Dr. Linda Podio

INAF, Italy

The chemical content of planet-forming discs: Towards a comparison with the Outer Solar System objects and exoplanets

Abstract: How have planets formed in the Solar System? And what chemical composition they inherited from their natal environment? Is the chemical composition passed unaltered from the earliest stages of the formation of the Sun to its disk and then to the planets which assembled in the disk? Or does it reflects chemical processes occurring in the disk and/or during the planet formation process?

A viable way to answer these questions is to study protoplanetary disks around young Sun-like stars. The impacting images recently obtained by millimetre arrays of antennas such as ALMA provided the first observational evidence of ongoing planet formation in 0.1-1 million years old disks, through rings and gaps in their dust and gas distribution. The chemical composition of the forming planets clearly depends on the location and timescale for their formation and is intimately connected to the spatial distribution and abundance of the various molecular species in the disk. The chemical characterisation of disks is therefore crucial.

This field, however, is still in its infancy, because of the small sizes of disks (~100 au) and to the low gas-phase abundance of molecules (abundances with respect to H2 down to 10-12), which requires an unprecedented combination of angular resolution and sensitivity. I will show the first pioneering results obtained as part of the ALMA chemical survey of protoplanetary disks in the Taurus star forming region (ALMA-DOT program). We recovered the radial distribution and abundance of diatomic molecules (CO, CS, and CN) as well as of simple organics (H2CO and CH3OH), which are key for the formation of prebiotic molecules, at ~20 au resolution. The CS and H2CO molecules show enhanced emission in the cold outer disk, which suggests efficient formation of organic molecules on the icy mantles of dust grain. This could be the dawn of ice chemistry in the disk, producing more complex organic molecules which still escape our observations.

The next step is the comparison of the molecules radial distribution and abundance gradients in disks with those observed in the Outer Solar System Objects (OSSOs), which are believed to preserve the pristine composition of the protosolar nebula. Further to this, with the advent of JWST and ARIEL it will be possible to characterise the atmospheres of extrasolar planets and to compare them with the chemical diversity observed in disks. Finally, the SKA will open us a new window in the cm to search for heavier molecules and to explore the inner disk regions which are obscured by the dust in the millimetre range covered by ALMA.