BETA-PICTORIS LIGHT VARIATIONS .1. THE PLANETARY HYPOTHESIS

The beta Pic disk is probably a young planetary system in the clearing -out phase and similar to ours 4 billion years ago. The understanding of that system may shed light on the origin and evolution of our own p lanetary system. A very important question is related to the presence of large bodies, from kilometer size to planets. It is shown that many indirect arguments seem to indicate that even large planets must be a lready formed within the system. Because it is seen nearly edge-on, ph otometric observations of the star were carried on in order to detect some signatures of inhomogeneities within the dust disk. This edge-on geometry is also very favorable to detect an eventual occultation by a n object orbiting the star. An exceptional and significant photometric event was observed on Julian Day 2444918 (Nov 10, 1981), when the lig htcurve shows a brightening during about 10 days with a central dip du ring less than one day. We discuss several possible explanations. The two most likely ones are: (1) occultation by a planet that is located in the dust disk, with a dust-free area around the planet, (2) the pas sage of a large cloud of dust with a highly forward peaked scattering in front of the star. In this paper we model the predicted lightcurve for the occultation by a planet in a dust ring. The model takes into a ccount the partial occultation phase and the limb-darkening effect. Ev en fine details of the light curve can be explained by this model. We find that the planet is of about Jupiter size and orbits the star at a distance of about 5 AU. We discuss the strong and the weak points of this model. The model of the forward scattering dust cloud is studied in a separate paper.