546 PART 4^ |^ THE SOLAR SYSTEM
The Family of Dwarf Planets
Perhaps the most interesting thing about Pluto is that it is not
alone. In Chapter 19, you read about the Kuiper belt objects that
orbit beyond Neptune. Th ere are many thousands of such
objects, and some of them are quite large. At least one is larger
than Pluto.
Th e object known as Eris, discovered in 2003, is 4 percent
larger in diameter than Pluto and 27 percent more massive. It
orbits about 1.7 times farther away from the sun than does Pluto.
Eris’s orbit is more eccentric and more highly inclined than
Pluto’s, but it seems to be a similar object. Th e discovery of Eris
led the International Astronomical Union to recognize a new
class of solar system objects called dwarf planets—objects that
orbit the sun and are large enough to assume a spherical shape
but not large enough to have suffi cient gravitational infl uence to
absorb or otherwise clear away remaining objects in the same
region. In contrast, the major planets, including Earth, were able
to do that type of regional clearing in the process of growing
from protoplanets into planets. Th ree solar system objects are
clearly dwarf planets—Pluto and Eris in the Kuiper belt, plus
Ceres, the 900-km-diameter asteroid that orbits within the aster-
oid belt between Mars and Jupiter and is nearly twice as big as
the next largest asteroids, Pallas and Vesta.
Roughly ten other Kuiper belt objects are known to have
sizes approaching that of Pluto and Eris, and there are proba-
bly others yet to be discovered. Two large objects named Sedna
and Orcus are each about 63 percent the size of Pluto. Another
object, called Quaoar (pronounced kwah-o-wahr), is 50 per-
cent the diameter of Pluto. Th e ten largest Kuiper belt objects
are considered candidate dwarf planets, pending determina-
tion of their shape. One strange object that may complicate
things is Haumea, with such a rapid spin—once every 4
hours—that it is shaped like a fl attened cigar. One of its
dimensions is nearly as large as Pluto’s diameter. Haumea is
not spherically shaped, but if it were not spinning so rapidly,
it might be a sphere and meet the defi nition of a dwarf planet.
Should the eff ect of Haumea’s rotation be counted in consider-
ing its status, or not?
Some astronomers argue that Charon, Pluto’s big spherical
moon, should be a member of the dwarf planets even though it
orbits another world and not the sun. Other astronomers are
upset that Ceres, a rocky asteroid, is included. Th e classifi cation
may seem arbitrary until you begin thinking about how the
dwarf planets formed. Some astronomers refer to them as oli-
garchs. Th e term oligarch is usually applied to business or politi-
cal leaders who are the biggest, meanest dudes in town. Th ey are
not alone, but they are the bosses. Th e dwarf planets appear to
have been bodies in the solar nebula that grew more rapidly than
their neighbors and became dominant, but never got big enough
to take over completely and sweep up all objects orbiting nearby.
Planets such as Earth and Jupiter cleared their orbital lanesobservations reveal that Pluto has a surprising amount of albedo
variation on its surface, but nobody knows what the dark and
light features might be. Understanding that will have to wait
until the New Horizons probe fl ies by in 2015.
Both Pluto and Charon go through dramatic seasons
much like those on Uranus as they circle the sun with their
highly inclined rotation axes. Th is should cause large changes
in Pluto’s atmosphere as the planet grows warmer when it is
closest to the sun, as it was in the late 1980s, and then freezes
as it draws away.
Earth to scalePlutoPlane of
Pluto’s orbitCharon19,640 km■ Figure 24-19
The nearly circular orbit of Charon is only a few times bigger than Earth. It
is shown here nearly edge-on, and consequently it looks elliptical in this
diagram. Charon’s orbit and the equator of Pluto are inclined 119.6° to the
plane of Pluto’s orbit around the sun.