Pluto 543
FIGURE 1 The Pluto system. This figure shows the orbits of
Charon, Nix, and Hydra around Pluto as they appeared from
Earth ca. 2005, the year Nix and Hydra were discovered.
(Adapted from Weaver et al., 2006,Nature 439 , 943.)
(described in Section 3) yielded a wealth of data on both
Pluto and Charon. Searches for other satellites of Pluto were
made in the early and mid 1990s by ground-based observa-
tories andHubble Space Telescope(HST) images obtained
for other Pluto studies, but no moons were detected.
In May 2005, however, a much more sensitive, dedicated
satellite search by anHSTobserving team led by H. A.
Weaver and S. A. Stern yielded the detections of two small
satellites. These bodies, which were subsequently named
Nix and Hydra, orbit in circular orbits in Pluto’s equatorial
plane, as Charon does. Nix and Hydra orbit Pluto somewhat
further out than Charon, with Nix being near 48,700 km
from Pluto’s center and Hydra near 64,800 km. These or-
bits are close to or in resonance with Charon, with the
Charon:Nix:Hydra periods being very close to or at 1:4:6.
Figure 1 depicts the satellite orbits of Nix and Hydra in re-
lation to Charon. Based on their observed magnitudes, we
can make reasonable assumptions about their albedos yield
size estimates of approximately 40–160 km diameters for
both. Initial color measurements made withHSTindicate
both satellites are neutrally reflecting, much like Charon.
No compositional or lightcurve results are available on ei-
ther satellite as of late 2006.
TheHSTimages that revealed Nix and Hydra have also
been used to search for other satellites; none were found.
From these data, it is possible to say that Pluto does not have
any other satellites close to Nix and Hydra’s brightness any-
where beyond Charon’s orbit; inside Charon’s orbit, such
bodies could remain undetected, but they are not expected
for theoretical reasons relating to Charon’s outward migra-
tion during tidal despinning.2. Pluto’s Orbit and Spin
2.1 Pluto’s Heliocentric Orbit
Relative to the eight previously discovered planets, Pluto’s
orbit is unusually eccentric (eccentricitye≈0.25), highly
inclined (inclinationi≈ 17 ◦), and large (semimajor axisa≈
39.4 AU). Pluto’s orbit period is 248 years, during which the
planet ranges from inside Neptune’s orbit (Pluto’s perihe-
lion is near 29.7 AU) to nearly 49.5 AU. The Pluto–Charon
barycenterpassed its once-every-248-year perihelion at
05.1±0.1 September 1989 UT; this will not occur again
untila.d.2236.
Current orbit integrations using osculating elements are
able to predict Pluto’s position to 0.5 arcsec accuracy over
timescales of a decade. The fact that Pluto’s perihelion is
closer to the Sun than Neptune’s orbit is quite unusual: No
other known planet in the solar system crosses the orbit of
another. The large change in Pluto’s heliocentric distance
as it moves around the Sun causes the surfaceinsolation
on Pluto and Charon to vary by factors of 3, which has im-
portant implications for Pluto’s atmosphere (see Section 6).
Pluto’s perihelion lies slightly inside Neptune’s orbit.
In the mid-1960s, it was discovered through computer
simulations that Pluto’s orbit librates in a 2:3 resonance with
Neptune, which prevents mutual close approaches between
the objects. This discovery has been verified by a series of
increasingly longer and more accurate simulations of the
outer solar system now exceeding 4× 109 years. It is likely
that Pluto was caught in this 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.
Pluto and Neptune can never closely approach one an-
other, owing to this resonance, and the fact that the ar-
gument of Pluto’s perihelion (i.e., the angle between the
perihelion position and the position of its ascending node)
librates (i.e., oscillates) about 90◦with an amplitude of ap-
proximately 23◦. This ensures that Pluto is never near peri-
helion when it is in conjunction with Neptune. Thus, Pluto is
“protected” because Neptune passes Pluto’s longitude only
near Pluto’s aphelion, never allowing Neptune and Pluto
to come closer than≈17 AU. Indeed, Pluto approaches
Uranus more closely than Neptune, with a minimum sep-
aration of≈11 AU, but still too far to significantly perturb
its orbit.
In the late 1980s, it was discovered that Pluto’s orbit ex-
hibits a high degree of sensitivity to initial conditions. This