1.1 Observations of protoplanetary discs
The presence of discs around newborn stars is a natural consequence of the collapse of the molecular cloud from which they form, as a mechanism to conserve angular momentum. The first evidence of the existence of protoplanetary discs came thanks to the Hubble Space Telescope (HST) in the mid-1990s, which was more or less at the same time as the first exoplanet discoveries. Figure 1 shows the HST images of the protoplanetary discs around four young stars in the Orion nebula, 1500 light-years from the Sun, compared with the much more detailed view of a protoplanetary disc in the same region obtained with the James Webb Space Telescope (JWST) in 2022. Protoplanetary discs have been observed mainly around young stars (with ages of about one to ten million years) that are close to the final stages of formation. This means that most of the material from the disc has been accreted by the central star, so the mass of the disc (Mdisc) is much lower than the mass of the star (M*).
The presence of planets in protoplanetary discs is strongly supported by observations, which have been supported by the development of ground-based instruments such as SPHERE.
Box 1 SPHERE
SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) is an instrument operating at near-infrared and visible wavelengths, installed on one of the four telescopes comprising the European Southern Observatory’s (ESO) Very Large Telescope (VLT) site in Paranal (Chile). SPHERE is one of the first dedicated direct-imaging instruments and its primary science goal is to directly detect and characterise young exoplanets and the discs in which they form.
Recent observations using VLT/SPHERE include the direct detection of two forming planets in the disc around the young T Tauri star PDS 70 (which gets its name from the Pico dos Dias Survey for young stellar objects). One of the directly imaged planets is shown in Figure 3.
