New work by Dr Harold Levinson and partners shows that many of the most famous comets may have originally been captured from other solar systems billions of years ago when the Sun was still escaping the nursery.  Conventional theory states that most comets come from the Oort cloud, a huge spherical mass of debris left over from the formation of the solar system located out past the orbit of Neptune.  A passing star’s gravity can nudge a body in the Oort cloud (which can be as small as a boulder or larger than Pluto) and disturb its orbit slightly.  After millions of years of slowly shifting around other Oort cloud objects, it falls down towards the inner solar system where the Sun’s heat warms and melts the ice, causing the characteristic Earth-sized coma and swooping tail of a comet.

However, new computer models tracing the path’s of famous comets back in time show that many familiar comets did not come from the Oort cloud at all.  Instead, billions of years ago when the Sun was brand new and still forming its planets, it would have had a number of nearby siblings which all formed from the same primordial gas and dust clouds.  Over time the combined solar winds from all these baby stars would blow away the clouds, and the stars would separate, but while they were still close to each other their Oort clouds would almost overlap.  In this way, they would steal Oort cloud bodies from each other which would then be trapped in unstable orbits, eventually to fall in and become such comets at Halley’s comet, Hale-Bopp, and many other famous bright examples.

For a more detailed break-down on the team’s work, visit Universe Today here: http://www.universetoday.com/2010/06/10/many-famous-comets-may-be-visitors-from-other-solar-systems/