Planet Nine Mystery Could Be Caused by a Black Hole, and Dark Matter 'Annihilation Signals' May Help Find It: Scientists

Could the effects of a hypothetical "Planet Nine" which astronomers have been searching for since 2016 actually be caused by a black hole lurking on the edge of the solar system?

That's the question two scientists have posed in a new paper published in the online pre-print website arXiv.

Five years ago, a study was published indicating the presence of a large, hypothetical world tens of millions of miles from the Sun, which has come to be known as "Planet Nine" or "Planet X."

The scientists who predicted the planet's existence did so based on the strange, highly elliptical orbits of around 30 so-called Trans-Netpunian objects (TNOs.) These are clustered together in a way that is extremely unlikely to have occurred by chance.

TNOs are any object in the Solar System beyond the orbit of Neptune, including those that make up the Kuiper Belt, a vast disc of small bodies that orbit the Sun.

The make up of the Solar System as we know it cannot explain the highly elliptical orbits. This is where the Planet Nine hypothesis comes in, describing a world about ten times more massive than the Earth, which exerts an influence on this handful of TNOs, thus explaining their peculiar clustering of orbits.

However, despite efforts to find it, Planet Nine has never been directly observed. And while some follow up studies have strengthened the case for its existence, some research—like the latest paper—has proposed alternative explanations for the weird orbits.

In the latest paper—authored by Jakub Scholtz from Durham University in the U.K. and James Unwin at the University of Illinois at Chicago and the University of California, Berkeley—the scientists suggest that if we accept the Planet Nine hypothesis as an explanation for the gravitational anomalies of the TNOs, this would imply that our models for planet formation may need to be updated.

They then suggest the "more exciting possibility" that the weird TNO orbits are the result of a primordial black hole—one formed at the very beginning of the universe—which has been captured by the gravitational influence of the solar system.

If this primordial black hole does indeed exist, Scholtz and Unwin propose that it would be surrounded by a halo of dark matter—a mysterious hypothetical substance, yet to be identified by scientists, which makes up 27 percent of the universe's mass.

This halo could extend hundreds of thousands of miles from the center of the black hole and may emit gamma ray radiation in the form of "annihilation signals"—which form as a result of interactions between dark matter and anti-matter. Researchers could look for these signals in order to confirm the black hole hypothesis.

The arXiv paper has yet to be published in a peer-reviewed scientific journal, so the findings should be viewed as preliminary.

Planet Nine
Artist's concept of a hypothetical planet orbiting far from the sun. Caltech/R. Hurt IPAC