184
See also: The Oort cloud 206 ■ Exploring beyond Neptune 286–87I
n 1943, Irish astronomer
Kenneth Edgeworth suggested
that beyond Neptune and Pluto,
there existed a disk of icy bodies
that were formed at the dawn of the
solar system, but were too small
and widely spaced to accrete into
a planet. From time to time, they
were nudged into the inner solar
system, where they appeared as
comets. He published his idea in
Journal of the British Astronomical
Association, a periodical not widely
read in the US.Kuiper belt
In 1951, in the more prestigious
Astrophysical Journal, a Dutch-
American astronomer named
Gerard Kuiper suggested that
such a disk once existed but had
long since been dispersed by the
effects of Pluto’s gravity. It came
to be called the Kuiper Belt,
though some astronomers now
use “Edgeworth-Kuiper Belt.”
Then, in 1980, Uruguayan
astronomer Julio Fernández
realized that a belt of cometary
nuclei beyond Neptune wasneeded to supply the numbers of
short-period comets seen in the
inner solar system. Photographs
of the region were taken, a few
hours apart, and these were then
examined to see if any of the
objects had moved, indicating
that they were much closer than
the stars. More than 1,000 objects
have now been found in the Kuiper
belt. Most are larger than 60 miles
(100 km) across, since anything
smaller is too faint to detect. ■A RESERVOIR OF
COMETS EXISTS
BEYOND THE PLANETS
THE KUIPER BELT
IN CONTEXT
KEY ASTRONOMER
Kenneth Edgeworth
(1880 –1972)BEFORE
1781 and 1846 The discovery
of Uranus and Neptune leads to
discussion of where the outer
edge of the solar system lies.1930 Pluto is discovered.
Astronomers Frederick C.
Leonard and Armin O.
Leuschner suggest there may
be similar bodies out there.AFTER
1977 Charles Kowal discovers
Chiron, an icy centaur (minor
planet) beyond Saturn.1992 A Trans-Neptunian
Object (TNO—an object
orbiting at a distance greater
than Neptune’s) is discovered
by David Jewitt and Jane Luu.
2005 The discovery of Eris,
a TNO of a similar size to
Pluto, and TNOs Haumea and
Makemake, lead to Pluto being
demoted to a dwarf planet.The comets have remained
what they were from the
beginning—astronomical
heaps of gravel without
any cohesion.
Kenneth Edgeworth