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Eccentricity has been redefined in our solar system with the measurements of a pair of objects that orbit each other out in the dark and cold stomping grounds of Pluto.

Last April, researchers announced that an object named 1998 WW31, a comet-like body orbiting the Sun in the solar system's fringes, had a companion. A new study, which will be published in the April 18 issue of the journal Nature, finds that the pair's common orbit brings them as close as 2,500 miles (4,000 kilometers) and as far as 25,000 miles (40,000 kilometers) from each other.

This relative discrepancy, called eccentricity, is the largest known for any two objects in the entire solar system, astronomers said.

Round and round

Everything in our solar system orbits something. Planets, asteroids and comets orbit the Sun. Moons orbit planets. Some asteroids even orbit each other.

"For the first time, we have a nice and clean orbit of a binary system in the Kuiper belt," said Christian Veillet of the Canada-France-Hawaii Telescope in Hawaii. Veillet led the new study, which combined data from the Hubble Space Telescope with ground-based observations.

The Kuiper Belt is a vast region of space beyond the orbits of Pluto and Neptune. Many astronomers include Pluto and its moon, Charon, in a class of giant, icy boulders called Kuiper Belt Objects, or KBOs. A half-dozen other binary KBOs have been found in the past year.

The newly studied KBOs orbit each other around a common gravitational midpoint every 570 days. Together, they also orbit the Sun.

It's not known how the two KBOs came to be a pair. They may have been created as twins, or one larger object might have been split up in a collision.

Knowing the orbital mechanics, however, yields a few clues about the objects, Veillet told The combined 1998 WW31 weigh about 5,000 times less than Pluto and Charon, the research shows.

If both objects have a similar mass and a similar surface -- neither of which is known for sure -- then Veillet said one is about 62-75 miles in diameter (100-120 kilometers) and the other is roughly 75-93 miles wide (120-150 kilometers).

Verge of knowledge

Many astronomers see the Kuiper Belt, which extends well beyond the orbits of Pluto and Neptune, as one of the last unexplored regions of our solar system. The objects there are thought to be leftovers from the birth of the Sun and planets, and exploring them will probably yield important clues about the formation and evolution of the entire solar system.

But little is known about KBOs because they are too far away to be studied in detail.

Researchers think KBOs may share physical characteristics with asteroids, which are primarily rock and metal, and also with comets, which are made mostly of ice and dust.

Veillet points out that if astronomers can find a binary KBO system in which one of the objects eclipses the other from our point of view, then an exact determination of size can be made by observing how their mutual reflected sunlight changes during the eclipse. Studying a pair during an eclipse would also provide an estimate for density and give some information about the surface composition, he said.

SOURCE:<a href="[] Objects at Solar System's Edge Redefine Eccentricity</a>

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Eccentricity Redefined 161 Tripp 24-Apr-02 07:09
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