If confirmed, as scientists at the University of Arizona's Spacewatch program and Harvard University expect, it would be Jupiter's 17th moon. It wasn't easy to find. The unnamed moon, which is probably only four km or less in diameter, is the smallest object orbiting any of the nine planets in our solar system, said Harvard astronomer Brian Marsden. The tiny rock is about 80 million km from Jupiter - about 200 times the distance between Earth and its moon. It circles the big planet in a long, slow orbit.
Astronomers look for new moons, asteroids and comets by tracking their motion.
That's what made the tiny moon tough to find. It moves so slowly, taking 774 days to circle Jupiter compared with our moon's 28 days, that it didn't seem to be moving around the planet at all when first spotted in October. Arizona astronomers took it for an asteroid, a celestial body that circles the sun. This week, Harvard astronomers, trying out a new computer program that calculates orbits, recognised it was a moon, a body that circles a planet. The discovery seems legitimate, said Bob Jacobson, a NASA Jet Propulsion Laboratory scientist.