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⊕). 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.
Jakub Scholtz & James Unwin
The possibility that out solar system is hiding another large planet, several times more massive that Earth, in its outer regions, has since lead to other research, some seeking to disprove the conclusion, others trying to find alternative explanations for the anomalous orbital patterns of TNOs. One of the most exciting – but also rather unlikely – is the idea that the Sun is orbited not by a distant Super‑Earth, but by… a tiny black hole! Tiny by diameter at least, an estimated 5 cm for a black hole of 5 Earth masses. The gravitational effect on the outer Solar System would be the same, but a black hole would not be detectable in the visible spectrum, but through gamma ray emissions and gravitational lensing.
A black hole so close-by would make an excellent target for observations and experiments, but an exploration mission could be difficult. A probe might take decades to reach it (after another decade of developing experiments and designing the probe) and entering orbit would probably prove challenging at that distance, with significant time lag; the probe would have to be autonomous enough to manage operations on its own. Personally, I still suspect there is no massive object shaping TNO orbits from a wide orbit around the Sun; most likely the effect will fade as we discover more and more small objects, or was caused in the distant past by the passage of a near-by star.
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