Using ESA's Herschel space observatory, astronomers have discovered vast belts of comets surrounding two nearby planetary systems known to host nothing larger than Earth-to-Neptune-mass worlds. The comet reservoirs could have delivered life-giving oceans to the innermost planets. The scientists publish their work in papers in Monthly Notices of the Royal Astronomical Society and Astronomy and Astrophysics.

Last year, Herschel found that the dusty belt surrounding the nearby star Fomalhaut must be maintained by collisions between comets. In the new Herschel study, two more nearby planetary systems - GJ 581 and 61 Vir - have been found to host vast amounts of cometary debris.

Herschel detected the signatures of cold dust at -200C (70 Kelvin), in quantities that mean these systems must have at least 10 times more comets than in our own Solar System's Kuiper Belt, a reservoir of cometary nuclei located beyond the orbit of Neptune.

GJ 581, or Gliese 581, is a low-mass red dwarf star, the most common type of star in the Galaxy. Situated in the constellation of Libra, earlier studies have shown that it hosts at least four planets, including one that resides in the 'Goldilocks Zone' - the distance from the central sun where liquid surface water could exist.

Two planets are now confirmed around the star 61 Vir, which is just a little less massive than our Sun and lies in the constellation of Virgo. The planets in both systems are known as 'super-Earths', covering a range of masses between 2 and 18 times that of Earth.

Interestingly, however, there is no evidence for giant Jupiter- or Saturn-mass planets in either system. The gravitational interplay between Jupiter and Saturn in our own Solar System is thought to have been responsible for disrupting a once highly populated Kuiper Belt, sending a deluge of comets towards the inner planets in a cataclysmic event that lasted several million years.

"The new observations are giving us a clue: they're saying that in the Solar System we have giant planets and a relatively sparse Kuiper Belt, but systems with only low-mass planets often have much denser Kuiper belts," says Dr Mark Wyatt from the University of Cambridge, lead author of the paper focusing on the debris disc around 61 Vir.

"We think that may be because the absence of a Jupiter in the low-mass planet systems allows them to avoid a dramatic heavy bombardment event, and instead experience a gradual rain of comets over billions of years."

"For an older star like GJ 581, which is at least two billion years old, enough time has elapsed for such a gradual rain of comets to deliver a sizable amount of water to the innermost planets, which is of particular importance for the planet residing in the star's habitable zone," adds Dr Jean-Francois Lestrade of the Observatoire de Paris who led the work on GJ 581.

However, 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, 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," says Dr Lestrade.

"Herschel is finding a correlation between the presence of massive debris discs and planetary systems with no Jupiter-class planets, which offers a clue to our understanding of how planetary systems form and evolve," says Goran Pilbratt, ESA's Herschel project scientist.