Sky & Telescope - July 2022 - 16

GREGG DINDERMAN / S&T
Wilds of the Solar System
icy bodies beyond Neptune and the ring of frozen relics in
the main Kuiper Belt. They noticed that all objects beyond a
certain distance from the Sun made their closest approach
to our star near where their orbits crossed the ecliptic, or the
midplane of the solar system.
That was odd, because subtle yet persistent tugs from the
known giant planets should make those orbits drift. Over
the age of the solar system, they should have slowly arranged
themselves in random orientations - unless something was
corralling them. And these orbits didn't appear to be strongly
influenced by the known giant planets. Perhaps, Sheppard
and Trujillo suggested, there was another planet out there,
holding these orbits in place.
Two years later, Batygin and Mike Brown (Caltech) took
a closer look. Zeroing in on a handful of " extreme " objects
- those that are far enough from the Sun to keep well clear
of Neptune - they not only confirmed what Sheppard and
Trujillo saw but also reported that the orbits were physically
aligned, all stretching out roughly in the same direction away
from the Sun. They agreed that the probable culprit was a
planet, roughly 10 times as massive as Earth, with an average
distance from the Sun of several hundred astronomical units
(a.u.) - about 10 to 30 times farther out than Neptune.
In the years since, this possibility has kept some planetary
scientists busy examining and reexamining the evidence.
" There've been three themes through which the Planet Nine
hypothesis has progressed, " Batygin says. " Data analysis,
theory, and actual observations. "
Analysis of available data has produced the most controversy.
" There are a lot of observational biases in discoveries
of the outer solar system, " says Samantha Lawler (University
of Regina, Canada). If those biases aren't accounted for, she
adds, " you can find some really weird things, like it looks like
there's clustering when there's not. "
In the case of Planet X, there appear to a bunch of objects
whose perihelia are near the ecliptic. But observers are biased
toward finding things when they are near perihelion -
because that's when they're closest to the Sun and therefore
DISTANT NEIGHBOR? The known planets inhabit the inner 30 astronomical
units of the solar system. Beyond that is the icy Kuiper Belt,
the main (or " classical " ) section of which spans about 20 a.u. before
transitioning to a region of objects on highly elongated, inclined orbits
that then fades into the Oort Cloud. Planet X is thought to dwell in this
extended Kuiper Belt, with an average solar distance of anywhere from
300 to 800 a.u. (estimates vary).
Planet X's
domain?
Inner solar
system
1
16 JULY 2022 * SKY & TELESCOPE
Main
asteroid belt
10
Distance (astronomical units)
100
1,000
Giant planet region
Classical
Kuiper
Belt
Extreme Kuiper Belt
brightest - and they tend to search along the ecliptic, because
that's where most solar system stuff resides.
There are more subtle biases as well, Lawler notes. Seasonal
weather patterns in Hawai'i and Chile - where many planetary
surveys are conducted - mean certain parts of the sky
have better telescope coverage than others, which could make
it look like some extreme Kuiper Belt objects are crowding up
on one side of the solar system.
Accounting for these biases is tricky. It requires knowing
everything about the observations, including where the
telescope pointed (including where it didn't find anything)
and how faint an object it could have seen. Unfortunately, for
many Kuiper Belt finds, much of that info is lost.
" What the community has been trying to do for a few
years now is not only try to find objects but try to understand
what objects a given survey can find, " says Pedro Bernardinelli
(University of Washington). This type of work involves
limiting analyses to just the Kuiper Belt objects found by a
single survey - one which has a good grasp of all its biases
- and seeing if the orbital clustering shows up there in some
statistically meaningful way.
These analyses keep saying there's no evidence for an extra
planet.
The Outer Solar System Origins Survey, or OSSOS, spent
four years focusing on two regions of sky near the ecliptic.
During that time, the project tallied more than 830 new Kuiper
Belt objects, four to eight of which fit some definition of
" extreme " - that is, some combo of perihelion distance and
orbit size that keeps them mostly detached from the gravitational
sway of Neptune. Based on computer simulations of
what OSSOS could find, these extreme objects could be part
of a larger, unseen population spread uniformly around the
Sun, the team reported in 2017.
" We can't say Planet Nine doesn't exist, " Lawler says. " But
we don't have evidence in favor of the reason for it existing. "
Recently, two other teams came to similar conclusions.
The Dark Energy Survey (DES) wasn't designed to look
for things in the solar system. It spent six years scanning
Sky & Telescope - July 2022

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