For the first time, astronomers say they have discovered signs of an exoplanet transiting a star outside of our galaxy.
The planet in question has been named M51-ULS-1b for its location within Messier 51 (M51), also called Whirlpool Galaxy for its distinctive swirls. Its candidacy as an exoplanet was announced in a recent paper published in the journal Nature Astronomy.
Background: Searching for an Exoplanet
As a planet outside of our solar system, M51 is defined as an exoplanet. While scientists and astronomers have found all other known exoplanets within the Milky Way, they have yet to find one outside of our own galaxy; that is, until the candidacy of M51.
To search for exoplanets, scientists use data from Earth-based and space telescopes. Those behind the discovery of M51-ULS-1b relied on NASA’s Chandra X-ray Observatory. Researchers will look for a “transit” event, when a planet’s orbit takes it in front of its star, causing the star to dim. The dimming of a star signals the presence of a planet.
“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, an astrophysicist at Harvard’s Center for Astrophysics, and first author of the study.
Analysis: What we know so far
The possible exoplanet discovery, M51-ULS-1, is located in a system that contains a black hole or a neutron star orbiting a companion star carrying a mass about 20 times that of the sun. Using the x-ray observatory, scientists found that the transit of the exoplanet lasted about three hours. From that, they deduced that its size would be roughly that of Saturn, orbiting the star at a distance twice that between Saturn and the Sun.
While the discovery has generated considerable buzz among astronomers and space lovers, verifying the existence of M51-ULS-1 could take a very long time, possibly down to generations of researchers.
“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.”
To gather the data needed to verify the existence of the exoplanet, researchers will have to wait for the next transit event, which—judging by the exoplanet’s large orbit size—would take around another 70 years.
How likely is it that the dimming of the star wasn’t caused by a planet? Could it possibly have been caused by a cloud of gas and dust? This seems unlikely, according to the researchers, who say that the characteristics of the event observed did not match that of a cloud passing before the star.
However, not everyone is convinced by the current evidence that M51-ULS-1 represents the exoplanet many astronomers are hoping it does.
“Certainly, M51-ULS-1 is an intriguing X-ray source, and it’s worth considering that there may be a planetary candidate orbiting this system,” wrote theoretical astrophysicist Ethan Siegel. “However, we have every reason to remain unconvinced by this assertion, at present,” he adds.
Until more data could be gathered to prove otherwise, researchers remain hopeful that M51-ULS-1 represents the valid discovery of an exoplanet existing beyond our galaxy.
Outlook: What now?
In the meanwhile, researchers can analyze past x-ray observations and old collections of data to find signs of M51-ULS-1.
“Since the next transit event is so uncertain (it could be as soon as decades from now, or much longer), there aren’t any plans in place to take follow-up observations of this particular planet candidate,” Theron Carmichael, a co-author of the paper, said.
“Instead, new X-ray observations and archival data of previous observations are more readily available to search for more planet candidates like this one.”
Though the existence of M51-ULS-1 may never be confirmed in our lifetimes, fret not! Frankly, it’s amazing that those of us now are witnessing the start of an epic scientific journey.
“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.
“We think we have a strong argument, and this process is how science works.”
Candy Chan is a journalist based in New York City. She recently graduated from Barnard College with a degree in History. Follow her reporting on her Twitter @candyschan.