Sky & Telescope - January 2022 - 16

Outer Solar System
The Cometary Cell Phone Lot
In the early 1970s, astronomers were still actively debating if
Jupiter-family comets had been captured from the Oort Cloud
or whether another, then-unobserved source region was
needed. (At that time, the only member of the Kuiper Belt
we'd found was Pluto.) In 1977, Charles Kowal
discovered a harbinger of new outer-solarsystem
populations: 2060 Chiron. This little
world is roughly 200 km (120 miles) across
and follows an orbit with a perihelion near
Saturn and an aphelion near Uranus. Early
calculations showed that gravitational influences
from the giant planets should relatively
quickly alter its orbit and send it elsewhere.
Further studies confirmed that most smallbody
orbits in the giant-planet region are only
stable for about a million years, much shorter
than the age of the solar system.
Some 15 years after Chiron's discovery, observers found
5145 Pholus, which has an orbit that spans from Saturn to
Neptune. Discoveries of several more objects followed soon
after. These so-called Centaurs are small bodies that, like
Chiron, all reside within the giant-planet region on unstable
orbits. Early observations of Pholus showed that its surface
is extremely red compared with objects in the main asteroid
belt, and subsequent measurements of its reflectance spectrum
(a more detailed measurement of the brightness as a function
of wavelength) also proved it to be uniquely red and missing
features commonly seen in asteroid spectra. Other Centaurs
also looked un-asteroid-like.
It was apparent that Centaurs are only temporary denizens
of the giant-planet region, and their surface compositions
don't match those in the asteroid belt. So where did they
come from?
In the 1980s and early 1990s, researchers had returned
Dictionary:
Centaur
A Centaur is a small
object that spends some
or all of its time in the
region between Jupiter
and Neptune. (Exact
definitions depend on
whom you ask.)
to the idea of a comet belt beyond Neptune's orbit, using
expanded computational capabilities and
improved numerical tools to show how such
a population could supply Jupiter-family
comets. In 1992 - the same year that other
observers found Pholus - David Jewitt and
Jane Luu discovered the first Kuiper Belt object
(KBO) residing entirely exterior to Neptune's
orbit. This discovery, along with more than a
dozen other KBO discoveries in the next three
years, finally provided direct evidence of the
hypothesized comet reservoir.
Follow-up observations of the newly
discovered Centaurs and KBOs found more examples of
distinctive red surfaces in both populations. This spectral
information provided important compositional links between
KBOs and Centaurs to back up the orbital modeling. It was
becoming clearer that Chiron and Pholus represented a new
population of objects closely related to the Kuiper Belt.
With these early KBO and Centaur discoveries, researchers
were finally able to demonstrate the path objects take from
the observed Kuiper Belt, through the giant planet region's
Centaur population, and into the JFCs. Our understanding of
the detailed orbital distribution and formation of the Kuiper
Belt has expanded dramatically in the last 30 years, but this
basic dynamical connection with the JFCs remains.
Although some KBOs have always lived beyond Neptune,
?
THE MAKING OF A COMET Years of observations and calculations led astronomers to deduce that primordial Kuiper Belt objects
like Arrokoth (left) become Jupiter-family comets like 67P/Churyumov-Gerasimenko (right). The question is, what do the objects in
transition look like?
16 JANUARY 2022 * SKY & TELESCOPE
ARROKOTH: NASA / JHU APL / SWRI; 67P: ESA / ROSETTA / MPS FOR
OSIRIS TEAM MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Sky & Telescope - January 2022

Table of Contents for the Digital Edition of Sky & Telescope - January 2022