Do missing Jupiters mean massive comet belts?
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 -200ºC (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 Göran Pilbratt, ESA's Herschel project scientist.
Images and captions
An expanded diagram of the debris disc and planets around the star known as Gliese 581, superimposed on a composite Herschel image assembled from separate observations made with its PhotoArray Camera and Spectrometer (PACS) at 70, 100 and 160 micrometre wavelengths.
The white region in the lower centre of the image is the emission that originates almost entirely from the disc, with only a small contribution from the unseen Gliese 581.
The line drawing superimposed on the Herschel image gives a schematic representation of the location and orientation of the star, planets and disc, albeit not to scale.
The black oval outline sketched onto the Herschel data represents the innermost boundary of the debris disc; the approximate location of the outermost boundary is represented by the outer set of dashed lines.
Gliese 581's planets have masses between 2 and 15 Earth masses and are all located within 0.22 Astronomical Units (AU, where 1 AU is the distance between Earth and our Sun or about 150 million km) of the central star. A vast debris disc extends from approximately 25 AU to 60 AU.
Background galaxies are also visible in the Herschel image, seen here as the tail-like feature, visible in yellow/red to the right of the disc, and the yellow/red objects in the upper left corner of the image.
An expanded diagram of the debris disc and planets around the star 61 Vir, superimposed on a composite Herschel PACS image assembled from separate observations at 70, 100 and 160 micrometre wavelengths.
The white region at bottom centre in the image is the emission that originates almost entirely from the disc, with only a small contribution from the unseen 61 Vir.
The line drawing superimposed on the Herschel image gives a schematic representation of the location and orientation of the star, planets and disc, albeit not to scale. The black oval outline sketched onto the Herschel data represents the innermost boundary of the debris disc; the approximate location of the outermost boundary is represented by the outer set of dashed lines. It is not possible to see that the part of the disc closest to the star is empty of dust due to smearing of the Herschel data.
The two planets around 61 Vir have masses between 5 and 18 Earth masses and are both located within 0.22 AU of the central star. A vast debris disc extends from approximately 30 AU to 100 AU.
The new work appears in two papers:
"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 journal 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 Canada, the USA, the UK, Spain, Germany, France, Switzerland and Chile.
This work was supported by the European Union through ERC grant number
279973 (Debris in Extrasolar Planetary Systems), though this press release reflects only the author's views and the Union is not liable for any use that may be made of the information contained therein.
Notes for editors
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