β Pictoris, a young planetary system? A review

Abstract

β Pictoris is a bright southern hemisphere star observed in 1983 by the IRAS satellite as presenting a large and unexpected IR excess. This excess was called the Vega-like phenomenon and quickly identified as due to circumstellar dust. Subsequently in 1984, using stellar coronography, dust was also directly seen as an edge-on disk extended to several hundreds of AUs. Since then, β Pictoris has been continuously observed. We present here a review of our present understanding of the β Pictoris circumstellar environment which still appears unique in the solar neighborhood. The circumstellar dust disk is predominantly made of relatively large particles (one micron or more) extending outward to more than 1000 AU and presenting a clearer (dust free) central region away to about 35 AU from the star. The gas is detected through stable and variable spectroscopic signatures revealing a permanent gas disk with sporadic inflows and also a few outflows. These are partially interpreted in terms of evaporation of kilometer-sized bodies very close to the star. Evaporation or destruction through collisions of kilometer-sized bodies seems to be needed also to explain both the dust as well as the very presence of the CO molecule detected in the circumstellar gas. Several indirect arguments along with the observation of a very peculiar photometric variation of the star suggests that even giant planets may have already formed in the β Pictoris system. β Pictoris is thus possibly the missing link between young stellar objects presenting proto-planetary circumstellar disks and much more evolved systems in which planets (at least giant ones) are already formed. β Pictoris is probably a unique place where we may now observe planetary formation as well as other phenomena that have taken place in the first 10 8 years of a young stellar system.