A map of the Solar System, with the orbits of 22 centaurs in red. The orbits of the giant planets are shown in bright blue and trans-Neptunian objects (TNOs) in yellow. Credit: Duncan Steel
The discovery of giant comets in the outer edges of the Solar System over the past two years indicates that these bodies pose a greater threat to life on Earth than asteroids, according to new research.
The centaurs, as they are called, are massive comets that follow unstable orbits, bringing them across the paths of the giant planets Jupiter, Saturn, Uranus and Neptune in the outer edges of the Solar System.
The gravitational pull of these planets can sometimes deflect the objects and send them on a path towards Earth.
Centaurs are usually about 50 to 100 kilometres across and a single body can contain more mass than the combined population of asteroids that have crossed Earth’s path to date.
A team comprising astronomers from the University of Buckingham and Armagh Observatory have calculated that one will be deflected onto a path crossing Earth’s orbit about once every 40,000 to 100,000 years.
As they approach Earth, they will disintegrate, the team’s research shows, forming large fragments of debris and dust that increase the chances of a section of the comet hitting Earth.
Analysis of previous impacts on Earth throughout history, combined with a growing knowledge of the matter that exists in space near our planet, have allowed the team to calculate that a centaur arrived in our Solar System about 30,000 years ago.
If true, such a comet would have littered the inner Solar System with debris, some of which could have included bodies up to several kilometres across.
Professor Bill Napier of the University of Buckingham says:
“In the last three decades we have invested a lot of effort in tracking and analysing the risk of a collision between Earth and an asteroid.
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.”
Iain Todd is BBC Sky at Night Magazine's Staff Writer. He fell in love with the night sky when he caught his first glimpse of Orion, aged 10.