Figure: Mid-IR spectra of the inner and outer disc regions of three young stars

This picture shows a schematic view of a circumstellar disc and the MIDI-spectra observed of the inner and outer regions of the discs around three young stars, HD 163296, HD 144432 and HD 142527 (black lines). In all of them, there are clear spectral differences between the inner and outer regions, indicating a difference in mineralogy. The general broadening of the spectral "mountain" in the inner discs is a sign of larger grains and the spectral peak at wavelength 11.3 μm indicates the presence of crystalline silicates. Also shown are best-fit model spectra (red lines), based on mixtures of the mentioned mineral species.

Spectral observations of silicate grains in the mid-infrared wavelength region (around 10 μm) will tell whether they are "pristine" or "processed". Earlier observations of discs around young stars have shown a mixture of pristine and processed material to be present, but it was so far impossible to tell where the different grains resided in the disc.

Thanks to a hundred-fold increase in angular resolution with the VLTI and the highly sensitive MIDI instrument, detailed infrared spectra of the various regions of the protoplanetary discs around three newborn stars, only a few million years old, now show that the dust close to the star is much more processed than the dust in the outer disc regions. In two stars (HD 144432 and HD 163296) the dust in the inner disc is fairly processed whereas the dust in the outer disc is nearly pristine. In the third star (HD 142527) the dust is processed in the entire disc. In the central region of this disc, it is extremely processed, consistent with completely crystalline dust.

An important conclusion from the VLTI observations is therefore that the building blocks for Earth-like planets are present in circumstellar discs from the very start. This is of great importance as it indicates that planets of the terrestrial (rocky) type like the Earth are most probably quite common in planetary systems, also outside the solar system.

Credit: European Southern Observatory (ESO)

http://www.eso.org/outreach/press-rel/pr-2004/pr-27-04.html