The Nobel Prize in Physics in Physics was presented to this year’s winner in 2019. Michel Mayor Didier Queloz51 Pegasi was recognized for establishing a new field of astronomy through the discovery and subsequent analysis of the first extrasolar planet. Over 4,000 exoplanets were discovered in our galaxy since 1991’s discovery. “We answered a very old question,” Mayor said, which was debated by philosophers since the ancient Greeks: “are there other worlds in the Universe?”
What is the First Known Planet of 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.
Astronomers have discovered all known exoplanets in the Milky Way galaxy. Almost all are less than 3,000 light-years away from Earth. A M51 exoplanet would be approximately 28 million light years away. This would mean that it would be thousands of miles farther than 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 Neues Ergebnis 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 others have instead looked for dips in X-ray brightness from X-ray binaries based upon observations by the Chandra X-ray Observatory. These luminous systems are often dominated by a neutron star, black hole, or a companion star pulling in gas. The material around the neutron star and black hole is superheated, and it glows in the X-rays.
The Size of Saturn’s Planet Orbiting a Neutron Star, or Black Hole
The region that produces bright Xrays is very small so a planet passing directly in front of it might block all or most of the Xrays. This makes transits easier because the Xrays may disappear completely. This could allow exoplanets can be detected at greater distances than the current optical light transit study, which cannot detect small decreases in light from the star.
This was the method used by the team in order to locate the exoplanet contender in a binary called M51–ULS-1. It’s located in Whirlpool Galaxy M51. This binary system is composed of a black hole, or neutron star, orbiting a companion Star with a mass approximately 20 times the Sun. The Xray transit that they discovered using Chandra data lasted approximately three hours. During this time, the Xray emission was reduced to zero. The researchers based on this information and others estimated that M51-ULS-1’s exoplanet-like candidate would be about the same size and orbit the neutron-star or black hole at around twice the distance as Saturn.
More Data Required
Although this is an intriguing study, it would require more data to confirm the identification as an extragalactic planetet. 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.”
Is it possible that the dimming could have been caused by a cloud made up of gas and dust, which passed in front of an X-ray generator? Researchers consider this unlikely as the events observed in M51-ULS-1 do not support the existence of such a cloud. The data is however consistent with the model for a planet candidate.
“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 BerndtssonPrinceton University. “We think we have a strong argument, and this process is how science works.”
A planet that exists in this system likely has a turbulent and violent history. The system’s exoplanets would have to survive supernova explosions that created the neutron stars or black holes. The future is also uncertain. The planet could be hit again by extremely high radiation levels if the supernova-emitting companion star explodes.
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 Send 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. Sharp images are necessary to avoid contamination by nearby Xray sources. This problem gets worse as a galaxy grows further out. 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. It became obvious that the object could be of any size. With such a bright, young X-ray source, it couldn’t have been a brown dwarf, or a low mass star. We had made the first discovery of a planet-like object within an extragalactic galaxy. I believe that there had to be something first. We were fortunate to find a sign of it quickly by serendipitously looking into an event. 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.
We will be able to discover new things 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 also be used to X-ray source in the Milky Way. This will be amazing, as it allows for a variety complementary methods to be applied to the same system in order to learn more about it, and eventually, to learn more regarding the history and fate of planets within X-ray active systmes.
“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. Although microlensing may eventually succeed, it won’t be able find planets within 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. While I was curious, I was skeptical for a while. It was an overwhelming feeling of joy when I realized that the planet was the most likely transiter. We went through a lengthy and rigorous review process after that. It felt like a surprise when people discovered the publication and started to share their excitement. Many people have offered to discuss the work, as it is something we can clearly explain. I feel privileged to have made this discovery.”
The authors will search Chandra and XMM–Newton archives for additional exoplanet candidate galaxies. Substantial Chandra datasets for at least 20 galaxies are available, including those like M31 or M33 that are more distant than M51. This allows shorter transits to detectable. A second interesting research area is the search for Xray transits in Milky Way Xray sources in order to find new planets in strange environments.
Image credit: A new Chandra imageNearly a million seconds of observation time are available for M51. 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 is composed of X-rays taken from Chandra (purple), and optical data from Hubble.
Maxwell Moe, Astrophysicist, NASA Einstein Fellow University of Arizona via Rosanne Di Stefano, Vinay Kashyap Astronomy of Nature
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Maxwell Moe (astrophysicist, NASA Einstein Fellow University of Arizona), Maxwell Moe Max is often found at Kitt Peak National Observatory two nights per week, exploring the mysteries of the Universe. Max earned his Ph.D. in Astronomy in 2015 from Harvard University.
