Breaking Degeneracies in Formation Histories by Measuring Refractory Content in Gas Giants

Chachan, Yayaati; Knutson, Heather A.; Lothringer, Joshua D.; Blake, G. A.

doi:10.3847/1538-4357/aca614

“…High-C/O, high-metallicity atmospheres require formation and/or substantial accretion in an environment that is enriched in both carbon and solids. Such conditions are consistent with models that incorporate pebble drift (Booth et al 2017), formation near CO or CO 2 snow lines (Öberg et al 2011), or with pollution by C-rich grains interior to the H 2 O snow line (Chachan et al 2023).…”

Section: Implications For Planet Formationsupporting

confidence: 85%

Keck Planet Imager and Characterizer Emission Spectroscopy of WASP-33b

Finnerty

¹

,

Schofield

²

,

Sappey

³

et al. 2023

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We present Keck Planet Imager and Characterizer (KPIC) high-resolution (R ∼35,000) K-band thermal emission spectroscopy of the ultrahot Jupiter WASP-33b. The use of KPIC’s single-mode fibers greatly improves both blaze and line-spread stabilities relative to slit spectrographs, enhancing the cross-correlation detection strength. We retrieve the dayside emission spectrum with a nested-sampling pipeline, which fits for orbital parameters, the atmospheric pressure–temperature profile, and the molecular abundances. We strongly detect the thermally inverted dayside and measure mass-mixing ratios for CO ( logCO MMR = − 1.1 − 0.6 + 0.4 ), H2O ( logH 2 O MMR = − 4.1 − 0.9 + 0.7 ), and OH ( logOH MMR = − 2.1 − 1.1 + 0.5 ), suggesting near-complete dayside photodissociation of H2O. The retrieved abundances suggest a carbon- and possibly metal-enriched atmosphere, with a gas-phase C/O ratio of 0.8 − 0.2 + 0.1 , consistent with the accretion of high-metallicity gas near the CO2 snow line and post-disk migration or with accretion between the soot and H2O snow lines. We also find tentative evidence for 12CO/13CO ∼ 50, consistent with values expected in protoplanetary disks, as well as tentative evidence for a metal-enriched atmosphere (2–15 × solar). These observations demonstrate KPIC’s ability to characterize close-in planets and the utility of KPIC’s improved instrumental stability for cross-correlation techniques.

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“…The atmospheric abundance of refractory species such as Fe is a particularly important quantity as it is an indicator of the rock composition accreted during the planet's formation (Lothringer et al 2021). Chachan et al (2023) further demonstrate how measuring the abundances of refractory species, specifically Fe, can break degeneracies that exist when trying to determine the formation location of an exoplanet when using only volatile species, such as C-and O-rich species. In the future, our results of measured refractory abundances can be combined with abundances of volatiles from near-infrared observations, to constrain how the population of ultrahot Jupiters formed and evolved, as was recently done with dayside spectra (Kasper et al 2023).…”

Section: Fementioning

confidence: 80%

“…Abundance constraints for the refractory species are of particular importance as they are expected to be accreted onto the planet as solids given their high sublimation temperatures, and hence provide a measure of the accreted rock during formation (Lothringer et al 2021;Knierim et al 2022;Chachan et al 2023). Future work can combine these optical observations with those in the infrared such as JWST and ground-based observatories such as IGRINS/Gemini-S and CRIRES+/VLT, which can constrain volatile species.…”

Section: Discussionmentioning

confidence: 99%

Retrieval Survey of Metals in Six Ultrahot Jupiters: Trends in Chemistry, Rain-out, Ionization, and Atmospheric Dynamics

Gandhi

¹

,

Kesseli

²

,

Zhang

³

et al. 2023

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Ground-based high-resolution spectroscopy (HRS) has detected numerous chemical species and atmospheric dynamics in exoplanets, most notably ultrahot Jupiters (UHJs). However, quantitative estimates on abundances have been challenging but are essential for accurate comparative characterization and to determine formation scenarios. In this work, we retrieve the atmospheres of six UHJs (WASP-76 b, MASCARA-4 b, MASCARA-2 b, WASP-121 b, HAT-P-70 b, and WASP-189 b) with ESPRESSO and HARPS-N/HARPS observations, exploring trends in eleven neutral species and dynamics. While Fe abundances agree well with stellar values, Mg, Ni, Cr, Mn, and V show more variation, highlighting the difficulty in using a single species as a proxy for metallicity. We find that Ca, Na, Ti, and TiO are underabundant, potentially due to ionization and/or nightside rain-out. Our retrievals also show that relative abundances between species are more robust, consistent with previous works. We perform spatially resolved and phase-resolved retrievals for WASP-76 b and WASP-121 b given their high signal-to-noise observations, and find the chemical abundances in each of the terminator regions are broadly consistent. We additionally constrain dynamics for our sample through Doppler shifts and broadening of the planetary signals during the primary eclipse, with median blueshifts between ∼0.9 and 9.0 km s−1 due to day–night winds. Furthermore, we constrain spectroscopic masses for MASCARA-2 b and HAT-P-70 b consistent with their known upper limits, but we note that these may be biased due to degeneracies. This work highlights the importance of future HRS studies to further probe differences and trends between exoplanets.

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“…While the initial formation locations of hot Jupiters may be obfuscated by postdisk dynamical sculpting, compositional tracers of their birthplaces may remain undisturbed. Chemical signatures of the gas giants' initial locations, such as the carbon-to-oxygen ratio (C/O; Öberg et al 2011) or siliconbased tracers of refractory content (Chachan et al 2023), may remain detectable and can potentially be used to distinguish the initial locations of Jovian planet formation in the future (e.g., Line et al 2021;Reggiani et al 2022).…”

Section: On Hot Jupiters' Initial Formation Locationsmentioning

confidence: 99%

Evidence for Hidden Nearby Companions to Hot Jupiters

Wu

¹

,

Rice

²

,

Wang

³

2023

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The first discovered extrasolar worlds—giant, “hot Jupiter” planets on short-period orbits—came as a surprise to solar system–centric models of planet formation, prompting the development of new theories for planetary system evolution. The near absence of observed nearby planetary companions to hot Jupiters has been widely quoted as evidence in support of high-eccentricity tidal migration, a framework in which hot Jupiters form further out in their natal protoplanetary disks before being thrown inward with extremely high eccentricities, stripping systems of any close-in planetary companions. In this work, we present new results from a search for transit timing variations across the full 4 yr Kepler data set, demonstrating that at least 12% ± 6% of hot Jupiters have a nearby planetary companion. This subset of hot Jupiters is expected to have a quiescent dynamical history such that the systems could retain their nearby companions. We also demonstrate a ubiquity of nearby planetary companions to warm Jupiters (≥70% ± 16%), indicating that warm Jupiters typically form quiescently. We conclude by combining our results with existing observational constraints to propose an “eccentric migration” framework for the formation of short-period giant planets through postdisk dynamical sculpting in compact multiplanet systems. Our framework suggests that hot Jupiters constitute the natural end stage for giant planets spanning a wide range of eccentricities, with orbits that reach small enough periapses—either from their final orbital configurations in the disk phase or from eccentricity excitation in the postdisk phase—to trigger efficient tidal circularization.

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Breaking Degeneracies in Formation Histories by Measuring Refractory Content in Gas Giants

Cited by 19 publications

References 178 publications

Keck Planet Imager and Characterizer Emission Spectroscopy of WASP-33b

Keck Planet Imager and Characterizer Emission Spectroscopy of WASP-33b

Retrieval Survey of Metals in Six Ultrahot Jupiters: Trends in Chemistry, Rain-out, Ionization, and Atmospheric Dynamics

Evidence for Hidden Nearby Companions to Hot Jupiters

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