Pluto's little brother

Posted: May 25, 2001

Astronomers have discovered that an object in the distant Kuiper Belt is as large as the largest asteroids, raising new questions about the classification of Pluto as a planet.

Astronomers from Hawaii and New York determined that the Kuiper Belt object (KBO) 20000 Varuna is larger than all but Ceres, the largest asteroid, and bigger than anything in the Kuiper Belt than Pluto and its moon Charon

Comparison of Trans-Neptunian Objects. Photo: Institute for Astronomy, University of Hawaii
Planetary scientists had speculated that Varuna, originally designated 2000 WR106, was particularly large when the object was discovered last November. However, past estimates of the size of Varuna and other KBOs were based on a guess for the albedo, or reflectivity, of such objects of 4 percent. If the objects were more reflective than believed, for example, then the objects would have to be correspondingly smaller to account for their observed brightness; likewise, a darker object would have to be larger.

In an effort to independently determine both the size and albedo of Varuna, astronomers David Jewitt and Herve Aussel of the University of Hawaii and Aaron Evans of the State University of New York at Stony Brook simultaneously observed the KBO using two telescopes atop Mauna Kea, Hawaii. One observed Varuna at visible wavelengths to record the amount of sunlight it reflected, while the other observed at submillimeter wavelengths -- a region of the electromagnetic spectrum beyond the infrared -- to measure the amount of blackbody radiation the body emitted.

Combined, the two data sets allowed astronomers to compute what diameter and albedo Varuna must have to explain the observations. They concluded that Varuna is approximately 900 kilometers in diameter, with an albedo of 7 percent. This is half again as large as the estimated size of the next-largest KBO, and approaches the size of Charon, which has a diameter of 1175 km. By comparison, Ceres, the largest asteroid, is 932 km in diameter; no other asteroid is more than 530 km across.

This measurement of Varuna's size may have an effect on arguments regarding the classification of Pluto as a planet. Supporters of planet status for Pluto have argued that Pluto, about 2275 km across, and Charon are much larger than any KBO, a gap that Varuna's discovery has closed somewhat.

"The results suggest that Pluto and Charon are not uniquely large objects, and that a continuum of sizes may exist," astronomers Stephen Tegler and William Romanishin note in an editorial that accompanied the Nature paper. "We can now imagine that bodies even larger and more distant than Pluto will be found."

Varuna's albedo has also raised new questions about differences among KBOs. While Varuna is somewhat more reflective that earlier estimated, it is march darker than Charon, which has an albedo of over 30%, even though the two objects have similar sizes. One possible explanation, according to Tegler and Romanishin, is that Charon's more reflective surface of water ice is the result of the process -- either collision or capture -- that made it a moon of Pluto.

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