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 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 PHB. 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 PHB hypothesis.
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