What if Planet 9 is a Primordial Black Hole?

TL;DR

This paper proposes that the mysterious Planet 9 could instead be a primordial black hole, which explains certain orbital anomalies and microlensing data, and suggests observational tests for this hypothesis.

Contribution

It introduces the novel idea that primordial black holes could account for Planet 9 and related phenomena, offering an alternative to the planetary hypothesis.

Findings

Primordial black holes can explain TNO orbital anomalies.

Capture probability of PBHs is comparable to that of planets.

Dark matter annihilation signals could confirm the PBH hypothesis.

Abstract

We highlight that the anomalous orbits of Trans-Neptunian Objects (TNOs) and an excess in microlensing events in the 5-year OGLE dataset can be simultaneously explained by a new population of astrophysical bodies with mass several times that of Earth ($M_\oplus$). We take these objects to be primordial black holes (PBHs) and point out the orbits of TNOs would be altered if one of these PBHs was captured by the Solar System, inline with the Planet 9 hypothesis. Capture of a free floating planet is a leading explanation for the origin of Planet 9 and we show that the probability of capturing a PBH instead is comparable. The observational constraints on a PBH in the outer Solar System significantly differ from the case of a new ninth planet. This scenario could be confirmed through annihilation signals from the dark matter microhalo around the PBH.