It seems we got off lightly in the cosmic lottery. Deadly comet impacts may be much rarer in our solar system than in others nearby.

We can't directly measure the rate of comet collisions in other solar systems but we can detect signs of the dust that such smashes kick up because the dust gets warmed by the star and so gives off infrared radiation. That radiation shows up as extra infrared in the spectrum of light coming from the star. Because such dust should dissipate quickly, it is thought to provide a good snapshot of the recent collision rate.

Jane Greaves of the University of St Andrews, UK, analysed observations by the Spitzer Space Telescope and found that the vast majority of sun-like stars near us have more dust than our solar system does and therefore have had more collisions in their vicinity. Our solar system may be one of the few that have been safe for life. Greaves presented her results at the Cosmic Cataclysms and Life symposium in Frascati, Italy, this month.
© C GSF/Caltech/JPL/NASA
This artist's concept illustrates a comet being torn to shreds around a dead star, or white dwarf, called G29-38.

About 25 per cent of the stars have a very strong dust signature. The rest of them have too little dust for it to be readily apparent when each spectrum is studied in isolation. Adding the measurements from these stars together, however, is like looking through a stack of slightly dusty windowpanes, making the total amount of dust easier to see. Greaves's analysis revealed that 90 per cent of solar systems are dustier and so more collision-ridden than our own.

Mark Wyatt of the University of Cambridge agrees that the rate of comet impacts is probably lower in our neck of the woods. But as the temperature of the dust found by Greaves indicates it tends to sit far from the parent stars, the impacts might not have affected life on habitable planets, which would sit closer to their star, he says.