ASTRONOMERS have discovered an object half the size of Pluto orbiting the Sun beyond Neptune which they say is threatening to rewrite theories about how the solar system was formed.

The body, which has been nicknamed Buffy, has an almost circular orbit which has baffled scientists as most objects in this area, known as the Kuiper Belt, have "highly eccentric" orbits.

The orbit is difficult to explain using previous theories about the solar system’s formation and has prompted a rethink.

Buffy was discovered by the Canada-France-Hawaii Telescope in 2004, but it was not until October this year that it started to become clear that it was something strange.

A statement by the Canada-France Ecliptic Plane Survey said: "Measurement of Buffy’s new position confirmed that Buffy was unlike any other previously-known object because it was on a nearly circular orbit while at a very large distance."

• The Royal Astronomical Society yesterday announced that Dr Clare Parnell, of St Andrews University, has won the Fowler Prize for Geophysics in recognition of "her outstanding contribution" to solar physics. "Dr Parnell’s research has had an international impact on the fundamental understanding of the heating of the solar corona, one of the major unsolved mysteries of solar physics in recent years," the RAS said.