In 2019 the Nobel Prize in Physics was awarded to Michel Mayor and Didier Queloz for pioneering a new field in astronomy with the discovery of the first planet beyond our solar system, 51 Pegasi b. Since the discovery in 1991, over 4,000 exoplanets have been found in our home galaxy. “We answered a very old question,” Mayor said, which was debated by philosophers since the ancient Greeks: “are there other worlds in the Universe?”
The First Known Planet in Another Galaxy?
Fast forward to 2021: astronomers using NASA’s Chandra X-ray Observatory may have found the first known planet in another galaxy, where an exoplanet appears to orbit a massive star and a dead star in the Whirlpool galaxy. Harvard Harvard & Smithsonian (CfA) Center for Astrophysics astronomers spotted what they believe may be the first known planet in another galaxy by studying the behavior of X-rays emitted by bright extragalactic X-ray sources in the Whirlpool, where an orbiting planet would temporarily block the X-rays and cause a brief detectable eclipse.
Until now, astronomers have found all other known exoplanets and exoplanet candidates in the Milky Way galaxy, almost all of them less than about 3,000 light-years from Earth. An exoplanet in M51 would be about 28 million light-years away, meaning it 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 Harvard CfA astronomer, Rosanne Di Stefano.
This new result is based on transits, events in which the passage of a planet in front of a star blocks some of the star’s light and produces a characteristic dip. Astronomers using both ground-based and space-based telescopes – like those on NASA’s Kepler and TESS missions – have searched for dips in optical light, electromagnetic radiation humans can see, enabling the discovery of thousands of planets.
Di Stefano and colleagues have instead searched for dips in the brightness of X-rays received from X-ray binaries based on observations made by the Chandra X-ray Observatory. These luminous systems typically contain a neutron star or black hole pulling in gas from a closely orbiting companion star. The material near the neutron star or black hole becomes superheated and glows in X-rays.
Planet the Size of Saturn Orbiting a Neutron Star or Black Hole
Because the region producing bright X-rays is small, a planet passing in front of it could block most or all of the X-rays, making the transit easier to spot because the X-rays can completely disappear. This could allow exoplanets to be detected at much greater distances than current optical light transit studies, which must be able to detect tiny decreases in light because the planet only blocks a tiny fraction of the star.
The team used this method to detect the exoplanet candidate in a binary system called M51-ULS-1, located in the Whirlpool Galaxy M51 pictured above. This binary system contains a black hole or neutron star orbiting a companion star with a mass about 20 times that of the Sun. The X-ray transit they found using Chandra data lasted about three hours, during which the X-ray emission decreased to zero. Based on this and other information, the researchers estimate the exoplanet candidate in M51-ULS-1 would be roughly the size of Saturn, and orbit the neutron star or black hole at about twice the distance of Saturn from the Sun.
More Data Needed
While this is a tantalizing study, more data would be needed to verify the interpretation as an extragalactic exoplanet. One challenge is that the planet candidate’s large orbit means it would not cross in front of its binary partner again for about 70 years, thwarting any attempts for a confirming observation for decades.
“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.”
Can the dimming have been caused by a cloud of gas and dust passing in front of the X-ray source? The researchers consider this to be an unlikely explanation, as the characteristics of the event observed in M51-ULS-1 are not consistent with the passage of such a cloud. The model of a planet candidate is, however, consistent with the data.
“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.”
If a planet exists in this system, it likely had a tumultuous history and violent past. An exoplanet in the system would have had to survive a supernova explosion that created the neutron star or black hole. The future may also be dangerous. At some point the companion star could also explode as a supernova and blast the planet once again with extremely high levels of radiation.
The Last Word –Could It Be Even More Exotic Than a Simple Eclipse by a Planet?
“I do think this is the first of many discoveries, and will indeed open up a new frontier in exoplanet detection,” Vinay Kashyap wrote in an email to The Daily Galaxy. “The difference between this kind of planet and the ones being found by Kepler and TESS is that these are in extreme environments, orbiting compact objects with very large X-ray fluxes,” he explained. “So with a suitably large population at hand hopefully soon, we can begin to gain a fuller grasp of planet formation scenarios and planet survivability in the face of cataclysmic events.
“What we need in order to detect more such extragalactic exoplanet systems is a high-resolution X-ray light bucket,” Kashyap summarizes. “We need X-rays because the technique relies on the emission source being as small or smaller than the obscuring object, and we are not likely to find such small emission sources easily in other wavelength regimes. We need sharp images to avoid issues with contamination by nearby X-ray sources, a problem that gets progressively worse the farther out a galaxy gets. Of course, the technique will also work for X-ray binaries within our own galaxy, and the requirements are correspondingly less stringent — fainter systems can be observed with blurrier telescopes than Chandra.”
“Another important factor is that long duration observations must be carried out, because these objects are invariably orbiting at larger distances than close-in hot Jupiters or other exoplanets caught in surveys.”
“My emotions when we realized what we detected were more like acknowledging an inevitability. Once the possible size of the object became apparent, and it became clear that for a young system such as this with such a bright X-ray source it could not be a brown dwarf or a low-mass star, it was clear that we had a first detection of a planet-like object in an extragalactic system. My feeling on this is that something had to be the first, and we got lucky in serendipitously investigating an event which led us quickly to a clear signature. All credit in finding this event and realizing its importance and pursuing it till it was properly modeled should go to Julia Berndtsson and Rosanne Di Stefano.”
“In the interests of completeness,” Kashyap concludes, “I should point out that the evidence for it being an eclipse by a planet is still circumstantial, and there is room for a more clever explanation of the data. I think, though, that if someone does come up with another explanation that fits all the data, that will be even more exotic (and interesting) than it being a simple eclipse by a planet.”
Di Stefano and her colleagues looked for X-ray transits in three galaxies beyond the Milky Way galaxy, using both Chandra and the European Space Agency’s XMM-Newton. Their search covered 55 systems in M51, 64 systems in Messier 101 (the “Pinwheel” galaxy), and 119 systems in Messier 104 (the “Sombrero” galaxy), resulting in the single exoplanet candidate described here.
Will Open a New Era of Discovery Beyond Our Milky Way
“I do think it likely that the discovery will open up a new era of planet discovery in external galaxies,” Harvard’s Rosanne Di Stefano wrote in an email to The Daily Galaxy. “The reason is simply that we have demonstrated that the transits of planet-size bodies can be discovered on X-ray light curves, and much additional X-ray data from external galaxies remains to be analyzed,” she explained. “As time goes on, new X-ray missions will be sensitive enough to collect more photons per unit time, making it easier to find the signatures of transits.
“It is also important to note, Di Stefano adds, “that the method works for any variable X-ray sources orbited by planets. It can therefore be applied to X-ray sources in the Milky Way. This will be fantastic, because then a variety of complementary methods may be applied to the same system to learn more about it and ultimately, more about the history and fates of planets in X-ray active systems.
“The new space technologies most useful to planet discovery via X-ray transit are going to be X-ray missions. They are the only ones that can provided the light curves needed for analysis,” De Stefano continued. “Other missions, including JWST may help to study counterparts or the population of stars that host the X-ray-discovered planets.
“One of the most interesting elements of discovery via X-ray is that is can be done because the transits can be deep (because planets are comparable in size to the X-ray sources), and also because X-ray binaries can be resolved even in distant galaxies,” Di Stefano observed. “The methods used to find nearby planets are many years away from being able to discover or study extragalactic planets. (Microlensing should eventually achieve some success, but it will not be able to find planets around specific systems.) The X-ray method did and will continue to find planets orbiting specific systems.”
Di Stefano said about how it felt to make the discovery is one with a time-dependent answer: “I was very happy when we first found a transit, but we didn’t know for sure that it was likely to correspond to a planetary transit for some time. On the one hand I was intrigued, on the other hand I remained skeptical for a long time. There was a feeling of elation when I was finally convinced that the most likely transiter by far was a planet. After that, of course, we went through a rigorous and long review process. Thus, by the time of publication, when other people learned about it and began to express their excitement and enthusiasm, it felt like a surprise. The many invitations to discuss this work have been welcome, because this is a project we can explain clearly. I feel privileged to have made this discovery.”
The authors will search the archives of both Chandra and XMM-Newton for more exoplanet candidates in other galaxies. Substantial Chandra datasets are available for at least 20 galaxies, including some like M31 and M33 that are much closer than M51, allowing shorter transits to be detectable. Another interesting line of research is to search for X-ray transits in Milky Way X-ray sources to discover new nearby planets in unusual environments.
Image credit top of page: A new Chandra image of M51 contains nearly a million seconds of observing time. The data reveal hundreds of point-like X-ray sources within what is nicknamed the “Whirlpool Galaxy.” Most of these point sources are X-ray binary systems with either a neutron star or black hole orbiting a Sun-like star. The composite image consists of X-rays from Chandra (purple) and optical data from Hubble (red, green, and blue).