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

Periodic Reporting for period 1 - DISCO (Decoding planetary compositions using observations and modelling of planet-forming disks)

Reporting period: 2017-07-03 to 2019-07-02

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.
"Planet-forming disks accrete material onto their central star. For stars somewhat more massive than our sun, internal mixing processes are slow and the freshly accreted material dominates the surface composition of the star, where its composition is easier to measure. The principle is depicted in the illustration accompanying this report. We developed the theoretical foundations of this ""CAM"" method and described the main applications (Jermyn & Kama 2018, MNRAS). In the first application paper (Kama et al. 2019, ApJ, in press), we used the new method to measure the fraction of sulfur atoms locked in rocky solids in protoplanetary disks. CAM now allows very precise quantitative measurements of the abundance and refractory fraction of nearly twenty individual elements in the disk regions where the vast majority of planets are later found.

Significant new data was acquired during the project to deepen and broaden our knowledge of the composition of gas and solids in planet-forming disks. Observing projects were carried out on the sub-millimetre ALMA and APEX telescopes to map carbon, oxygen, sulfur, and other elements in the gas phase on scales of tens to hundreds of astronomical units in several planet-forming disks. A separate effort was focussed on obtaining new stellar spectra for analysis with the CAM-method. We built up a dataset of 16 previously unstudied stars, chosen to maximise their scientific impact in terms of using CAM to characterise the composition of protoplanetary or debris disk material.

The chemical element content of protoplanetary disks is generally measured relative to the total amount of mass present. A molecule closely linked to the dominant but invisible H2, hydrogen deuteride (HD), can provide a very robust total mass measurement. In DISCO, we were able to apply a fresh perspective to archival data obtained before 2013 to constrain the total mass in 15 disks (Kama et al. submitted), most notably HD 163296 where five giant planets have recently been discovered. We can now connect them to the mass reservoir from which they are being born.

A major part of DISCO was focussed on measuring and understanding the carbon and oxygen abundance in a sufficient number of planet-forming regions to allow for general conclusions to be drawn. As a result of this work, we find that planet-forming disks fall into two clearly distinct chemical categories. These results make testable predictions for the chemical composition of planets around specific types of stars, and will be submitted for publication later this year. Predictions from this work can be tested with exoplanet composition measurements from the James Webb Space Telescope once it launches, and from other forthcoming space telescopes. We are on the path to a deeper understanding of the origins of planetary chemical composition, and to unlocking the full scientific potential of major space missions targeting exoplanets.

In addition to the specialist publications already mentioned, DISCO science was communicated at two public talks at the Institute of Astronomy, and at lectures at the Science Society and the Postdoctoral Society at Trinity College. In connection to astrochemistry, planets and life in the Universe, the PI was interviewed for a popular science programme on Estonian national television."
Prior research on the elemental composition of planet-forming disks had been largely restricted to measuring the presence of a few different chemicals in the form of gas particles, using sub-millimetre spectroscopy. In DISCO, we built on this prior work, but most importantly developed an entirely new, complementary tool for studying the behaviour and reservoir of chemical elements in planet-forming disks: CAM. The results from CAM provide input for planet formation simulations, which can then include a wider range of chemical elements than just C and O. Next-generation telescopes capable of characterising exoplanets, such as the James Webb Space Telescope and ARIEL, will allow to measure the abundance of compounds bearing sulfur and other elements which have thus far been unobservable in such environments. With DISCO, we are now positioned to tease out new clues about the formation histories of planets.

The PI has joined the consortium of the European Space Agency's ARIEL exoplanet mission, to apply DISCO in taking us closer to understanding the origins of our habitable Earth and the properties of planets all across the Galaxy.
Illustration of how the chemical composition of a star's surface relates to material in its disk.