Astronomers using the European Space Agency’s (ESA) Herschel space observatory have discovered massive comet belts orbiting two nearby planetary systems. The systems, named GJ 581 and 61 Vir, are known to host planets that range in mass from Earth-sized to Neptune-sized.

Neither of the systems shows evidence of planets that are close to Jupiter’s or Saturn’s mass. Scientists believe that Jupiter is responsible for disrupting our own Kuiper Belt in the past, sending comets raining toward the inner planets millions of 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,” said Mark Wyatt, lead author of a paper describing observations of the debris disc around 61 Vir and an Astronomer at the University of Cambridge. “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.”

Astronomers measured signatures of cold dust at -200 degrees Celsius in the two systems, suggesting that they must have 10 times or more comets than are found in our own solar system’s Kuiper Belt.

“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,” said Jean-Francois Lestrade, who led the work on GJ 581 at the Observatoire de Paris.

Lestrade also pointed out, however, that the amount of dust detected around GJ 581 suggests collisions between comets, which could be caused by a Neptune-sized planet that is too far the star to be currently detected.

A recent NASA study using data from the Spitzer space telescope suggest that Jupiter-sized planets just outside a system’s “snow line” might also be key to shaping a system where planets can sustain life as we know it.

“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,” said Göran Pilbratt, ESA’s Herschel project scientist.