596 Encyclopedia of the Solar System
FIGURE 5 The dynamical lifetime for test
particles with initial eccentricity of 0.01
derived from 1 billion year integrations by
M. Duncan, H. Levison, and M. Budd.
This plot is similar to that of Fig. 4 except
that coordinates are semimajor axis and
inclination (instead of semimajor axis and
eccentricity) and a different color table was
used for the solid bars. In addition, the red
and yellow curves show the locations of
Neptune longitude of perihelion secular
resonances (v 8 ) and the Neptune
longitude of the ascending node secular
resonances (v 18 ), respectively. The green
lines show the location of the important
Neptune mean motion resonances.4. Correlations Between Physical and
Orbital Properties
The existence of two distinct classical Kuiper Belt popu-
lations, called the hot (i> 4 ◦) and cold (i< 4 ◦) classical
populations, could be caused in one of two general man-
ners. Either a subset of an initially dynamically cold popu-
lation was excited, leading to the creation of the hot classical
population, or the populations are truly distinct and formed
separately.
One manner in which we can attempt to determine
which of these scenarios is more likely is to examine the
physical properties of the two classical populations. If the
objects in the hot and cold populations are physically differ-
ent, it is less likely that they were initially part of the same
population.
The first suggestion of a physical difference between the
hot and the cold classical objects came from the obser-
vation that the intrinsically brightest classical belt objects
(those with lowestabsolute magnitudes) are preferen-
tially found with high inclination. Figure 6 shows the distri-
bution of the classical objects in an inclination vs. absolute
magnitude diagram. As one sees, for an absolute magnitude
H> 5 .5, there is a given proportion between the number
of objects discovered in the cold and the hot populations
respectively. This ratio is completely different forH<5.5,
where cold population objects are almost absent. All the
biggest classical objects, such as, for instance, 50000 Quaoar,
20000 Varuna, 19521 Chaos, 28978 Ixion, 2005 FY 9 , and
2003 EL 61 have inclinations larger than 5◦. Their median
inclination is 12◦. It has been argued that this is a result
of an observational bias because the brightest objects have
been discovered in wide field surveys not confined around
the ecliptic, which are thus more likely to find large incli-
nation objects than the deep ecliptic surveys that detected
the fainter bodies. However, a recent survey for bright ob-
jects, which covered∼70% of the ecliptic, found many hot
classical objects but few cold classical objects, confirming
that the effect illustrated in Fig. 6 is real.
The second possible physical difference between hot and
cold classical Kuiper Belt objects is their colors. With the
name “color” astronomers generically refer to the slope of
the spectrum of the light reflected by a trans-Neptunian ob-
ject at visible wavelengths, relative to that of the light emit-
ted by the Sun. “Red” objects reflect more at long than at
short wavelengths, while “gray” objects have a more or less
uniform reflectance. Colors relate in a poorly understood
manner to objects’ surface composition. It has been shown
and repeatedly confirmed that, for the classical belt, the in-
clination, and possibly the perihelion distance, is correlated
with color. In essence, the low inclination classical objects
tend to be redder than higher inclination objects. More in-
terestingly, colors naturally divide into distinct red and gray
populations at precisely the location of the divide between
the inclinations of the hot and cold classical objects. These
populations differ at a 99.9% confidence level. Interest-
ingly, the cold classical population also differs in color from
the Plutinos and the scattered objects at the 99.8 and 99.9%
confidence level, respectively, while the hot classical popu-
lation appears identical in color to these other populations.
The possibility remains, however, that the colors of the
objects, rather than being markers of different populations,