Dust is a preponderant component of planetary systems, and often is the principal
source of luminous radiation close to the central star. In our solar system, the zodiacal light seen from the Earth is its most well-known manifestation. This diffuse glow seen as the Sun sets was already known in Ancient Egypt : its origin was clearly identified in the XVIIth century as being due to the scattering of solar light by dust grains situated in the asteroid belt between Mars and Jupiter. « It is supposed that these grains are supplied continuously through the evaporation of comets, and as a consequence of collisions between asteroids. » explains Jean-Charles Augereau, coordinator of the EXOZODI project at the Grenoble Institute of Planetology and Astrophysics (see the insert). Other than the source of these dust grains, the question of the nature of the zodiacal light has been understood for four centuries. In the 1990s the question of their presence in the newly discovered extrasolar sytems was raised, as was their possible nuisance value for the detection of other habitable Earths, a major challenge for the astronomy of the XXIst century.
The detection of the extra-solar equivalents of the zodiacal dust was an observational
challenge, since they are both very close to, and are much fainter than, the central
star which tends to "blind" the instrument. « We had to combine the very high resolving power of CHARA with the high dynamic range of FLUOR to be able to detect them ",
explains Vincent Coudé du Foresto, astronomer at the Paris Observatory, who
developed the instrument. This interferometric technique furnishes a resolving power
similar to that of a giant 330 m telescope, by combining the light collected by
telescopes spread over tens to hundreds of metres. It has enabled us to distinguish the
feeble « exo-zodiacal » light of the hot dust grains, from the intense light of the star.
By studying a sample of about forty stars, over eight years, the franco-belgian research
collaboration has provided for the first time an estimate of the number of close stars
which have e o-zodiacal dust. According to Olivier Absil, scientist at Liège University
and principal author of the paper, « Close to 20% of Sun-like stars have hot dust in the
proximity or within their habitable zone; this fraction rises to 30% when stars up to
twice as hot, such as Vega, are included in the sample. ».
The exo-zodiacal phenomenon is thus relatively common in solar-type systems, and this has to be taken into account seriously by future space programs designed to detect directly exo-Earths.
Over and above this first important statistical study on the presence of exo-zodiacal
light around stars in our neighborhood, the astronomers have begun to suggest the
origin of the hot dust grains responsible for the scattered light. For the stars like our
Sun, the presence of exo-zodiacal dust seems to be related to the presence of cold
dust, which has in particular been detected by ESA’s Herschel satellite; this dust is
situated in external rings similar to the Kuiper belt, a relic from the formation of the
Solar System. According to Olivier Absil, « This correlation suggests that the hot dust is necessarily related to the cold dust which is presumably within enormous reservoirs of dust and small bodies situated at the limits of these stellar systems, and which drive the phenomenon. » The dust is presumably furnished via a constant injection of comets into the central parts of the systems from the outer belts. For this scenario to work, the outer belts must be subject to important perturbations so that the small bodies which are there are sent into the inner parts of the system: here, perhaps, is an indirect proof that these close stars are accompanied by planets.
These results are the fruit of a systematic 8 year observational program targetting
about forty close stars, using FLUOR, an instrument developed in France and installed since 2002 on the stellar interferometer CHARAMark, at the Mont Wilson Observatory (California).1
Reference
- « A near-infrared interferometric survey of debris discs stars : First statistics based on 42 stars observed with CHARA/FLUOR », Absil et al. 2013, A&A, 555, A104
1 CHARA (Center for High Angular Resolution Astronomy) at the State University of Georgia enables astronomers, using six 1 m telescopes situated on Mount Wilson in California, to simulate a giant telescope of close to 330 m, and to distinguish details of only 200 microsecondes of arc in size, hardly larger than a football seen from the Moon! The light collected by the CHARA array is recombined by the FLUOR (Fiber Linked Unit for Optical Recombination) instrument, developed by the Laboratoire d’études spatiales et d’instrumentation en astrophysique - LESIA of the Observatoire de Paris (Unité de recherche Observatoire de Paris / CNRS / Université Pierre et
Marie Curie / Université Paris Diderot), with strong support from the Institut National des Sciences de l’Univers of the CNRS.
Last update on 21 December 2021