8 ASTRONOMY • MARCH 2020
QUANTUM GRAVITY
The strange orbits of distant
space rocks suggest there’s a 5- to
15-Earth-mass world dubbed Planet
Nine lurking in the outskirts of the solar
system. But now, a team of scientists is
proposing something far stranger may
be inf luencing the orbits of these distant
bodies: a primordial black hole.
Primordial black holes are believed
to have popped into existence within
the first few fractions of a second after
the Big Bang. Their existence has yet
to be confirmed. But, according to
research posted September 24 to the
preprint site arXiv, if primordial blackholes exist, there’s no reason the solar
system couldn’t have captured one
whose gravity would mimic the effects
of the proposed Planet Nine. And
because black holes are incredibly adept
at crushing down matter, the black hole
equivalent to 5 Earth masses would be
only slightly bigger than a baseball.
As part of the Optical Gravitational
Lensing Experiment (OGLE),
astronomers have spent almost three
decades monitoring the sky in search
of gravitational microlensing events,
which occur when a massive fore-
ground object (such as a planet or tinyblack hole) passes directly in front of
a background object (such as a star). If
the alignment of the objects is perfect,
the foreground object acts as a sort
of lens, distorting and amplifying the
light from the object behind it.
After digging through five years
of OGLE observations, researchers
uncovered six microlensing events that
seem to have occurred when objects
roughly 0.5 to 20 Earth masses acted
as gravitational lenses. These lensing
objects, located about 26,000 light-years
away toward the Milky Way’s galactic
bulge, could correspond to free-f loatingIS PLANET NINE A TINY BLACK HOLE?
Oddly orbiting objects hint at a massive planet in the outer solar
system. But some researchers think a black hole could fit the bill.
PRIMORDIAL IMPOSTOR. Tiny primordial black
holes born in the first few moments after the Big
Bang are thought to speckle the universe. New
research posits that a primordial black hole could
mimic the effects of the proposed world dubbed
Planet Nine. NAGUALDESIGN/TOM RUEN/WIKIMEDIA COMMONS