There may be a tiny black hole lurking in our Solar System

Astronomers J. Scholtz and J. Unwin in a new study investigated the anomalous orbits of trans-Neptunian objects (TNOs) and excess in microlensing events using the 5-year Optical Gravitational Lensing Experiment (OGLE) and came to the conclusion that these gravitational anomalies can be explained if a tiny primordial black hole with a mass few times that of the Earth were to be lurking in the outskirts of the Solar System. According to the leading Planet 9 hypothesis, it is believed that a super Earth-sized rogue planet must be present to shepherd the orbits of the TNOs. However, the new study suggested that the probability of the Sun capturing a primordial black hole is comparable to the capturing of a rogue planet and suggested ways by which the scenario could be verified or falsified.

An Artist's illustration of the Solar system. Credit: pixel4k

Until now, two distinct but seemingly interconnected gravitational anomalies have been detected with no consensus regarding its source. Firstly, there is the unusual clustering of orbits for a group of TNOs (TNOs are Solar System bodies found orbiting the Sun beyond the orbit of Neptune), and secondly, the gravitational anomaly linked with an excess of six ultrashort microlensing events with crossing times of 0.1–0.3 days. Interestingly, both of these gravitational irregularities can be elegantly explained if either a hidden planet (Planet 9) or a group of densely clustered objects (also called free-floating planets) or a tiny primordial black hole (PBH) had a mass equivalent to 0.5 to 20 times that of the earth present beyond the orbit of Neptune (at roughly 200 astronomical units).

PBHs are exciting hypothetical objects. Their existence could very well solve many of the mysteries of modern cosmology such as dark matter and dark energy. PBH could also explain how supermassive black holes form and grow and therefore could explain the evolution of different types of galaxies. Since PBH does not form from the conventional stellar collapse, but from the primordial density fluctuations, they can therefore be substantially lighter than the Earth. Under certain circumstances, PBH could also accrete dark matter (DM) from its surroundings and can form a microhalo. As a result, the density of DM around PBH increases and can lead to potentially detectable DM annihilation signals. If such signals are indeed detected, it could provide concrete evidence for the existence of PBHs.

An Artist's illustration of Planet 9: Credit: R. Hurt (IPAC)/Caltech

The study also discusses more exotic possibilities to explain observations such as DM microhalos without PBHs, Bose stars, or DM stars. However, as the study suggests, each of these scenarios would lead to completely different experimental signatures, distinct from those of a rocky or gas planet.

The following YouTube video discusses the Planet 9 hypothesis.

The study, therefore, had (tried to) shed light on this intriguing scenario by demonstrating that the probability of the Sun capturing a PBH is not that improbable, and given the right set of conditions, tiny but distinct DM annihilation signals could very well be detected using dedicated searches for moving sources in x rays, gamma rays, and other high energy cosmic rays. The authors conclude by noting that if conventional searches fail to find Planet 9 and the evidence for TNO anomalies continues to grow, the PBH Planet 9 hypothesis would become a compelling explanation. The study has been published in Physical Review Letters.

Article Information: J. Scholtz and J. Unwin, "What If Planet 9 Is a Primordial Black Hole?", Phys. Rev. Lett.125, 051103 (2020)

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