Comets from beyond the Solar System

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.