542 Encyclopedia of the Solar System
TABLE 1 Pluto’s Heliocentric OrbitOrbital Element ValueSemimajor Axis,a 39.44 AU
Orbital period,P 247.688 year
Eccentricity,e 0.254
Inclination,i 17.14◦
Longitudinal ascending node,ω 110.29◦
Longitudinal perihelion,ω 223.94◦
Perihelion epoch,T 05.1 September
1989 UTNote: Osculating elements on JD 2449000.5, referred to the mean
ecliptic and equinox of J2000.0.detection, and then study of a set of once-every-124-year
mutual eclipse events between Pluto and Charon; and the
occultation by Pluto of a bright star, confirming the pres-
ence of an atmosphere. In addition, the 1989Voyager 2en-
counter with the Neptune system gave us detailed insights
into the object believed to be Pluto’s closest analog in the
solar system, Triton, thereby showing how complex and sci-
entifically interesting Pluto would be under close scrutiny
by spacecraft. In the 1990s, it was discovered that Pluto’s
surface consists of a complex mixture of low-temperature
volatile ices, that this surface displays large-scale bright and
dark units, and that Pluto’s atmosphere consists primarily of
nitrogen gas, with trace amounts of carbon monoxide and
only a trace of methane. Additionally, Pluto’s small moons
Nix and Hydra were discovered, and Pluto’s context in the
solar system became understood only after the discoveries
of many smaller objects in the region of the solar system
beyond Neptune called the Kuiper Belt.
1.2 The Discovery of Pluto’s Three Satellites
Charon (pronounced correctly as “Kharon,” but more
colloquially pronounced as “Sharon”) was discovered by
J. W. Christy and R. S. Harrington on a series of pho-
tographic plates made in 1978 at the U.S. Naval Obser-
vatory’s Flagstaff Station in Arizona. Interestingly, these
images were taken less than 4 miles from Lowell Obser-
vatory, where Pluto had been discovered 48 years before.
Charon was apparent on the 1978 Naval Observatory im-
ages as a bump or elongation in Pluto’s apparent shape. This
elongation of Pluto had occasionally been seen on photo-
graphic plates made in the 1960s, but it had not been rec-
ognized to be a satellite. This was because the elongation
of Pluto’s image by Charon was attributed to turbulence in
the Earth’s atmosphere causing a distortion of Pluto’s point-
like image (the two are< 1 arc second(arcsec) apart, and
blended together by atmospheric seeing). What Christy and
Harrington recognized in 1978 was that although Pluto was
Regarding the 2006 IAU Planetary
Definition and Pluto
In August 2006, a motion passed the International Astro-
nomical Union’s (IAU’s) General Assembly in Prague which
defined “dwarf planets” as those bodies in heliocentric orbit
that are large enough to be rounded by self gravity and thus
reach a state of approximate hydrostatic equilibrium. The
IAU further required that “planets” fit a context-dependent
criterion, in that a planet must have cleared its orbital neigh-
borhood. Since Pluto and all dwarf planets fail this test, the
IAU currently does not consider Pluto a planet. However,
based on the inclusion of the dynamical clearing clause and
its restriction to planets being objects that orbit the Sun
and not other stars, the IAU definition of planethood has
been criticized as narrowly constructed, technically flawed,
poorly worded, biased against size with increasing heliocen-
tric distance, and at odds with other classification schemes in
astronomy that rely only on the intrisic properties of the ob-
ject in question. These and other criticisms have come from
planetary scientists, astronomers, teachers, and lay people,
many of whom have elected to neglect the IAU definition.
It is not known if the IAU definition will be widely adopted,
or how long the IAU definition will survive before it is modi-
fied. For those reasons, here we continue to refer to Pluto as
a planet in this chapter. An international scientific congress
to further assess the definition of planets is planned for
2007.distorted, none of the stars in the photographs were! This
led them to look for a periodicity in the elongations. The
recognition that the bump was in fact a close-in satellite
was made when it was determined that this bump regularly
cycled around Pluto in a 6.39-day period, which matched
Pluto’s rotation period, implying that the elongation was due
to an object that circled Pluto.
In the first few months after Charon’s discovery, Christy
and Harrington determined that Charon’s orbit is syn-
chronous with Pluto’s rotation and also in Pluto’s equato-
rial plane, and therefore highly inclined to the plane of the
ecliptic. During that same year, 1978, Leif Andersson rec-
ognized that Pluto’s orbital motion would cause Charon’s
orbital plane to sweep through the line of sight to the Earth
for a period of several years every half Pluto orbit, or 124 ter-
restrial years. Mutual eclipses (also called mutual events)
would then begin occurring every 3.2 days (half Charon’s
orbit period). These eclipses were predicted to progress
over a period of 5 to 6 years, from shallow, partial events to
central events lasting up to 5 hours, then to recede again to
shallow grazing events. It was widely recognized that such
a series of mutual eclipses and occultations would be scien-
tifically valuable events. Fortuitously, these mutual events
began occurring in 1984 and ended in 1990. These events