Elemental abundances sub-WG

Context and Objectives

"It has been established that as soon as the protostellar cloud collapses, the protoplanetary disc cools down and the snowline of the various volatile species (e.g. H2O, CO2, CO, CH4, N2) move towards the central star, causing the chemical composition of both solids and gas to evolve as a function of time and their location in the disc (Eistrup et al. 2016, 2018; Booth & Ilee 2019; Madhusudhan 2019). While in the inner part of the snowline the various species contribute to the gas composition, beyond the snowline, they contribute to the solids composition. Thus, assuming that the planet migrates inside the disc during its formation, the net planetary composition is provided in first approximation (i.e. without accounting for chemical evolution of the disc) by the cumulative accretion history over the migration trail across both gas and solids. As a consequence, the present-day atmospheric chemical composition of exoplanets can be used to retrieve key information on the planet’s birthplace and on the time the planet migrated to its present orbit (Cridland et al. 2019, 2020; Schneider & Bitsch 2021a,b; Turrini et al. 2021; Pacetti et al. 2022; Khorshid et al. 2022).

In the light of the facts presented above, one of the main focus of the last decade of the exoplanet community has been the determination of the carbon-to-oxygen ratio (C/O, e.g. Moses et al. 2013b,a; Notsu et al. 2020), used as a formation location proxy of giant planets (see Öberg et al. 2011; Madhusudhan et al. 2016; Madhusudhan 2019 and references therein). When a super-solar C/O ratio is detected, the planet is theorised to have formed in a low-metallicity environment and hence to have accreted most of C and O in gas state. On the other hand, when a sub-solar C/O ratio is found, the planet is expected to have formed in a high metallicity environment where C and O were dominated by the accretion of solids. However, recent theoretical developments showed that the C/O ratio alone provides limited information with regard to the formation region, and that the inclusion of both carbon-to-nitrogen (C/N, Turrini et al. 2021; Pacetti et al. 2022) and nitrogen-to-oxygen (N/O, e.g. Piso et al. 2016; Turrini et al. 2021; Ohno & Fortney 2023) ratios enable breaking the degeneracy in the information provided by C/O (Turrini et al. 2021; Fonte et al. 2023).

An important aspect about the various planetary elemental ratios, however, is the fact that they cannot be directly used for comparison purposes between objects in different planetary systems, due to the variety of scale range they present. Consequently, when performing population studies and/or comparison planetology, it is extremely important to normalise these ratios to the elemental abundance ratios of their own star (Turrini et al. 2021). Normalised elemental ratios open up the possibility to directly compare the formation and migration histories of giant planets that reside in different planetary systems, always in the event that stellar abundances are homogeneously derived among the stellar sample (Turrini et al. 2022; Kolecki & Wang 2022)."

The purpose of this sub-WG is to derive the  elemental abundances of several refractory a  siderophiles, silicate, and volatile elements which are important to link the stellar chemistry with the origin of planets.

What does this mean for the planet characterisation, and in particular for inferring the planetary formation location ? 

Since the stellar C/N systematically decreases with respect to the solar one for increasing stellar metallicities (left plot), while N/O and C/O systematically increase (right plot), the adoption of solar values (for the host star of the planet you are analysing) means that we systematically underestimate (overestimate) the planetary C/N (N/O and C/O) values for stars of supersolar metallicity, the opposite being true for stars of subsolar metallicity. This leads to incorrect constraints on the formation region of the giant planets from their deviations from the stellar values, for instance, causing giant planets around supersolar metallicity stars to appear to have formed closer to their host stars than they actually have (see e.g. Figs. 7 and 8 in Turrini et al. 2021 or Figs. 4–7 in Pacetti et al. 2022).