Planet Nine could be a primordial black hole

The hypothetical Planet Nine, assumed to be lurking somewhere
in the outskirts of our solar system, may not be a planet at all.
A new study, published September 24 on the arXiv pre-print server,
suggests (https://bit.ly/3vXuae1) that the mysterious and still
undiscovered object might be a primordial black hole.
Primordial black holes (PBHs) are old and relatively small black
holes that emerged soon after the Big Bang. They are thought to
have been formed as a result of density fluctuations in the very
early universe. It is believed that PBHs with the lowest mass have
likely evaporated. However, those with larger masses may still
exist, evaporating at the present epoch-even though they have been
never directly observed.
Astronomers Jakub Scholtz of Durham University and James Unwin
of University of Illinois at Chicago, assume that PBHs could reside
even closer to us than we think. In a recently published paper, they
ponder the possibility that the elusive Planet Nine, theorized to be
orbiting the sun at a distance between 300 and 1,000 AU, could
be such an old and compact black hole.
Explaining their intriguing hypothesis, the researchers focus on two
unsolved gravitational anomalies of similar mass: anomalous orbits
of trans-Neptunian objects (TNOs) and an excess in microlensing
events. What is interesting is that both events are due to objects
with masses estimated to be between 0.5 and 20 Earth masses.
The anomalies of TNO orbits are assumed to be triggered by a new
gravitational source in the outer solar system. While it is widely
accepted that this source could be a free-floating planet, Scholtz
and Unwin argue that the PBH scenario is not unreasonable and
should be taken into account.
"Capture of a free-floating planet is a leading explanation for the
origin of Planet Nine, and we show that the probability of capturing
a PBH instead is comparable," the astronomers wrote in the paper.
However, it could be difficult to confirm this theory, as such
a hypothetical PBH, with a mass of around five Earth masses and
a radius of about five centimeters, would have a Hawking temperature
of approximately 0.004 K, making it colder than the cosmic microwave
background (CMB). Therefore, the power radiated by a typical PBH
alone is minuscule, which makes it hard to detect.
In order to overcome this obstacle, the authors of the paper propose
to search for annihilation signals from the dark matter microhalo
around a PBH. Such a dark matter halo, if annihilating, is thought
to be able to provide a powerful signal that could be identified by
observations. Hence, the astronomers suggest dedicated searches
for moving sources in X-rays, gamma-rays and also other high-energy
cosmic rays, which could provide more evidence supporting the PBH
hypothesis.