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Astron. Astrophys. 328, 311-320 (1997)

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1. Introduction

Despite many observational advances over the past decades (see e.g., Schneider 1995), our Solar System remains the only well-studied planetary system around a main sequence star. It contains objects formed 4.5 billion years ago, from comets and asteroids to massive planets like Jupiter. Surprisingly, three planets have tentatively been detected around the millisecond pulsar PSR 1257+12 (Wolszczan 1994), a Jupiter-mass companion to the star 51 Pegasi at only 0.05 AU from the star is inferred from radial velocity measurements (Mayor & Queloz 1995), and planets has been discoved around other stars (e.g., Marcy & Butler 1996). These examples reveal the probable diversity of plausible planetary systems yet to be discovered as suggested by Lissauer (1995).

On the other hand, there is ample evidence that disks are common around young stars (see e.g., Beckwith & Sargent 1993). Theoretical studies suggest that planetary systems can form within the typical observed lifetime of these disks (Lissauer 1993) implying that possibly planetary systems around main sequence stars could be relatively common.

From a large body of observational approaches, an unique case of a main sequence star emerges: [FORMULA] Pictoris, surrounded by a disk of gas and dust (Aumann et al. 1984; Smith & Terrile 1984; Hobbs et al. 1985; Kondo & Bruhweiler 1985; Vidal-Madjar et al. 1986). This system may offer indirect clues to the presence of a large range of sizes of bodies orbiting the star.

Since 1984 a lot of work, both observational and theoretical, has been done to understand the [FORMULA]  Pic system (Ferlet & Vidal-Madjar 1995). Although only the presence of micron-sized particles is largely accepted (see the review by Artymowicz 1995), there is also evidence for the existence of larger bodies from kilometer size to planetary sizes. We will review in Sect. 2 the chain of independent arguments which seem to suggest very strongly that massive bodies are already formed around [FORMULA]  Pic. In Sect. 3 we will present and discuss the photometric observations completed in 1981, before [FORMULA]  Pic was identified as a possible young planetary system. In Sect. 4 and 5 we will discuss the different plausible scenarios that might explain the photometric variations, both the unlikely ones (Sect. 4) and the likely ones (Sect. 5). Of the two likely explanations, we discuss in this paper the model of occultation by a planet located within an empty sphere of influence, that is located in the dust disk of [FORMULA]  Pic. The alternative model of a large forward-scattering cloud is discussed in a separate paper (Lamers et al. 1997).

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998