'He ain't heavy, he's my planet'

When does an asteroid become a planet? That question has astronomers debating the status of their latest discovery. Tucked secretively between Neptune and Pluto, EB 173 or "plutino," as researchers are calling it, is a newfound sphere of rock and ice that could be our 10th planet.

EB 173 joins a cast of thousands of objects orbiting the sun. Most of these asteroids and comets are too small to qualify for planet status, but EB 173 appears to make the grade, if only because of its gravity.

Most astronomers agree that to qualify as a planet an object should be orbiting a star, have enough gravity to form its own sphere, and weigh 1017 tons - that's more weight than 2 trillion Empire State buildings. EB 173 orbits the Sun and has gravity, but it's slightly underweight.

Plutino may be slim by planetary standards but it's no slouch. Bright enough to be spotted by amateur astronomers, the new planet is 373 miles in diameter and is only slightly smaller than Ceres, the largest asteroid in our solar system, which lies between Mars and Jupiter. The planet's discovery will be published in Astronomical Journal Letters later this month.

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