556 Encyclopedia of the Solar System
Since late 1992, however, our concept of the outer so-
lar system has evolved considerably, owing to a rapid set
of discoveries of faint (i.e., 22nd–25th astronomical mag-
nitude) largish bodies orbiting between 30 and 50 AU in
what is known as the Edgeworth–Kuiper Belt. The first
such objects were detected by David Jewitt and Jane Luu
using the University of Hawaii’s 2.2 m telescope on Mauna
Kea.
As of this writing at the end of 2006, over 1000 small
worlds with diameters of 100 to 2000 km have been discov-
ered in the Kuiper Belt, including some objects that have
clearly been scattered out of the giant planets’ region. Many
of these are apparently in the 2:3 mean-motion resonance
with Neptune that Pluto also occupies. The largest discov-
ered Kuiper Belt body is almost Pluto’s size. Some have
satellites. Beyond the Kuiper Belt, in the so-called scat-
tered belt, lie other large bodies, including 2003 UB313
(EoS) which is slightly larger than Pluto.
Because the Kuiper Belt census obtained to date has
covered only a tiny fraction of the ecliptic sky, it is estimated
that many times the discovered population exists. Current
models of the population of the region between 30 and
50 AU from the Sun now indicate that some 100,000 or more
objects with diameters larger than approximately 100 km
and perhaps several billion comets 1–20 km in diameter
reside there. The total mass of bodies currently in the 30–50
AU zone may amount to as much as 0.01M⊕, exceeding
the mass of the Asteroid Belt by more than an order of
magnitude.
Interestingly, various collisional evolution models that
have been developed by Don Davis (PSI), Paolo Farinella
(deceased), Alan Stern (SwRI), and Scott Kenyon (Harvard)
have provided strong evidence that the 100 km diameter
and larger bodies detected in the Edgeworth–Kuiper Belt
could not have grown there in the age of the solar system,
unless the mass of the primordial Kuiper Belt region was
many times higher—in the range of 10 to perhaps 50 Earth
masses.
Both the discovery of the rapidly expanding cohort of
objects found in the 30–50 AU zone, and the circumstan-
tial evidence that this region of the solar system was much
more heavily populated when the solar system was young,
finally provide a context for Pluto (and the putative Charon-
progenitor as well). We now see that Pluto did not form in
isolation and does not exist so today. Instead, Pluto is simply
one of a large number of significant miniplanets that grew
in the region beyond Neptune when the solar system was
young. Pluto’s presence there today is in large measure due
to its location in the stable 2:3 resonance with Neptune. The
question now has moved from why a small planet like Pluto
formed in isolation, to why a large population of objects
hundreds and thousands of kilometers in diameter formed
in the 30–50 AU zone without progressing to the forma-
tion of a larger planet there. Perhaps the answer lies in the
influence of “nearby” Neptune.Bibliography
Binzel, R. P. (1990). Pluto.Sci. Am., 252 (6), 50–58.
Stern, S. A. (1992). The Pluto–Charon system.Ann. Rev. As-
tron. Astrophys. 30 , 185–233.
Stern, S. A., and Mitton, J. (2005). “Pluto and Charon: Ice-
Dwarfs on the Ragged Edge of the Solar System,” 2nd Ed. John
Wiley & Sons, New York.
Stern, S. A., and Tholen, D. J., eds. (1997). “Pluto and Charon.”
Univ. Arizona Press, Tucson.
Tombaugh, C. W., and Moore, P. (1980). “Out of the Darkness:
The Planet Pluto.” Stackpole Books, Harrisburg, Pennsylvania.
Whyte, A. J. (1980). “The Planet Pluto.” Pergamon Press Ltd.,
Toronto, Canada.