DISCO · Decoding planetary compositions using observations and modelling of planet-forming disks

Decoding planetary compositions using observations and modelling of planet-forming disks

We now know of over 4000 planets. Among these, small planets like the Earth outnumber the Jupiter-like gas giants. Preliminary evidence suggests there are large variations in the abundance of elements such as carbon, iron, and others. Understanding this emerging diversity is important. For example, the Earth's elemental budget is key to various aspects of its habitability. Therefore, we must understand the origin of planetary compositions if we are to scientifically estimate the number of habitable worlds or to determine the false-positive likelihood of any proposed biosignatures. Also of wide relevance in the field is the potential to use bulk elemental composition to link a planet to its formation location in a protoplanetary disk. A high-profile example of this is the hypothesis that the carbon-to-oxygen ratio in the atmospheres of gas giants directly reflects a gas-phase composition that is unique with radial location in a disk. However, it is currently an untested hypothesis and much more work on planet-forming disks is needed to place it on a firm footing. The DISCO project aimed to build the fundamental knowledge to address these and other questions about planetary composition, bu developing innovative techniques for measuring the chemical element content planet-forming environments. We applied these techniques to chemical elements starting from carbon and oxygen, whose importance for tracking planet formation locations is already of major interest, to sulfur and others, which are fundamental importance to rocky planets. The major published outcomes of DISCO include the innovative CAM-technique for measuring the chemical element content of planet-forming material (Jermyn & Kama 2018; see also the illustration); its application to first measurement of the content of sulfur, zinc, and sodium in planet-forming rocks (Kama et al. 2019); and new gas mass constraints for 15 protoplanetary disks (Kama et al. submitted). In addition to this, contributions were made to several other papers, and new work on the carbon-to-oxygen ratio and other elemental abundances in disks is being prepared for publication, including several projects where summer and Master's students made significant contributions. Work done in the DISCO project is substantially feeding into the science consortium activities of the European Space Agency's ARIEL space telescope, where it is being well received by the exoplanet research community. In the coming decade, we will witness major advances in our knowledge of the chemical composition of solar system objects and planets around other stars. This will be driven by solar system exploration and sample return missions, and by new space- and ground-based telescopes, such as the James Webb Space Telescope, the UK-led ARIEL, and the European Extremely Large Telescope (E-ELT). Work done in the DISCO project on the foundations of planetary composition will help to realise the full scientific potential of these exciting new instruments.

Data: CORDIS, © European Union