Pluto 555
bodies were together caught up in the outward tidal migra-
tion of Charon following its formation closer in.
8.2 The Origin of Pluto Itself
The presence of volatile ices, including methane, nitro-
gen, and carbon monoxide on Pluto, and water and other
ices on Charon, argues strongly for their formation in the
outer solar system. The average density and consequent
high rock content of these two bodies also argues for for-
mation from the outer solar nebula, rather than from plan-
etary subnebula material. As described earlier, it is thought
that the two objects (or more precisely Pluto and a Charon-
progenitor) formed independently and subsequently col-
lided, thus forming the binary either through direct, inelas-
tic capture or through the accretion of Charon from debris
put in orbit around Pluto by the impact.
The first widely discussed theory for Pluto’s origin was
R.A. Lyttleton’s 1936 suggestion, which was based on the
fact that Pluto’s orbit is Neptune-crossing. In Lyttleton’s
well-remembered scenario, Pluto was formerly a satellite
of Neptune, ejected via a close encounter between itself
and the satellite Triton. According to Lyttleton, this en-
counter also reversed the orbit of Triton. Variants on the
“origin-as-a-former-satellite-of-Neptune” hypothesis were
later proposed. However, all these scenarios were dealt a
serious blow by the discovery of Charon, which severely
complicates the Pluto-ejection problem by requiring either
(1) Charon to also be ejected from the Neptune system in
such a way that it enters orbit around Pluto, or (2) Charon to
be formed far beyond Neptune where Pluto currently orbits
and then captured into orbit around Pluto (presumably by
a collision).
Other strong objections to scenarios like Lyttleton’s also
exist. First among these is the fact that any object ejected
from orbit around Neptune would be Neptune-crossing
and therefore subject to either accretion or rapid dynam-
ical demise. It is implausible that such an object would
be transferred to the observed 2:3 Neptune:Pluto reso-
nance, because stable 2:3 libration orbits are dynamically
disconnected in orbital phase space from orbits intersecting
Neptune. Further, because Pluto is less massive than Triton
by about a factor of 2, it is impossible for Pluto to reverse
Triton’s orbit to a retrograde one, as is observed. Further
still, Pluto’s rock content is so high that it is unlikely that
Pluto formed in a planetary subnebula. Of course, none of
these facts were known until decades after Lyttleton made
his original (and then quite logical) suggestion that Pluto
might be a former satellite of Neptune.
As described in Section 2, it is likely that Pluto was caught
in the 2:3 resonance and had its orbital eccentricity and
inclination amplified to current values as Neptune migrated
outward during the clearing of the outer solar system by the
giant planets.
The heliocentric formation/giant collision scenario de-
scribed earlier for the origin of the satellites can account
for most of the major attributes of the system, including the
elliptical, Neptune-crossing orbit, the high axial obliquities,
and the≈8.5:1 mass ratio. Further, the present tidal equilib-
rium state would naturally be reached by Pluto and Charon
in 10^8 –10^9 years—a small fraction of the age of the solar
system.
Still, such a scenario begs two questions. First, why is
Pluto so small? And, second, how could Pluto and the
Charon-progenitor, alone in over 10^3 AU^3 of space, “find”
each other in order to execute a mutual collision? That
is, the giant impact hypothesis still fails to explain (1) the
existence of Pluto and Charon themselves; (2) the very
small masses of Pluto and Charon compared to the gas
giants in general, and Neptune and Uranus in particular;
(3) the fact that the collision producing the impact was
highly unlikely; and (4) the system’s position in the Neptune
resonance.
In 1991, Alan Stern of the Southwest Research Institute
suggested that the solution to (1)–(3) lies in the possibil-
ity that Pluto and Charon were members of a large pop-
ulation (300–3000) of small (∼ 1025 g)ice dwarfplanets
present during the accretion of Uranus and Neptune in the
20–30 AU zone. Such a population would make likely the
Pluto–Charon collision, as well as three otherwise highly
unlikely occurrences in the 20–30 AU region: the capture
of Triton into retrograde orbit and the tilting of Uranus and
Neptune. Similar conclusions based on different consider-
ations were reached by William McKinnon of Washington
University in the late 1980s. According to this work, the
vast majority of the ice dwarfs were either scattered (with
the comets) to the Oort cloud or ejected from the solar sys-
tem altogether by perturbations from Neptune and Uranus.
Only Pluto–Charon and Triton remain in the 20–30 AU zone
today, specifically because they are trapped in unique dy-
namical niches that protect them against loss to such strong
perturbations.
If this is correct, it implies that Pluto, Charon, and Triton
are important “relics” of a very large population of small
planets, dubbed ice dwarfs first by Stern, which by number
(but not mass) dominate the planetary population of the
solar system. As such, these three bodies would no longer
appear as isolated anomalies in the outer solar system and
would be genetic relations from an ancient, ice dwarf en-
semble, and therefore worthy of intense study as a new and
valuable class of planetary body unto themselves.8.3 The Context of Pluto in the Outer Solar System
When the existence of the ice dwarf population was first
suggested, the solar system beyond Neptune appeared to
only be inhabited by Pluto and the numerous comets scat-
tered out of the planetary region during the accretion of the
giant planets.