Earth could be at higher risk of being hit by a comet than widely thought, according to astronomers who have suggested keeping a closer eye on distant objects.



Most studies of potential hazards involved objects in the asteroid belt roughly between Mars and Jupiter, the researchers said. But they noted that the discovery in the last two decades of hundreds of giant comets dubbed “centaurs” requires expanding the list of potential hazards.



These balls of ice and dust, typically 30-60 miles (50-100km) wide, have unstable, elliptical orbits that start far beyond Neptune, the most distant planet from the Sun.

Their orbits cross those of Jupiter, Saturn, Uranus and Neptune, whose gravity fields occasionally deflect a comet towards Earth – once about every 40,000 to 100,000 years.

As they draw closer to the Sun the comets gradually break up, which is what causes the trademark cometary debris tail – “making impacts on our planet inevitable”, the research team write in Astronomy and Geophysics, journal of the Royal Astronomical Society.

“The disintegration of such giant comets would produce intermittent but prolonged periods of bombardment lasting up to 100,000 years.”

Royal Astronomical Society diagram of the solar system, with the orbits of seventeen trans-Neptunian objects shown in yellow, out of several thousand known, and 22 ‘centaur’ comets out of about 400 that have been identified. Photograph: Duncan Steel/AFP/Getty Images

They argue that “assessment of the extraterrestrial impact risk based solely on near-Earth asteroid counts underestimates its nature and magnitude”.

A single centaur contains more mass than the entire population of Earth-crossing asteroids discovered to date, they write.

“In the last three decades we have invested a lot of effort in tracking and analysing the risk of a collision between the Earth and an asteroid,” said co-author Bill Napier of the University of Buckingham.

“Our work suggests we need to look beyond our immediate neighbourhood too, and look out beyond the orbit of Jupiter to find centaurs.

“If we are right then these distant comets could be a serious hazard and it’s time to understand them better.”

Scientists believe a comet bombardment may have kickstarted life on Earth by bringing water and organic molecules.

A comet strike is also a leading contender for having ended the reign of the dinosaurs 65 million years ago.