the neutron star or black hole at about twice the distance of Saturn from the Sun.

The study uncovered fascinating findings of this potential new planet. However, researchers say they need more data to confirm the possibility of an exoplanet. Unfortunately, the planet’s large orbit will keep it from crossing its companion star for about 70 years. So, researchers can’t validate the research for quite some time.

The Future of Uncovering New Exoplanets

“Unfortunately to confirm that we’re seeing a planet, we would likely have to wait decades to see another transit,” said co-author Nia Imara of the University of California at Santa Cruz. “And because of the uncertainties about how long it takes to orbit, we wouldn’t know exactly when to look.”

The team posited that a dust or gas cloud passing in front of the X-ray may have caused the dimming. However, they concluded that this wouldn’t have been likely due to the characteristics of the planetary event in M51. They believe the model of a new planet aligns with the data since the event wasn’t consistent with the passage of a cloud.

“We know we are making an exciting and bold claim, so we expect that other astronomers will look at it very carefully,” said co-author Julia Berndtsson of Princeton University in New Jersey. “We think we have a strong argument, and this process is how science works.”

If a new planet exists in the binary system, it probably had a turbulent and violent past. An exoplanet surviving in this system would have experienced a supernova explosion which created the neutron star or black hole. Its future may also hold some tumultuous events. Scientists say that the companion star may explode as a supernova eventually, unleashing vast amounts of radiation.

Di Stefano and her colleagues searched for X-ray transits in three galaxies beyond the Milky Way galaxy for the study. They used both Chandra and the European Space Agency’s XMM-Newton observatories. The search included 55 systems in M51, 64 systems in Messier 101 (the “Pinwheel” galaxy), and 119 systems in Messier 104 (the “Sombrero” galaxy). After combing the galaxies for a new planet, it led to the single candidate in M51.

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Final Thoughts on Researchers Discovering a New Planet Beyond Our Galaxy

Humans have been intrigued by the vastness of outer space for millennia. Today, astronomers have made huge strides in helping us understand the universe and uncover its many mysteries. Our curiosity about the world around us leads to fascinating discoveries, such as new stars or planets. Recently, astronomers have detected an unknown planet outside the Milky Way for the first time.

In the future, the authors will continue to search for more exoplanets in other galaxies. Chandra holds essential data for at least 20 galaxies, including much closer than M51, like M31 and M33. This would make it easier to detect transits should they occur. They also want to search for X-ray transits in Milky Way X-ray sources to uncover new planets in unconventional environments.