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Low-mass planets make good neighbours for debris discs
by Staff Writers
Paris, France (ESA) Nov 29, 2012

This image shows the debris disc around the star 61 Virginis as seen with Herschel at far-infrared wavelengths. 61 Virginis is a G-type star that is known to host at least two planets, which have masses equivalent to about five and 18 times the mass of Earth and orbit their parent star at 0.05 and 0.22 AU, respectively - much closer than Mercury is to the Sun. The debris disc discovered with Herschel extends between 30 and 100 AU from the star - well beyond the orbits of its known planets. A sketch of the debris disc and the orbits of the two known planets is superimposed on the image. The relative sizes of the disc and planetary orbits are not drawn to scale: the disc is about 100 times larger than the orbit of the outermost planet. The image is based on data gathered with the PACS instrument on board Herschel at wavelengths of 70 microns (blue), 100 microns (green) and 160 microns (red), respectively. Credit: ESA/Herschel/PACS/Mark Wyatt, University of Cambridge, UK.

Astronomers using ESA's Herschel Space Observatory have detected massive debris discs around 61 Virginis and Gliese 581, two nearby stars that are known to host super-Earth planets.

The study also reveals that debris discs are preferentially found in planetary systems with low-mass planets rather than in those hosting high-mass planets.

This suggests that debris discs may survive more easily in the absence of very massive planets, and highlights the importance of debris discs in the study of planet formation.

The formation of planets around a newly born star is a dynamical process that may last several hundreds of millions of years. Debris discs are a by-product of planetary formation.

They consist of everything orbiting a star that is not a planet: asteroids, comets, planetesimals and the dust that derives from them. In the Solar System, the debris disc is mainly concentrated in two belts: the asteroid belt, between the orbits of Mars and Jupiter, and the Kuiper Belt, well beyond the orbit of Neptune.

Debris discs around stars other than the Sun were first detected in the early 1980s via observations at infrared wavelengths. Several hundreds of them are known to date and a few dozen have been resolved.

Astronomers are now using ESA's Herschel Space Observatory to search for debris discs around a variety of stars in our Galaxy, the Milky Way, deeper and more thoroughly than it has been possible so far.

By exploiting the telescope's unprecedented sensitivity and resolution, it is possible to detect very faint discs and to image them in great detail. In particular, the Disc Emission via a Bias-free Reconnaissance in the Infrared/Submillimetre (DEBRIS) Open Time Key Programme has been designed to seek discs in over 400 of the stars closest to the Sun.

Two new studies based on data from the DEBRIS survey have successfully detected debris discs around a handful of nearby stars, known to host planets. What's more, these stars all appear to host super-Earths - planets with low masses, between the mass of Earth and that of Neptune.

The results of these studies hint that the presence of debris discs which are bright enough to be detected with current observatories could be related to whether their parent star has low-mass planets in orbit around it.

"One of the debris discs we have resolved with Herschel surrounds the star 61 Virginis, which is very similar to our Sun in terms of its mass, temperature and age," comments Mark Wyatt from the Institute of Astronomy in Cambridge, UK. Wyatt led the analysis of G-type stars - the same spectral type as the Sun - in the DEBRIS survey.

The G-type star 61 Virginis is known to host at least two planets. These have masses equivalent to about five and 18 times the mass of Earth and orbit their parent star at 0.05 and 0.22 AU, respectively - much closer than Mercury is to the Sun.

"The debris disc extends between 30 and 100 AU from 61 Virginis - well beyond the orbits of its known planets," explains Wyatt. "Since planets and debris discs occupy such different scales, one would not necessarily expect a correlation between their properties.

However, material in the debris disc is also a fossil from the epoch of planet formation so it may carry information about the processes that contributed to build up the planetary system," he adds.

Wyatt and his collaborators took a closer look at the 60 G-type stars that are nearest to the Sun. Out of this sample, they found eleven with planets. Five of these host high-mass planets, with masses of the order of Jupiter's, and the remaining six host low-mass planets.

"Four of the six stars hosting low-mass planets also show debris discs: that's quite a high fraction. In contrast, none of the stars that host high-mass planets appears to have a disc," Wyatt notes.

Since the data suggest that debris discs are preferentially found in planetary systems with low-mass planets, the presence of high-mass planets seems to represent a hindrance to the survival of a debris disc.

A similar result has emerged from another study based on the M-type stars in the DEBRIS survey. M-type stars have very low masses and temperatures, and are the most abundant kind of stars in the Milky Way.

So far, only one M-type star is known to possess a debris disc - the very young star AU Mic, which is about 12 million years old. Given the low surface temperature of these stars, astronomers expect them to retain debris discs more easily than hotter stars, where the radiation pressure may drive the debris away.

However, M-type stars have a different internal structure from their higher-mass counterparts, which causes them to have very intense magnetic fields and to radiate plenty of X-rays - two factors that may contribute to disperse a possible disc, instead.

"With Herschel, we have found a new debris disc around an M-type star, known as Gliese 581," notes Jean-Francois Lestrade from the Observatoire de Paris, France, who led the study on M-type stars in the DEBRIS survey.

"With an age of over two billion years, Gliese 581 is much older than AU Mic, the other M-type star known to possess a debris disc. This is the first proof that discs are able to survive for a very long time around this kind of star, too," Lestrade adds.

Gliese 581 is known to host at least four planets, all of them with low masses - between twice and 15 times the mass of Earth - and with orbits within 0.22 AU from the star. Given Gliese 581's low temperature, two of these super-Earths may even be located in the so-called habitable zone - the distance from a star where water may be found in liquid form.

"The debris disc around Gliese 581 extends from 25 to 60 AU from the star," explains Lestrade. "This massive disc, which also harbours icy bodies such as comets, represents an enormous reservoir of water and other volatiles that can be delivered to the planets, in much the same way as it likely happened during the early days of the Solar System."

Lestrade and his collaborators analysed the other M-type stars in the DEBRIS sample: out of three stars with known planets, Gliese 581 is the only one where Herschel detected a debris disc. It was also the only star in the sample known to have low-mass planets - the other two stars, which host Jupiter-sized planets, showed no sign of a debris disc.

"If there is a correlation between debris discs and low-mass planets, as suggested by the study of G-type stars, this seems to continue also in the realm of less massive stars," notes Lestrade.

These two studies show that debris discs appear to survive more easily around stars that host low-mass planets as opposed to more massive planets, suggesting that gravitational perturbations induced by massive planets may cause the disc to disperse. Something similar might have happened in the early days of the Solar System.

The presence of Jupiter or Saturn may have acted on the debris disc, which was probably more massive in the past, and scattered most of its material away. With its present mass, the Kuiper belt could not be observed even from the closest star to the Sun using state-of-the-art instrumentation.

"The discovery of massive debris discs in another two nearby planetary systems is a unique result achieved with Herschel," comments Goran Pilbratt, Herschel Project Scientist at ESA. "These studies yet again highlight the importance of debris discs in the understanding of planetary systems," he concludes.

The data for this study were gathered as part of the Disc Emission via a Bias-free Reconnaissance in the Infrared/Submillimetre (DEBRIS) Open Time Key Programme, a volume-limited survey to detect and characterise dusty debris discs around nearby main-sequence stars with the PACS and SPIRE instruments on board Herschel. The survey targets the nearest ~90 stars to the Sun of each of the following spectral types: A, F, G, K, M.

The Herschel observations allowed astronomers to detect and resolve debris discs around two stars in the vicinity of the Sun: the G-type 61 Virginis (or 61 Vir), at a distance of about 28 light-years, and the M-type Gliese 581 (or GJ 581), at a distance of about 20 light-years.

61 Virginis has a mass of 0.88 solar masses and a surface temperature of 5602 K, slightly lower than the Sun's; its age is about 4.6 billion years, and it hosts at least two planets (with minimum masses of 5 and 18 Earth masses and orbital distances of 0.05 and 0.22 AU, respectively).

Gliese 581 has a mass of 0.28 solar masses, a surface temperature of about 3500 K and an age of at least 2 billion years. It is known to host four planets (with minimum mass of 1.9, 15.6, 5.4 and 7.1 times the mass of the Earth and orbital distances of 0.03, 0.04, 0.07 and 0.22 AU, respectively), two of which are located in the so-called habitable zone.

Debris discs have also been detected around three more G-type stars - HD 20794, HD 69830 and HD 38858 - which all host low-mass planets. In particular, HD 20794 hosts three planets of 2.8, 2.5 and 5 Earth masses, respectively; HD 69830 hosts three planets of 11, 13 and 19 Earth masses, respectively; and HD 38858 hosts one planet of 32 times the mass of the Earth.

In comparison, the mass of Neptune is equivalent to 17 MEarth, the mass of Saturn to 95 MEarth and the mass of Jupiter to 318 MEarth.

DEBRIS is an international collaboration of over 30 researchers from Canada, the U.S.A., the U.K., Spain, Germany, France, Switzerland, and Chile. The project is led by Brenda Matthews (Herzberg Institute of Astrophysics, National Research Council of Canada, and Department of Physics and Astronomy, University of Victoria, Victoria, BC, Canada).

Related publications: M. C. Wyatt, et al., "Herschel imaging of 61 Vir: implications for the prevalence of debris in low-mass planetary systems", 2012, Monthly Notices of the Royal Astronomical Society, 424, 1206-1223. DOI:10.1111/j.1365-2966.2012.21298.x; J.-F. Lestrade, et al., "A DEBRIS Disk Around The Planet Hosting M-star GJ 581 Spatially Resolved with Herschel", 2012, Astronomy and Astrophysics.


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