Astronomy

Planet Nine plus five
Planet Nine
Distant Kuiper Belt objects (magenta)
Distant perpendicular objects (blue)
A predicted outcome
of Planet Nine’s
existence is that a
second set of confined
objects should exist.
They are forced
into positions at
right angles to Planet
Nine and into orbits
perpendicular to the
solar system. Five
known objects fit this
modeling perfectly
(shown in blue-green).
CALTECH/R. HURT (IPAC)
that it, too, was pulled in the same direction as all of
the rest was again overlooked as mere coincidence.
Trujillo and Sheppard did notice what they thought
were some unusual orbital alignments, though, and, as
was now common, suggested that a yet unseen planet
— perhaps twice the mass of Earth and lying at a distance
of 200 AU — was the natural explanation.
At this point, Konstantin Batygin, a newly hired
assistant professor at Caltech, and I became interested.
We were pretty sure that the Trujillo and
Sheppard suggestion wouldn’t work and quickly convinced
ourselves that we were right (results that were
also reached by Meg Schwamb, working in Taiwan).
No planet could cause the alignments that Trujillo
and Sheppard thought they were seeing.
Looking carefully at the data, however, we became
intrigued by these distant objects whose perihelia had
been tugged away from the Kuiper Belt and by the fact
that they were all coincidentally aligned. We quickly
calculated that the probability of such an alignment
occurring just due to chance would be only about 1
percent, a small, but not overwhelmingly small,
chance. I recall saying to Batygin, “This is interesting,
but we really need one more object to be aligned to
make it statistically convincing.”
In a seemingly unrelated analysis, Rodney Gomes
in Brazil noticed the existence of an unusually large
number of objects with distant orbits whose closest
approach to the Sun had been pushed inward even
closer than the orbit of Saturn and whose orbits were
twisted such that they were nearly perpendicular to the
disk of the solar system.
No one really had any viable suggestion for the origin
of these peculiar objects. But Gomes had an interesting
hypothesis: Perhaps a distant, massive planet
was twisting these orbits perpendicularly and pushing
their closest approaches inward.
For anyone paying close attention to the outer solar
system, it was 1820 all over again. Bouvard simply had
Uranus to guide his way. This time there were multiple
lines of evidence suggesting that perhaps an undiscovered
planet was out there. Just as in 1820, the size, the
orbit, and the mass were all completely unknown, but
the hints that something was out there seemed to be
coming from all directions.
Meanwhile, Batygin and I saw the new object we
had been waiting for when, in 2014, astronomers
reported the discovery of 2013 RF 98
, whose orbit is distant,
elongated, and aligned nearly precisely like the
rest. All of the most distant orbits were aligned. All of
the distant orbits whose perihelia had been pulled out
of the Kuiper Belt were aligned in the same direction.
This time we calculated that the probability that this
alignment was just due to coincidence was down to
0.007 percent. The signs in the sky were clear:
Something was out there.
Batygin and I got to work. With months of penand-paper
calculations, and then more months of
detailed computer simulations, we realized that everything
we were seeing could be explained by a planet a
little less massive than Neptune on an eccentric orbit
that takes it from around 200 AU at its perihelion out
to 1,200 AU at its aphelion — its farthest point from
the Sun — over an approximately 20,000-year orbital
period. Such a planet would capture Kuiper Belt
objects with distant elongated orbits into stable orbits
elongated in the opposite direction from the planet.
Moreover, it would pull the perihelia of these
24 ASTRONOMY • JUNE 2016