Sky & Telescope - January 2022 - 17

ralo s
Only about a third of Centaurs will
make it through the entire giant planet
region and past Jupiter to become
JFCs in the inner solar system.
many of the objects in today's Kuiper Belt were placed onto
their orbits when the giant planets formed and then migrated
to their current positions (S&T: Mar. 2021, p. 22). A variety of
gravitational effects can change KBOs' orbits. On long timescales,
the chaotic combination of orbital alignments with
Neptune and the gravitational pull from all the giant planets
can dramatically tilt or elongate a KBO's orbit, allowing it
to make closer and closer approaches to Neptune. Smaller
gravitational tugs from the most massive objects embedded
within the Kuiper Belt might even contribute to changes in
neighboring objects' orbits, too. All of these factors slowly
feed KBOs onto Neptune-crossing orbits over many millions
or even billions of years.
Close encounters with Neptune will then scatter these
objects, sending some of them to more distant parts of the
outer solar system and others onto smaller orbits in the Centaur
region. Many Centaurs will interact with just our outer
ice giants, Neptune and Uranus, before being tossed back into
the Kuiper Belt. But about half will continue their journey
inward and become Saturn-crossing Centaurs.
Every time a Centaur encounters a giant planet, its orbit
either gets smaller or larger. The exact outcome depends
on how closely the Centaur approaches the planet and the
direction of that approach. Only about a third of Centaurs
will make it through the entire giant-planet region and past
Jupiter to become JFCs in the inner solar system. The time
it takes an object to either traverse the giant-planet region
or be kicked back out into the Kuiper Belt is typically a few
million years.
Charting the Territory
Given that Centaurs represent a middle state in the continuum
between the outer solar system's Kuiper Belt populations
and the inner solar system's JFC population, it is perhaps
unsurprising that there isn't one single agreed-upon definition
for what a Centaur is. The commonly used definitions
place Centaurs as spending some or all of their time in the
giant-planet region, with their closest approach to the Sun
lying exterior to Jupiter's orbit but interior to Neptune's orbit.
Most definitions also require that their average distance from
the Sun be in the giant-planet region; the strictest require
that the object's orbit lies entirely inside that of Neptune.
Depending on which definition you use, the current
census of observed Centaurs ranges from about 250 to 350
objects. On this list are objects discovered before Chiron,
including 29P/Schwassmann-Wachmann 1, which was discovered
in 1927 and classified as a comet due to its bursts of
activity (S&T: Sept. 2021, p. 57). It wasn't until we had obserof
pl
na
e
wo
e
o
f
s o l a r
s y s t e
486958 Arrokoth
m
2060 Chiron
Neptune
el
Uranus
Neptune
Saturn
Neptune
Comet NEOWISE (C/2020 F3)
ENTRY Many Kuiper Belt objects such as Arrokoth (top) follow paths
minimally tilted with respect to the planets' orbits, as do Centaurs like
Chiron (center). But Oort Cloud comets such as C/2020 F3 (bottom)
often zoom through on highly elongated orbits at steep angles.
vations of more distant Centaurs and KBOs that we were better
able to recognize the Centaurs as a distinct population.
The Centaurs provide a critical link between the nearly
primordial icy bodies in the Kuiper Belt and the active, highly
evolved comets in the inner solar system. Like their namesakes,
Centaurs are hybrid creatures: part active comet, part
frozen planetesimal. They offer us an opportunity to watch
as objects experience their first significant thawing since they
were tossed into outer-solar-system cold storage.
The partly cometary nature of Centaurs became apparent
within a decade of Chiron's discovery. Large variations in its
brightness over time suggested it was venting icy material
from its surface; follow-up observations confirmed the presence
of a coma. We now have many more examples of activity
amongst Centaurs, ranging from relatively quiescent outgassing
to extremely bright outburst events throwing off large
amounts of dust and gas. Approximately 10% of the observed
Centaurs have confirmed cometary behavior - and this is
likely a lower limit, because not all Centaurs are amenable to
a search for activity!
The sublimation of water ice largely drives activity on comets
in the inner solar system, but for objects beyond Jupiter,
the Sun is too far away to heat up the water ice they contain.
Other ices that sublimate at lower temperatures, such as carbon
dioxide and carbon monoxide, instead must contribute to
Centaur activity - although some of the observed activity is
sk yand tele scope .o r g * JANUARY 2022 17
m
ets
y s
v
B
e
p l a n
A
b
o
GREGG DINDERMAN / S&T
https://skyandtelescope.org/

Sky & Telescope - January 2022

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

Contents
Sky & Telescope - January 2022 - Cover1
Sky & Telescope - January 2022 - Cover2
Sky & Telescope - January 2022 - 1
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Sky & Telescope - January 2022 - Cover3
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