Astronomers using ESA's Herschel space observatory have discovered enormous comet belts surrounding two nearby planetary systems known to host only Earth-to-Neptune-mass worlds.
The comet reservoirs could have delivered life-giving oceans to the innermost planets, they say.
Scientists already knew that the dusty belt surrounding Fomalhaut must be maintained by collisions between comets. But the new study reveals that two more nearby planetary systems - GJ 581 and 61 Vir - also host vast amounts of cometary debris.
Signatures of cold dust at 200ºC below freezing indicate that these systems must have at least 10 times more comets than in our own solar system's Kuiper Belt.
GJ 581, or Gliese 581, is a low-mass M dwarf star, and is known to host at least four planets, including one that could potentially support life. Two planets have been confirmed around G-type star 61 Vir.
The planets in both systems are known as 'super-Earths', between two and 18 times that of Earth.
Interestingly, though, there's no evidence for giant Jupiter- or Saturn-mass planets in either system. And it's the gravitational interplay between Jupiter and Saturn in our own solar system that's thought to have been responsible for disrupting a once highly populated Kuiper Belt, sending a deluge of comets towards the inner planets.
In order to produce the vast amount of dust seen by Herschel, collisions between the comets are needed, which could be triggered by a Neptune-sized planet residing close to the disc.
"Simulations show us that the known close-in planets in each of these systems cannot do the job," says Dr Jean-Francois Lestrade of the Observatoire de Paris.
"But a similarly-sized planet located much further from the star - currently beyond the reach of current detection campaigns - would be able to stir the disc to make it dusty and observable."