London, July 28 : Astronomers have suggested that comets might be fragments of larger bodies that crumbled as they entered the inner solar system, seeing the puzzling abundance of comets in short solar orbits.
Short-period comets take less than 200 years to circle the sun and are thought to originate in the Kuiper belt of icy objects beyond Neptune.
Some Kuiper-belt objects (KBOs) are in vulnerable orbits that allow the gravity of the outer planets to tug them inwards, where the sun's heat turns them into comets.
However, there seem to be too few KBOs in these vulnerable orbits to maintain the 10,000-strong population of short-period comets, which is continually thinned as they burn out, fall into the sun or get ejected from the solar system.
According to a report in New Scientist, when Kathryn Volk and Renu Malhotra at the University of Arizona in Tucson, US, calculated the rate at which Kuiper-belt objects should be drawn inwards, their result was just 1/500th of the rate needed.
This disparity would disappear if some of the largest KBOs crumble into 100 to 1000 pieces as they are drawn into the inner solar system.
The idea is supported by the fact that comets have been seen to break up from the stress of encounters with planets.
"Comets are thought generally to be pretty fragile," said Malhotra.