Astronomers from Lowell Observatory, the Massachusetts Institute of
Technology, and the Large Binocular Telescope Observatory have discovered
an icy planetary body orbiting the Sun beyond Neptune in the Kuiper Belt
roughly equal in size to Pluto’s moon Charon.

“This object is intrinsically the brightest Kuiper Belt Object found
so far,” says Lowell Observatory Director Robert Millis, leader of the
survey team. “The exact diameter of 2001 KX76 depends on assumptions
that astronomers make about how its brightness relates to its size.
Traditional assumptions make it the biggest by a significant amount,
while others make it larger by at least 5 percent.”

Assuming a reflectivity (or albedo) of 4 percent, 2001 KX76 would have
a diameter of approximately 1,270 kilometers (788 miles), bigger than
Ceres, the largest known asteroid. For comparison, Pluto’s moon Charon
has an estimated diameter of 1,200 kilometers (744 miles).

Earlier this year, a Kuiper Belt Object (KBO) called 20000 Varuna was
announced with an estimated diameter of 900 kilometers, based on a
calculated reflectivity of 7 percent. Applying this albedo to 2001 KX76
gives it a diameter of roughly 960 kilometers (595 miles).

2001 KX76 was discovered in the course of the Deep Ecliptic Survey,
a NASA-funded search for KBOs being conducted by the Lowell-MIT-LBT team
using the National Science Foundation’s telescopes at Kitt Peak National
Observatory near Tucson, AZ, and Cerro Tololo Inter-American Observatory
in Chile. The team spotted 2001 KX76 in deep digital images of the
southern sky taken with the 4-meter Blanco Telescope at Cerro Tololo
on May 22 by James L. Elliot of MIT and Lawrence H. Wasserman of Lowell
Observatory.

2001 KX76 is currently at a distance of just over 6.4 billion kilometers
(4 billion miles) from the Sun. Its orbit is inclined by approximately
20 degrees with respect to the orbital plane of the major planets, but
the detailed shape of its orbit remains uncertain. Available evidence
suggests that the newly discovered KBO may be in an orbital resonance
with Neptune, orbiting the Sun three times for each time that Neptune
completes four orbits.

The brightness and colors of 2001 KX76 have been measured by Elliot,
Susan Kern, and David Osip, all of MIT, with the Raymond and Beverly
Sackler Magellan Instant Camera (MagIC) on the 6.5-meter Magellan
Telescope at Las Campanas Observatory in Chile. The object has a
distinctly reddish color typical of many primitive bodies in the outer
Solar System.

“2001 KX76 is so exciting because it demonstrates that significant
bodies remain to be discovered in the Kuiper Belt,” Millis explains.
“We have every reason to believe that objects ranging up to planets
as large or larger than Pluto are out there waiting to be found.
Until the Kuiper Belt has been thoroughly explored, we cannot pretend
to know the extent or the content of the Solar System.”

The existence of the Kuiper Belt was postulated by J. A. Fernandez and
by M. Duncan, T. Quinn, and S. Tremaine in the 1980s to explain the
origin of short-period comets. These comets move around the Sun in the
same direction as the planets, and are found in orbits that are tipped
only modestly with respect to the ecliptic plane. These researchers
showed that short-period comets could not have originated from the more
distant spherical Oort Comet Cloud as originally believed. They
predicted that a second, more flattened reservoir of “proto-comets”
must lie beyond the orbit of Neptune.

The first Kuiper Belt Object was found in 1992 by David Jewitt and
Jane Luu of the University of Hawaii. Since then, astronomers have
found over 400 KBOs, but tens of thousands likely remain to be
discovered. These objects are believed to be remnants from the
formation of the Solar System, and consequently are among the most
primitive and least-evolved objects available for study by planetary
astronomers.

About one-quarter of the known KBOs have been found by the Deep
Ecliptic Survey Team. Other members of the team are Marc Buie of
Lowell and Mark Wagner of the Large Binocular Telescope Observatory
on Mount Graham, AZ. The Deep Ecliptic Survey was recently awarded
formal survey status at the National Optical Astronomy Observatory
(NOAO), assuring that this reconnaissance of the outer Solar System
will continue for the next three years.

Much more precise measurement of KBO diameters will be possible with
NASA’s upcoming Space Infrared Telescope Facility (SIRTF) mission, due
for launch in 2002.

Kitt Peak and Cerro Tololo Inter-American Observatory are part of NOAO,
which is operated by the Association of Universities for Research in
Astronomy (AURA), Inc., under a cooperative agreement with the National
Science Foundation.

The survey team’s research is supported by the NASA Planetary Astronomy
Program through grants to Lowell Observatory, Flagstaff, AZ,
and MIT in Cambridge, MA.