What if Planet 9 is a Primordial Black Hole?

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.

• The paper posits that anomalous TNO orbits and microlensing events can be explained if Planet 9 is a primordial black hole with a mass ranging between 0.5 to 20 Earth masses.
• The paper employs statistical analysis to show that the capture probability of primordial black holes by the solar system is comparable to that of free-floating planets.
• The findings imply that gamma-ray signals from dark matter annihilation could serve as a novel observational method to confirm the presence of a primordial black hole.

What if Planet 9 is a Primordial Black Hole?

The paper, "What if Planet 9 is a Primordial Black Hole?" by Jakub Scholtz and James Unwin, explores a novel hypothesis that links several astrophysical anomalies to the potential existence of primordial black holes (PBHs) within our solar system. The authors present a compelling argument that ties together unexpected orbital dynamics of Trans-Neptunian Objects (TNOs) and anomalies observed in microlensing events to the presence of either a new ninth planet or PBHs with masses several times that of Earth.

Key Observations and Hypotheses

1. Anomalous TNO Orbits: The observed clustering of TNO orbits, high perihelia of selected objects such as Sedna, and substantial inclinations pose challenges to existing models explained solely by the known giant planets. These observations have previously motivated the Planet 9 hypothesis, suggesting an unseen massive planetary body may be influencing these orbital characteristics.
2. Microlensing Anomalies: The Optical Gravitational Lensing Experiment (OGLE) has detected microlensing events suggestive of massive objects with properties unlike typical planets. These events provide evidence for either free floating planets (FFPs) or PBHs, with masses ranging from 0.5 Earth masses to 20 Earth masses.
3. Primordial Black Holes as Candidates: The authors propose that the observed anomalies in both TNO orbits and microlensing could be explained if Planet 9 is not a typical planet but rather a PBH. They estimate the capture probability of PBHs by the solar system to be comparable to that of capturing FFPs, making this hypothesis statistically plausible.

Discussion and Implications

The implications of identifying Planet 9 as a PBH are significant. The presence of a PBH in our solar system would challenge existing paradigms of planetary formation and dynamics. Further, it could imply that the nature of dark matter might be partially composed of PBHs, potentially explaining both astrophysical phenomena traditionally only approached through the lens of dark matter as weakly interacting massive particles (WIMPs).

1. Potential for Detection via Dark Matter Annihilation: PBHs, if embedded within dark matter halos, can produce distinguishable signals. The paper outlines that detectable annihilation signals in the form of gamma rays could serve as a feasible detection method, providing a unique observational window to confirm the presence of a PBH within the solar system.
2. Astrodynamical Constraints and Observations: Planet 9 as a PBH would necessitate new models for understanding solar system dynamics, especially considering the different gravitational influences and observational signatures compared to a typical rocky or gaseous planet. This alternative shifts the focus of search strategies from conventional optical and infrared methods to those that can identify high-energy cosmic emissions.
3. Future Work and Exploration: This hypothesis opens up avenues for further theoretical and observational work. Future projects could involve:
   • Dedicate searches for gamma-ray sources with temporal-spatial correlations consistent with PBH dynamics.
   • Simulations and modeling to further constrain the possible orbital parameters of a PBH-based Planet 9 scenario.
   • Continued surveys for microlensing events to better understand the mass and distribution compositions of potential PBH populations.

Conclusion

While Planet 9, as conceptualized in this framework, stands as an intellectually intriguing proposition, confirming such a hypothesis would require a significant shift in both observational techniques and theoretical models. However, the indirect benefits, such as insights into the properties of dark matter and the structure of early universe dynamics, validate the pursuit of further exploration. This work exemplifies how examining existing anomalies through unconventional lenses can lead to profound shifts in scientific understanding, maintaining the dynamic nature of astrophysical research.