Pluto has suffered another demotion with scientists discovering a very bright, icy body possibly even larger than the tiny ice-planet's moon Charon and orbiting the Sun in the same celestial neighborhood.
The object, called 2001 KX76, orbits beyond Neptune in the Kuiper Belt of early solar system ice-and-rock smidgeons.
"This object is intrinsically the brightest Kuiper Belt Object found so far," says Lowell Observatory Director Robert Millis, leader of the survey team. And brightness is how astronomers calculate size.
Under one assumption about the object's reflectivity (albedo), 2001 KX76 is probably 788 miles (1,270 kilometers) in diameter, making it 44 miles larger across than Charon and even bigger than Ceres, the largest known asteroid.
"2001 KX76 is so exciting because it demonstrates that significant bodies remain to be discovered in the Kuiper Belt," Millis said.
"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."
Varuna verity
The truth is that 2001 KX76 could be a bit smaller than Charon. It all depends on how reflective such objects are.
For instance, earlier this year, a Kuiper Belt Object (KBO) called 20000 Varuna was announced with an estimated diameter of 558 miles (900 kilometers), based on a calculated reflectivity of 7 percent.
Applying this albedo to 2001 KX76 gives it a diameter of only 595 miles (960 kilometers).
Reddish and resonant
2001 KX76 was discovered in the course of the Deep Ecliptic Survey, a NASA-funded search for KBOs.
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 the Massachusetts Institute of Technology (MIT) and Lawrence H. Wasserman of Lowell Observatory.
2001 KX76 currently is just over 4 billion miles (6.4 billion kilometers) 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 object has a distinctly reddish color typical of many primitive bodies in the outer Solar System.
KBOs
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.
The Deep Ecliptic Survey
About one-quarter of the known KBOs have been found by the Deep Ecliptic Survey Team.
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.
The survey team's research is supported by the NASA Planetary Astronomy Program through grants to Lowell Observatory and MIT. The team includes astronomers from Lowell Observatory, MIT and the Large Binocular Telescope Observatory.
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