Minor Planet Found Near Pluto
An asteroid recently discovered between Neptune and Pluto might be larger than almost all others in the solar system.
by Vanessa Thomas
Astronomers from the United States and Venezuela have spotted an asteroid that may nearly be the largest known. Lying between the orbits of the two outermost planets, 2000 EB173 is estimated to be about 370 miles (600 kilometers) wide, approximately one-quarter the size of Pluto. If the estimated size of this far-out “minor planet” is correct, 580-mile-wide Ceres would be the only asteroid that could claim superiority over it.
The discoverers of the asteroid designated 2000 EB173 are members of the Quasar Equatorial Survey Team (QUEST) from Yale University, Indiana University, Centro de Investigaciones de Astronomía (CIDA), and Universidad de Los Andes. QUEST primarily uses the 1-meter (39-inch) CIDA telescope to repeatedly scan sections of the sky to search for quasars, supernovae, and other variable objects. But this technique can easily be adapted to look for Trans-Neptunian Objects (TNOs) as well.
On the night of March 15, the team made two scans four hours apart of a patch of sky about 67 degrees wide. The astronomers looked near the ecliptic for matching objects separated by 5 to 20 arcseconds (the distance TNOs at opposition appear to move in four hours). A computer program removed stationary objects and those not significantly bright compared to background “noise,” leaving only one candidate.
After spotting EB173, the astronomers examined scans from previous nights and found the asteroid in images dating back to February 11. From the asteroid’s motion during those two months, the team calculated its orbit. When they looked for the asteroid again in June, they successfully found EB173 where they predicted it would be.
Like Ceres, most asteroids lie between Mars and Jupiter. However, other minor planets (another term astronomers use for asteroids) roam throughout the solar system, from the sun-hugging Atens inside the orbit of Earth to the Kuiper Belt Objects beyond Pluto. According to the QUEST team, the characteristics of EB173's orbit are typical of a Plutino, which is a type of TNO that has an orbit mimicking Pluto’s. Like Pluto, EB173 and other Plutinos travel around the sun in about 250 years and participate in a resonance dance with Neptune, in which they pass inside Neptune’s orbit for a short time and complete two orbits for every three Neptunian years.
EB173 is the brightest of all the nearly 300 TNOs discovered so far. Assuming that all TNOs have a common albedo, or reflectivity, the QUEST astronomers say that EB173’s magnitude corresponds to an object with a 370-mile diameter. But based on the possible variation of the compositions and albedos of TNOs, other astronomers caution that EB173 could be anywhere from 180 to 440 miles across.
Even if later observations determine that EB173 is not big enough to bump 360-mile-wide Vesta out of its second-place spot, the newly discovered asteroid will still be among the largest known in the solar system. And some astronomers suspect there may be many more of similar size yet undetected. In a paper describing EB173’s discovery submitted to the Astrophysical Journal Letters, the QUEST astronomers explain that an extrapolation of the area of sky covered in their one-night search to the entire ecliptic indicates that more than 100 TNOs with magnitudes and sizes like EB173’s could exist. They predict that “A dedicated search with a telescope of similar capabilities to our own would discover most of these bodies in a few years.”
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