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CHAPTER 29

Pluto

S. Alan Stern

Southwest Research Institute

Boulder, Colorado

1. Historical Background 6. Pluto’s Atmosphere


2. Pluto’s Orbit and Spin 7. Charon


3. The Mutual Events 8. The Origin of Pluto’s Satellite System


4. Pluto’s Surface Properties and Appearance Bibliography


5. Pluto’s Interior and Bulk Composition

P

luto is the ninth planet and the prototype of the dwarf
planets so common in the Kuiper Belt and beyond. It
is in an elliptical, 248 year orbit that ranges from 29.5 to
49.5Astronomical Units(AU) from the Sun. Its largest
satellite, Charon, is close enough to Pluto in size that the
pair are widely considered to be a double planet. Pluto’s two
other known satellites, Nix and Hydra, which orbit beyond
Charon but in Charon’s orbital plane, are both relatively
small. Almost nothing is known about Nix and Hydra save
their orbits, approximate sizes, and their neutral, Charon-
like colors. Both Pluto and Charon are rich in ices, but
their surface compositions,albedos, and colors are very
different. Unlike Charon, Pluto is known to possess distinct
surface markings, polar caps, and an atmosphere. Major
questions under study about the Pluto system include the
fate of Pluto’s atmosphere, the degrees of internal activity
Pluto and Charon exhibit, and the origin of the system.

1. Historical Background

1.1 Overview

Pluto was discovered in February 1930, at Lowell Ob-
servatory in Flagstaff, Arizona. This discovery was made
by Clyde Tombaugh (1906–1997), an observatory staff

assistant working on a search for a long-suspected perturber

of the orbits of Uranus and Neptune. That search, which

was first begun in 1905 by the observatory’s founder, Perci-

val Lowell, never located the large object originally being

searched for because the positional discrepancies of Uranus

and Neptune which prompted that search were fictitious.

Still, the search for Lowell’s “Planet X” resulted in the dis-

covery of the tiny planet Pluto, which itself heralded the

discovery of the Kuiper Belt some 70+years later.

Within a year of Pluto’s discovery, its orbit was well de-

termined. That orbit is both eccentric and highly inclined

to the plane of theecliptic, compared to the orbits of the

other planets (see Table 1). However, no important discov-

eries about Pluto’s physical properties were made until the

early 1950s. This lack of information was largely due to the

difficulty of observing Pluto with the scientific instruments

available in the 1930s and 1940s. Between 1953 and 1976,

however, technological advances in photoelectric photome-

try made possible several important findings. Among these

were the discovery of Pluto’s∼6.387-day rotation period,

the discovery of Pluto’s reddish surface color, and the dis-

covery of Pluto’s high axial tilt, orobliquity.

Between 1976 and 1989, the pace of discoveries in-

creased more dramatically. In rapid succession, there was

the discovery of methane (CH 4 ) on Pluto’s surface; the de-

tection of Pluto’s largest satellite Charon; the prediction,

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