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Summary

BOWIE-ALIGN (PIs: Kirk & Ahrer) is using JWST to compare the atmospheric compositions of aligned and misaligned short-period gas giant exoplanets (“hot Jupiters”). Aligned hot Jupiters are the likely result of migration through a natal protoplanetary disc while misaligned hot Jupiters likely migrated to their current positions after the dispersal of the disc, via dynamical interactions with other bodies in the system. The difference in the evolution of aligned versus misaligned hot Jupiters is expected to lead to a dichotomy in their atmospheric compositions (Figure below). In Penzlin et al., 2024, MNRAS, 535, 171 we present a detailed modelling investigation into the predicted differences between aligned and misaligned hot Jupiters. In Kirk et al., 2024, RASTI, 3, 691 we present an overview of the JWST observational test of these predictions. With the total observational sample of eight of the best observable hot Jupiters (four aligned and four misaligned), BOWIE-ALIGN will robustly test the link between an exoplanet’s composition and its evolution.

Detailed description

It has long been proposed that measuring an exoplanet’s atmospheric composition (specifically its carbon-to-oxygen ratio, C/O) can reveal information regarding where a planet formed with respect to different ice lines (e.g., Oberg et al., 2011; Madhusudhan et al., 2014; Booth et al.., 2017; Schneider et al., 2021). The basic principle that the C/O of a planet’s atmosphere is dependent on where it accreted its atmosphere relative to different C- and O-bearing molecular ice lines is robust, and there is little doubt atmospheric composition will lead to insights into planet formation and evolution. However, there are many uncertainties when relating an individual planet’s composition to its formation location, which, combined with our lack of sensitivity to carbon-bearing molecules in the pre-JWST era, have prevented a detailed investigation of how atmospheric composition depends on planet formation and evolution.

The challenges to our understanding of the link between formation and composition include the uncertain and evolving locations of ice lines within discs (e.g., Morbidelli et al., 2016; Panic et al., 2017; Owen et al., 2020), the observed diversity of protoplanetary discs (e.g., Law et al., 2021), how much solid versus gaseous material is accreted during planet formation (e.g., Espinoza et al., 2017), and the drift of solids relative to the gas in the disc (e.g., Booth et al., 2017). Furthermore, transit spectroscopy observations of exoplanets’ atmospheres probe the atmospheric composition at the planetary limb which might hold inhomogeneities caused by, e.g., local atmospheric mixing (e.g., Zamyatina et al., 2024) or cloud formation (e.g., Helling et al., 2016) therefore not necessarily reflecting the bulk planet’s atmospheric composition (Muller et al., 2014).

In Penzlin et al., 2024, MNRAS, 535, 171 we used simulations to demonstrate that the unconstrained values of key disc and planet formation parameters such as dust-to-gas mass, disc temperature, and the relative drift of dust to gas (Stokes-to-alpha number), create a degeneracy between C/O and [O/H] (which we refer to as metallicity, Z, throughout this paper) over a wide dynamic range. These uncertainties are hard to constrain robustly through independent observations. Therefore, we propose the best way to determine whether differences in formation history lead to a measurable difference in atmospheric C/O and metallicity is by comparing populations of planets for which we have independent evidence that they underwent different evolutionary pathways. Specifically, planets that have undergone disc-free (high eccentricity) migration should have different C/O and metallicity to planets that have undergone disc migration. The idea behind this is that disc-migrated planets will accrete solids from the inner disc during their migration while disc-free, high-eccentricity migrated planets will not since they complete their migration after disc dispersal (Figure 1).

With the advent of JWST’s revolutionary precision and wavelength coverage of carbon-bearing molecules, we are able to test these predictions for the first time against a well-designed target sample. To this end, we are undertaking a survey with JWST to compare the C/O and metallicity of four disc migrated hot Jupiters with four high eccentricity migrated hot Jupiters. We focus specifically on hot Jupiters, and not smaller planets, since their giant masses likely necessitate formation beyond ice lines and hence subsequent migration (e.g., Lin et al., 1996; Rafikov et al., 2006; Dawson & Johnson, 2018}. Furthermore, their massive H/He envelopes retain the primordial composition, without being changed by atmospheric loss (Owen et al., 2018). We will combine the transmission spectra of five planets from our new observational programme (GO 3838, 49.2 hours, PIs: Kirk & Ahrer) with spectra of three planets from other programmes (GTO 1274, PI: Lunine; GTO 1353, PI: Lewis; GO 3154, PI: Ahrer). Our programme is called BOWIE-ALIGN, with BOWIE corresponding to the core institutions of our collaboration (Bristol, Oxford, Warwick, Imperial, Exeter) and ALIGN standing for A spectral Light Investigation into hot gas Giant origiNs.

We distinguish disc migrated from high eccentricity migrated hot Jupiters via their sky-projected orbital alignments (“obliquities”) around F stars where tidal realignment is thought to be inefficient (Albrecht et al., 2012). Disc migration is expected to lead to a slowly shrinking planetary orbit and the accretion of gas, dust and planetesimals in the migrating planet’s path (Dawson & Johnson, 2018). This results in little change in the eccentricity and inclination of a planet’s orbital plane, which remains aligned with the stellar spin axis (Figure 1). High eccentricity migration likely occurs after disc dispersal. Under this mechanism, it is thought that an initially cold Jupiter is perturbed into an eccentric orbit via interactions with a third body (e.g., Rasio et al., 1996; Wu et al., 2003}, which drive up the eccentricity and inclination of the planet (Kozai et al., 1962; Lidov et al., 1962; Munoz et al., 2016). This method of migration is believed to result in misalignments between a planet’s orbital plane and the stellar spin axis. Therefore, by comparing aligned and misaligned hot Jupiters we can test the predicted impacts of migration method on atmospheric C/O and metallicity.

The key with our survey is that since formation models are unable to a priori predict the specific values of C/O and metallicity for individual scenarios, they robustly predict a difference. As we show in Penzlin et al., 2024, the sign of this difference is even uncertain owing to uncertainties in formation models. Thus, by comparing one sample to another we can test this difference, along with narrowing down the range of uncertain disc parameters currently rendering the models unpredictive. Hence, rather than is the common expectation of measuring an atmospheric composition and comparing it to formation models to determine how the planet formed, we are proposing an opposite approach. Namely, testing the idea that different formation scenarios predict different atmospheric compositions, then using our measured compositions to constrain the formation and evolution models.

The sample

Figure 2 shows the planetary and stellar parameters of our sample of eight hot Jupiters, following the cuts detailed in Kirk et al., 2024. In particular Figure 2 shows the obliquity distribution of our planets while Figure 3 shows that the host stars are all above the metallicity-dependent Kraft break.

The results

- WASP-15b (Kirk et al., 2024)

The first result we presented was the 2.8–5.2 micron transmission spectrum of the misaligned hot Jupiter WASP-15b (Figure 4). Our high signal to noise data, which has negligible red noise, revealed significant absorption by H2O (4.2 sigma) and CO2 (8.9 sigma). From independent data reduction and atmospheric retrieval approaches, we inferred that WASP-15b’s atmospheric metallicity is super-solar (>~ 15X solar) and its C/O is consistent with solar, that together imply planetesimal accretion. Our GCM simulations for WASP-15b suggest that the C/O we measure at the limb is likely representative of the entire photosphere due to the mostly uniform spatial distribution of H2O, CO2 and CO. We additionally see evidence for absorption by SO2 and absorption at 4.9 micron, for which the current leading candidate is OCS, albeit with several caveats. If confirmed, this would be the first detection of OCS in an exoplanet atmosphere and point towards complex photochemistry of sulphur-bearing species in the upper atmosphere. This is was the first result in our mission to perform a comparative study of aligned vs misaligned hot Jupiters.

News

Our description of the BOWIE-ALIGN survey (Kirk et al., 2024) was picked up by the Royal Astronomical Society’s media team..