ABSTRACT
Discs are a natural part of the star formation process – the collapse of a rotating cloud core plus the conservation of angular momentum imply that material can not fall directly onto a growing protostar. Instead, material drifts down from the protostellar envelope to a smaller flattened disc of orbiting gas and dust particles, and this material then accretes, building up the young star towards its final mass somewhere in the pre-main sequence stage. Residual material in the disc is the reservoir for planet formation, and the disc origin is confirmed by finding the Sun’s planets orbiting in a plane. Discovering extrasolar discs has taken surprisingly long. In the 1960’s and 1970’s, discs were inferred from infrared emission around young stars (i.e. from orbiting dust grains re-emitting absorbed stellar photons) and from scattering patterns of polarized light. The first true discovery was made with the coronographic imaging of the thin edge-on disc of beta Pictoris (Smith & Terrile 1984). At an age of about 10 Myr, we see the characteristic flattened disc plane, but on a much larger scale than in the solar system – particles are spread out to hundreds of AU from the star (possibly as a remnant of the protostellar envelope epoch), compared to the Kuiper Belt of cometary planetesimals in a solar system, the bulk of which lies within 50 AU. More overview and historical aspects may be found in Greaves (2005) and references therein.
