Europe's 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 Director - Technology, Science and Exploration for 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.
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."
Notes to Editors
"Herschel imaging of 61 Vir: implications for the prevalence of debris in low-mass planetary systems," by M. Wyatt et al., is published in the Monthly Notices of the Royal Astronomical Society 424, 2012.
"A DEBRIS disk around the planet hosting M-star GJ 581 spatially resolved with Herschel," by J.-F. Lestrade et al., is accepted for publication in Astronomy & Astrophysics. The observations were carried out as part of the DEBRIS (Disc Emission via a Bias-free Reconnaissance in the Infrared/Submillimetre) key project for Herschel, using both PACS and SPIRE instruments. DEBRIS is an international collaboration with researchers from UK, Canada, the USA, Spain, Germany, France, Switzerland and Chile.
Dr Mark Wyatt
Institute of Astronomy
University of Cambridge, UK
Tel: +44 (0)1223 337 517
Prof. Matt Griffin
SPIRE Principal Investigator
School of Physics and Astronomy
Tel: +44 (0)2920 874 203
Dr Chris North
UK Herschel Outreach Officer
School of Physics and Astronomy
Tel: +44 (0)2920 870 537
UK Space Agency
Tel: +44 (0)1793 418069