Herschel delves into dusty space debris to reveal the secrets of solar system formation

Herschel image of Gliese 581 with sketch
Herschel images of Gliese 581, with sketches of the planetary system and newly-discovered debris disc shown (not to scale). Image credit: ESA/Herschel/PACS/DEBRIS consortium.

The Herschel Space Observatory has discovered vast dust rings surrounding two nearby planetary systems known to host only relatively small worlds, with masses between Earth and Neptune. The discovery of these comet reservoirs, which could have delivered life-giving oceans to the system’s innermost planets, suggests that debris discs may survive more easily in planetary systems that don’t include massive, Jupiter-like planets.

Part of the data for the study was gathered by SPIRE (Spectral and Photometric Imaging Receiver) – a UK-led instrument on board Herschel which operates at wavelengths that have not been exploited before to study the star formation history of the Universe.

Dr Chris Castelli, Acting Chief Executive of the UK Space Agency, said, "Amazing results like this perfectly illustrate why Herschel is a flagship mission for the UK. Herschel and its many UK scientists continue to push the boundaries of space exploration to uncover the secrets of our cosmos."

The Herschel astronomers discovered the debris discs, or comet belts, around the nearby planetary systems called Gliese 581 and 61 Virginis. Both systems are known to host super-Earth planets - extrasolar planets with a mass higher than Earth's, but substantially below the mass of the Solar System's gas giants.

Herschel detected signatures of cold dust at 200oC below freezing, in quantities that mean these systems must have at least 10 times more comets than in our own Solar System's Kuiper Belt.

Gliese 581 is a low-mass M dwarf star, the most common type of star in the Galaxy, and has less than a third the mass of the Sun. Earlier studies have shown that it hosts at least four planets, including one that resides in the 'Goldilocks Zone' – the distance from the central star where liquid water could exist on the surface.

Herschel image of 61 Virginis with sketch
Herschel images of 61 Virginis, with sketches of the planetary system and newly-discovered debris disc shown (not to scale). Image credit: ESA/Herschel/PACS/DEBRIS consortium.

Two planets are also confirmed around G-type star 61 Virginis, which is just a little less massive than our Sun. Interestingly, 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, a belt of icy bodies out beyond the orbit of Neptune. This disruption, which occurred over 3 billion years ago, sent 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 Virginis.

"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."

Professor Matt Griffin, of Cardiff University and lead scientist of Herschel's SPIRE instrument, added, "With Herschel we are now able to study the way planetary systems are built up and the interaction between the planets and the huge clouds of comets and debris around the parent star. This is great news for our quest to understand how our Solar System formed, and how it compares to the vast numbers of planetary systems in the Galaxy."

These observations were made as part of the DEBRIS Herschel observing programme, and are published in the journals Monthly Notices of the Royal Astronomical Society and Astronomy and Astrophysics.