Planet 9 Might Be a Black Hole

The weird orbits of Trans-Neptunian Objects (TNOs) and an excess amount of 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. Researchers propose that 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 orbits of trans-Neptunian Objects (TNOs) have been taken as evidence of a new ninth planet in our solar system, called Planet 9 (P9), with mass about 5 to 15 times the Earth and they would be orbiting around the Sun at a distance of 300−1000 AU. There is set of gravitational anomalies recently observed by the Optical Gravitational Lensing Experiment (OGLE). OGLE reported an excess of six ultrashort microlensing events with crossing times of 0.1 − 0.3 days. The lensing objects are located towards the galactic bulge, roughly 8000 parsecs away. These events correspond to lensing by objects of mass of half of the Earth to 20 times the Earth. They could be interpreted as an unexpected population of free-floating planets (FFPs) or as Primordial Black Holes (PBHs).

These two anomalies correspond to a similar mass scale. Perhaps the most natural explanation is that they are caused by the existence of an unknown population of planets.

However, IF the OGLE events are due to a population of PBHs then it is possible that the orbital anomalies of TNOs are also due to one of these PBHs that was captured by the Solar System.

There are three alternative hypotheses for the origin of Planet 9:
a) P9 formed on its current orbit (‘in situ’);
b) P9 formed in the inner Solar System and has been up-scattered into its current orbit; or
c) P9 has formed outside of the Solar System and has been captured.

While all three scenarios are unlikely, they are still favorable compared to the chance alignment of TNOs. In case of the in situ formation, at a ∼ 500 AU there is typically insufficient time and material to build an Earth mass planet. The prospect of a planet forming near Uranus and Neptune before being scattered to its present orbit is low since in order to fall into a stable orbit the planet would need to be appropriately influenced by a passing star (or another mechanism) [4, 29]. The probability of capturing a free-floating planet (FFP) is estimated to be similarly improbable, with estimates differing by orders of magnitude depending on assumptions.

The anomalous orbits of TNOs and OGLE’s short microlensing events could have the same origin. They could be caused by a population of Primordial Black Holes with five times the mass of the Earth. While the principal search strategies for a planet is to employ optical and infrared/microwave surveys, the signals could be very different for a PBH (or another exotic object). Thus, the PBH hypothesis expands the required experimental program to search for the body responsible for TNO shepherding and motivates dedicated searches for moving sources in x-rays, gamma rays and other high energy cosmic rays. Conversely, if conventional searches fail to find Planet 9 and the evidence for TNO anomalies continues to grow, the PBH P9 hypothesis will become a compelling explanation.

Arxiv – What if Planet 9 is a Primordial Black Hole?

SOURCES – Arxiv

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