302 The Poetry of Physics and The Physics of Poetry
The Dwarf Planets
There are five known dwarf planets in the Solar Systems, Ceres, in the
Asteroid Belt: Pluto, Haumea, and Makemake in the Kuiper Belt and
Eris in the Scattered Disk.
Ceres, the Solar System’s smallest dwarf planet and the only one in
the Asteroid Belt, has a radius of 475 km. It has a slightly elliptical orbit
with its distance to the Sun varying from 2.58 to 2.99 AU. Its orbital
period is 4.6 years. It accounts for one third of the total mass of the
Asteroid Belt. It is spherical with a rocky core and an icy mantle of water
and hydrated minerals.
Pluto, once thought to be a planet was reclassified as a dwarf planet,
the second largest in the Solar System with a radius 0.18 that of Earth’s
and a mass only 0.002 Earth masses. The search for Pluto was motivated
by the slight perturbations of Uranus and Neptune. Its orbit is highly
inclined and eccentric with its distance to the Sun varying from 30 to
49 AU bringing it within the orbit of Neptune. Its orbital period is
248 years and it apparently rotates on its axis once every 6.39 days.
Pluto is composed primarily of rock and ices of water, nitrogen and
methane. Pluto has three moons, Charon, the largest and two much
smaller moons Nix and Hydra. Charon has a radius one half that of Pluto
but only 11.6% of Pluto’s mass. Hydra and Nix are tiny objects with
diameters of 100 ± 20 km.
Haumea has a mass that is one-third the mass of Pluto. It has a rather
elliptical orbit with its distance to the Sun varying from 34.7 to 51.5 AU.
Its orbital period is 283 years. It has an ellipsoid shape with its long axis
twice that of its short axis, which is due to its rapid rotation. Haumea is
thought to be composed almost entirely of solid rock and is the remnant
of the breakup of a larger object that suffered a catastrophic impact.
Haumea has two known moons.
Makemake’s orbit is rather elliptical with its distance to the Sun
varying from 38.5 to 53 AU. Its orbital period is 310 years. Its diameter
is roughly three-quarters that of Pluto and its mass is one-third the mass
of Pluto. Makemake has no known satellites.
Eris is the largest dwarf planet in the Solar System. Its orbit is very
elliptical with its distance to the Sun varying from 37.7 to 97.6 AU. Its
orbital period is 557 years. Its diameter is roughly 1.13 times that of
Pluto and its mass is 1.27 times the mass of Pluto. Makemake has one
known satellite, Dysnomia, whose radius is only 85 ± 35 km.