The Solar System and Its Place in the Galaxy 3
In the 1980s, dynamical calculations suggested the exis-
tence of a belt of many small objects in orbits beyond Nep-
tune. In the early 1990s the first of these objects, 1992 QB 1
was discovered at a distance of 40.9astronomical units
(AU). More discoveries followed and over 1000 bodies have
now been found in the trans-Neptunian zone. They are col-
lectively known as theKuiper belt. All of these bodies
were estimated to be smaller than Pluto, though a few were
found that were about half the diameter of Pluto.
The existence of the Kuiper belt suggested that Pluto,
like Ceres, was simply the largest body among a huge swarm
of bodies beyond Neptune, again calling Pluto’s status into
question. Then came the discovery of Eris (2003 UB 313 ), a
Kuiper belt object in a distant orbit, which turned out to be
slightly larger than Pluto.
In response, the IAU, the governing body for as-
tronomers worldwide, formed a committee to create a for-
mal definition of a planet. The definition was presented at
the IAU’s triennial gathering in Prague in 2006, where it
was revised several times by the astronomers at the meet-
ing. Eventually the IAU voted and passed a resolution that
defined a planet.
That resolution states that a planet must have three qual-
ities: (1) it must be round, indicating its interior is in hydro-
static equilibrium; (2) it must orbit the Sun; and (3) it must
have gravitationally cleared its zone of other debris. The last
requirement means that a planet must be massive enough
to be gravitationally dominant in its zone in the solar sys-
tem. Any round body orbiting the Sun that fails condition
(3) is labeled a “dwarf planet” by the IAU.
The outcome left the solar system with the eight major
planets discovered through 1846, and reclassified Ceres,
Pluto, and Eris as dwarf planets. Other large objects in the
asteroid and Kuiper belts may be added to the list of dwarf
planets if observations show that they too are round.
Although most astronomers have accepted the new IAU
definition, there are some who have not, and who are ac-
tively campaigning to change it. There are weaknesses in
the definition, particularly in condition (3), which are likely
to be modified by an IAU committee tasked with improv-
ing the definition. However, the likelihood of the definition
being changed sufficiently to again classify Pluto as a planet
is small.
In this chapter we will use the new IAU definition of a
planet. For an alternative view of the new definition, the
reader is directed to the chapter Pluto.
3. The Architecture of the Solar System
The solar system consists of the Sun at its center, eight
planets, three dwarf planets, 165 known naturalsatellites
(or moons) of planets and dwarf planets (as of September
2006), four ring systems, approximately one million aster-
oids (greater than 1 km in diameter), trillions of comets
(greater than 1 km in diameter), thesolar wind, and a large
cloud of interplanetary dust. The arrangement and nature
of all these bodies are the result of physical and dynamical
processes during their origin and subsequent evolution, and
their complex interactions with one another.
At the center of the solar system is the Sun, a rather
ordinary,main sequencestar. The Sun is classified spec-
trally as a G2 dwarf, which means that it emits the bulk of
its radiation in the visible region of the spectrum, peaking
at yellow-green wavelengths. The Sun contains 99.86% of
the mass in the solar system, but only about 0.5% of the an-
gular momentum. The low angular momentum of the Sun
results from the transfer of momentum to the accretion disk
surrounding the Sun during the formation of the planetary
system, and to a slow spin-down due to angular momentum
being carried away by the solar wind.
The Sun is composed of hydrogen (70% by mass), he-
lium (28%), and heavier elements (2%). The Sun produces
energy through nuclear fusion at its center, hydrogen atoms
combining to form helium and releasing energy that even-
tually makes its way to the Sun’s surface as visible sunlight.
The central temperature of the Sun where fusion takes place
is 15.7 million kelvins, while the temperature at the visible
surface, the photosphere, is∼6400 K. The Sun has an outer
atmosphere called the corona, which is only visible during
solar eclipses, or through the use of specially designed tele-
scopes called coronagraphs.
A star like the Sun is believed to have a typical lifetime
of 9 billion to 10 billion years on the main sequence. The
present age of the Sun (and the entire solar system) is esti-
mated to be 4.56 billion years, so it is about halfway through
its nominal lifetime. The age estimate comes from radioiso-
tope dating of meteorites.3.1 Dynamics
The planets all orbit the Sun in roughly the same plane,
known as theecliptic(the plane of the Earth’s orbit), and
in the same direction, counterclockwise as viewed from the
north ecliptic pole. Because of gravitational torques from
the other planets, the ecliptic is not inertially fixed in space,
and so dynamicists often use the invariable plane, which
is the plane defined by the summed angular momentum
vectors of all of the planets.
To first order, the motion of any body about the Sun
is governed by Kepler’s laws of planetary motion. These
laws state that (1) each planet moves about the Sun in an
orbit that is an ellipse, with the Sun at one focus of the
ellipse; (2) the straight line joining a planet and the Sun
sweeps out equal areas in space in equal intervals of time;
and (3) the squares of the sidereal periods of the planets
are in direct proportion to the cubes of the semimajor axes
of their orbits. The laws of planetary motion, first set down
by J. Kepler in 1609 and 1619, are easily shown to be the
result of the inverse-square law of gravity with the Sun as the