The Key Program DEBRIS (Disc Emission via a Bias-free Reconnaissance in the Infrared/Sub-mm) on the Herschel Space Observatory is an unbiased flux-limited survey to search for dust emission at λ = 100 and 160μm toward the nearest ∼89 stars of each spectral type A, F, G, K, M as evidence of debris disks.
As part as the observational program, we have spatially resolved the second debris disk around a M-type star at 70, 100 and 160 μm: GJ 581 which is hosting multiple planets. Contrary to the first M-type star dust disk detected surrounding AU Microscopii (Kalas et al. 2004) extending from 50 to 210 AU, GJ 581 is much older (2-8 Gyr), and X-ray quiet in the ROSAT data.
We fit an axisymmetric model of the disk to the three PACS images using a power-law and gaussian densities and found that the best fit model is for a disk extending radially from 25 ± 12 AU to more than 60 AU.
Such a cold disk is reminiscent of the Kuiper Belt but it surrounds a low mass star (0.3 M⊙ ) and its fractional dust luminosity Ldust /L∗ of ∼ 10−4 is much higher. This may be explained by the fact that dust cannot be expelled from the system by radiation or wind pressures because of the low luminosity and low X-ray luminosity of GJ 581.
Hosting at least 4 known planets of low masses and orbiting within 0.3 AU from the star, we suggest that the correlation between low-mass planets and debris disks recently found for G-type stars also applies to M-type stars.