PARIS (TIP): Planet Earth could be at higher risk of a space rock impact than widely thought, according to astronomers who today suggested keeping a closer eye on distant giant comets.

Most studies of potential Earth-smashers focus on objects in the asteroid belt roughly between Mars, Earth’s outside neighbour, and Jupiter on its other flank, said the researchers.

But they noted that the discovery in the last two decades of hundreds of giant comets dubbed centaurs, albeit with much larger orbits, requires expanding the list of potential hazards.

These balls of ice and dust, typically 50-100 kilometres wide, have unstable, elliptical orbits that start way beyond Neptune, the most distant planet from the Sun. Their orbits cross those of the giant planets Jupiter, Saturn, Uranus and Neptune, whose gravity fields occasionally deflect a comet towards Earth – once about every 40,000-100,000 years.

As they draw closer to the Sun, the comets would gradually break up, which is what causes the trademark cometary debris tail – “making impacts on our planet inevitable”. “The disintegration of such giant comets would produce intermittent but prolonged periods of bombardment lasting up to 100,000 years,” the research team wrote in the Royal Astronomical Society journal, Astronomy and Geophysics.

And they argued that “assessment of the extraterrestrial impact risk based solely on near-Earth asteroid counts, underestimates its nature and magnitude.”