Future solar systems

Widely regarded as pathological variable stars – with erratic photometric and spectroscopic behavior of unknown physical origin – 20 years ago, T Tauri stars (TTSs) turned out in the last decade to be promising laboratories for observing the formation of solar systems such as ours. This is because circumstellar, presumably protoplanetary disks were found to surround a large fraction of them. While evidence for disks was primarily indirect until 1995, recent high resolution imaging confirmed earlier claims that the infrared (IR) and ultraviolet (UV) excesses seen in the spectral energy distribution (SED) of these stars were due to disk emission. The activity displayed by young stellar objects at all wavelengths is due to the interaction between the circumstellar disk and the magnetized star and to non-stationary accretion/ejection phenomena. In this review, we briefly summarize properties of these young solar-type stars and describe their circumstellar disks in some detail, focusing on current optical, infrared and millimeter high angular resolution observations that now allow us to resolve the disks.