In the search for worlds like—or, more often than not, very much unlike—our own, astronomers have looked to the sky. Given the extent that our current eyes to the sky can see, it’s no surprise to most people that all the planets that we’ve spotted thus far have been our galactic “neighbors,” so to speak.
You see, finding planets isn’t as simple as pointing your best set of binoculars to the sky and just hoping that you end up spotting a round marble with clouds or sand dunes. Mind you, we’re talking about thousands of light-years of distance, and more often than not, even more than that; at that distance, visible light won’t be of much help to scientists wanting to spot hidden worlds.
Thus, scientists resort to inference to identify these planets. One of the methods they use to do so is called the transit method, where scientists notice and track the slight dips in starlight they observe from a star for extended periods of time; the notion behind it is the fact that the planet, which hopefully intercepts the path between its host star and us here on Earth, interrupts its starlight at regular intervals due to its orbit.
Of course, there are more methods and circumstances out there—including the very method used to identify this new potential extragalactic planet, tentatively called M51-ULS-1b. The team that found the new class of planet was led by Harvard & Smithsonian Center for Astrophysics astrophysicist Roseanne Di Stefano, whose findings were published in the journal Nature Astronomy.
Di Stefano and team found the suspected extragalactic planet as an update to Di Stefano’s previous work together with Di Stefano’s co-author for both papers, University of California Santa Cruz astrophysicist and famous science artist Nia Imara. In their 2018 study, published in The Astrophysical Journal, they discuss the possibility of finding exoplanets revolving around a specific type of star, called an X-ray binary star.
In an X-ray binary star, the two stars are predicted to be so close to each other that one’s material is predicted to be siphoned off into the other. According to the 2018 study by Di Stefano and Imara, this very process produces X-rays. Given that the size of the binary star component gathering material from the other is sufficiently small—like, say, if the accreting star was a neutron star or a black hole—its X-rays would be periodically blocked by a sufficiently large exoplanet, similar to how our eclipsing Moon blocks off nearly all visible light from the Sun.
With these principles in mind, Di Stefano and team set out to search for “eclipsing” exoplanets across three distant galaxies: the Whirlpool Galaxy, otherwise known as M51; the Pinwheel Galaxy, M101; and the Sombrero Galaxy, M104. After combing through several thousand X-ray binary light curves, they found just one which fits their exoplanet-eclipsing criteria: an ultra-bright X-ray binary called M51-ULS-1, whose X-rays remained relatively consistent, save for the three hours where it saw a significant dip in its intensity.
As the brightness before and after the dip were relatively the same, Di Stefano and team suspect that whatever it was that caused the dip in intensity was likely an external object rather than an internal process that occurs between the two components of the M51-ULS-1 X-ray binary. Analysis of the data afterwards left the team with the conclusion that the cause of the dip was a Saturn-sized planet, making their find the first potential exoplanet ever found outside our own Milky Way Galaxy.
“At first we recognized the intriguing possibility, but there were many reasons to be cautious. We had to carefully consider all alternatives, and the full analysis involved several different pieces, each of which had to show consistency with a planet interpretation, and rule out other possibilities,” Di Stefano said in a statement.
Now, given the distance between the team here on Earth and the Whirlpool Galaxy—which is estimated to be around 31 million light-years, or 293 billion billion km—gathering more information about what may be the most distant world we’ve ever discovered will be difficult. For instance, the rate at which it is expected to orbit its binary stars is far too long for our telescopes to keep watching it for further proof to the findings by Di Stefano and team. Thus, it is unlikely that we will ever get true confirmation of the status of the mysterious object in the sky.
Despite this, Di Stefano and team stand by their stance that all the data they obtained so far all lead to M51-ULS-1b being a true exoplanet. Their methodology also paves the way for identifying more potential extragalactic exoplanets, perhaps on other galaxies.
In a statement to news website ScienceAlert, Di Stefano mentioned that they feel “privileged” to have been able to contribute to this landmark find. “It demonstrates a new method that can be used by the community to find new planets both in the Milky Way and in other galaxies. And, we feel humble about the fact that the discovery of a candidate extroplanet (extragalactic exoplanet) in another galaxy connects this work with a long and interesting history.”
(For more exoplanet finds, check out earlier news regarding the potential new exoplanet hiding in the nose of the constellation Orion. For more space anomalies, check out our piece on the brown dwarf which was found “by accident” by a citizen scientist.)
References
- Di Stefano, R., Berndtsson, J., Urquhart, R., Soria, R., Kashyap, V. L., Carmichael, T. W., & Imara, N. (2021). A possible planet candidate in an external galaxy detected through X-ray transit. Nature Astronomy, 1–11. https://doi.org/10.1038/s41550-021-01495-w
- Gohd, C. (2021, October 26). 1st exoplanet outside the Milky Way possibly discovered. LiveScience. https://www.livescience.com/first-planet-outside-milky-way-discovery
- Imara, N., & Di Stefano, R. (2018). Searching for exoplanets around x-ray binaries with accreting white dwarfs, neutron stars, and black holes. The Astrophysical Journal, 859(1), 40. https://doi.org/10.3847/1538-4357/aab903
- Starr, M. (2021, October 25). In an astonishing feat, astronomers present evidence of an extra-galactic planet. ScienceAlert. https://www.sciencealert.com/astronomers-have-detected-what-they-think-is-a-planet-in-a-whole-other-galaxy
