592 Encyclopedia of the Solar System
than Neptune. Normally, this configuration would, sooner
or later, lead to close encounters between the two planets
that would eventually scatter Pluto away. However, close
encounters do not occur because Pluto is locked in a mean-
motion resonance where it goes around the Sun twice every
time Neptune goes around three times. So, every time Pluto
crosses the trajectory of Neptune, the giant planet is always
in one of three specific locations on its orbit, all very far
away from the crossing point. This resonance is known as
the 2:3 mean-motion resonance.
The other type of resonance that is important in the
Kuiper Belt is called a secular resonance. There are actually
two types of secular resonances. The first, which was dis-
cussed earlier, is a resonance between the precession rates
of the longitudes of perihelion. As discussed, this can lead to
changes in eccentricity. These resonances are identified by
the Greek lettervwith a numbered subscript that indicates
the resonant planet (1 for Mercury through 9 for Pluto).
In the Kuiper Belt, the perihelion secular resonance with
Neptune, orv 8 , is most important. The other type of secular
resonance occurs when the small body’s nodal precession
rate is the same as for a planet. This type of resonance can
cause significant changes in the inclination of the orbit of the
small body. These resonances are identified byv 1 x, where
xis the number of the resonant planet. For example, the
nodal resonance with Neptune is thev 18.
The dynamical structure of the Kuiper Belt has been
sculpted by a combination of mean-motion and secular res-
onances and by the evolution of these resonances during
the formation of Uranus and Neptune. We come back to
this issue in Sections 3 and 7.3. Orbital and Dynamical Structure of the
Trans-Neptunian Population
Figure 2 shows the distribution of the objects with semima-
jor axis larger than 30 astronomical units (AU) whose orbits
have been determined from observations spanning over at
least 3 years.
A glance at the figure reveals that the orbits of the
trans-Neptunian objects can be very diverse. The majorityFIGURE 2 The distribution of the objects with well-determined orbits, as to February 1, 2005. The
upper and lower panels show respectively the inclination and the eccentricity vs. semimajor axis. Two
different semimajor axis scales are used to illustrate the Kuiper Belt (left panels) and the scattered disk
(right panels) distributions. Red dots correspond to the scattered disk, magenta dots to the extended
scattered disk, blue dots to the classical Kuiper Belt, and green dots to the resonant populations. The big,
crossed circle denotes the orbit of Pluto. The vertical lines labeled 3:4, 2:3, and 1:2 mark the location of
the corresponding mean-motion resonances with Neptune. The two dotted curves on the lower panels
correspond to perihelion distancesq=30 AU andq=35 AU on the left, andq=30 AU and
q=38 AU on the right. These curves approximately bound the scattered disk orbital distribution.