Could Planet 9 be a primordial black hole?

For several years now, astronomers and cosmologists have been theoretically discussing the existence of an extra planet with a mass 10 times larger than that of Earth, located in the extreme regions of the solar system. This hypothetical planet, called 9 Planet, could be the source of gravitational effects that would explain the unusual patterns in the orbits of trans-Neptunian objects (TNOs) by existing cosmological data. TNOs are celestial bodies that revolve around the Sun and lie outside Neptune.

Building on research conducted in recent years, Jakub Scholtz and James Unwin, two researchers from Durham University and the University of Illinois at Chicago, recently conducted a study examining the possibility that Planet 9 has a primordial black hole is. Their paper, published in Physical review letters, hypothesizes that the anomalous orbits of TNOs and an excess in microlensing events observed in the 5-year Optical Gravitational Lensing Experiment (OGLE) dataset can be simultaneously explained by the existence of a specific population of astrophysical bodies (one of which would be Planet 9 ). More specifically, it introduces the idea that Planet 9 and the rest of these bodies may be primordial black holes (PBHs).

“Our work began when James and his wife Laura went to the Chicago planetarium and watched a short documentary about Planet 9,” said Jakub Scholtz, one of the researchers who conducted the study, told Phys.org. “It must have captured James’ attention because he called me the other day and we started to find out if there was another object that could be a planet mimicry. We came up with some fun scenarios: Bose stars, ultra-compact dark dust halos, primary black holes – and several other possibilities. “

A few months after they began researching hypotheses about the nature of Planet 9, another research team at the University of Tokyo re-analyzed data collected as part of the OGLE experiment. OGLE is a research project carried out at the University of Warsaw, which means that the images of the sky were made with advanced telescopes over long periods.

The reanalysis of the OGLE datasets tentatively indicated the existence of a population of PBHs with a mass similar to what astronomers predicted would be the mass of Planet 9. When Scholtz and Unwin learned about these tentative findings, they began to specifically consider the possibility that Planet 9 may, in fact, be a primordial black hole.

“The final pieces really came together when we realized that the dark dust halos surrounding primordial black holes could be a way to observe Planet 9 if it were a black hole, because of the X-ray / gamma ray signal it emits. , “Said Scholtz. “In some sense, the purpose of our study was really to convey the message that the idea of ​​a primary black hole around the sun is not as absurd as it seems, and that we may need to pay more attention to it.”

The hypothesis that the unusual orbits of TNOs observed in cosmological data in the past could be explained by the existence of an additional planet (Planet 9), has already been explored by several researchers, including a team at the California Institute of Technology led by Michael Brown and Konstantin Batygin. The team at the University of Tokyo that reanalyzed the OGLE dataset, on the other hand, was the first to introduce the idea that the excesses in microlensing events observed within the OGLE data were evidence could of the existence of a population of PBHs.

In essence, the study by Scholtz and Unwin connects these two hypotheses, suggesting that the long theoretical extra planet may, in fact, be a black hole belonging to the population of PBHs proposed by Nikura and her colleagues at the University of Tokyo. In addition, the researchers showed that one of the earlier theorized scenarios for the origin of Planet 9, known as the ‘capture of a free-floating planet’, is just as likely as a scenario involving the capture of a PBH of the population marked by the team in Japan.

“I think our study has two major significant outcomes,” Scholtz said. “First we managed to inspire other scientists, who were initially skeptical (as they should be) about this scenario, and some very nice ideas came out. Edward Witten, for example, suggested that we do it. “Planet 9 investigates the existence of a small space probe based on the Starshot program, and Loeb et al. pointed out that a population of primordial black holes would sometimes cause flashes when they encounter material on their orbit.”

The recent article by Scholtz and Unwin introduces a new, fascinating hypothesis about the nature of what has hitherto been referred to as Planet 9. This hypothesis could be further investigated as testing in new research studies. In addition, the two researchers began to look closely at moving gamma-ray and X-ray sources in the air, a topic that has so far been largely ignored, despite the large amount of available data that researchers can study.

“Our future research will focus primarily on exploring various existing datasets and looking for evidence (or lack thereof) of moving sources in the air,” Scholtz said. “We have identified a very promising method that can help us see a moving source, as long as we detect about 10 source photons per year with the FERMI-large area telescope (in the GeV range).”

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