Most of the comets that periodically appear in Earth's neighbourhood come from outside the Solar System, from a regions beyond Neptune called the Kuiper Belt and the Oort cloud. However, astronomers have recently discovered a group of comets among the asteroids lying between the orbits of Mars and Jupiter. Physicists David Jewitt and Henry Hsieh of the University of Hawaii have labelled these as "main belt" comets.

Their  proximity to the inner planets seems to lend credence to a theory that Earth's water came from comets that smashed into the surface billions of years ago. It was formerly thought that comets from the outer regions had been the source of Earth's water, but chemical analyses have since ruled out that possibility.

According to Jewitt, the presence of these objects will allow for additional studies of the chemical makeup of the solar system, and may even shed new light on its formation. "They're a window to some early epoch, back when objects were accreting," he adds.