Toward Chemical Constraints on Hot Jupiter Migration

Abstract

The origin of hot Jupiters -gas giant exoplanets orbiting very close to their host stars -is a long-standing puzzle. Planet formation theories suggest that such planets are unlikely to have formed in-situ but instead may have formed at large orbital separations beyond the snow line and migrated inward to their present orbits. Two competing hypotheses suggest that the planets migrated either through interaction with the protoplanetary disk during their formation, or by disk-free mechanisms such as gravitational interactions with a third body. Observations of eccentricities and spin-orbit misalignments of hot Jupiter systems have been unable to differentiate between the two hypotheses. In the present work, we suggest that chemical depletions in hot Jupiter atmospheres might be able to constrain their migration mechanisms. We find that sub-solar carbon and oxygen abundances in Jovian-mass hot Jupiters around Sun-like stars are hard to explain by disk migration. Instead, such abundances are more readily explained by giant planets forming at large orbital separations, either by core accretion or gravitational instability, and migrating to close-in orbits via disk-free mechanisms involving dynamical encounters. Such planets also contain solar or super-solar C/O ratios. On the contrary, hot Jupiters with super-solar O and C abundances can be explained by a variety of formation-migration pathways which, however, lead to solar or sub-solar C/O ratios. Current estimates of low oxygen abundances in hot Jupiter atmospheres may be indicative of disk-free migration mechanisms. We discuss open questions in this area which future studies will need to investigate. Subject headings: planetary systems -planets and satellites: general

A Comparison of Simulated JWST Observations Derived from Equilibrium and Non-equilibrium Chemistry Models of Giant Exoplanets

“…The bulk composition of a planet's atmosphere plays a major role in the chemistry at work in the planet's atmosphere. The bulk composition of exoplanet atmospheres will depend on its formation history (Öberg et al 2011;Madhusudhan et al 2014;Wakeford et al 2017). Wakeford et al (2017) clearly demonstrates this with the analysis of HAT-P-26b which departs from the Kr14 relationship.…”

KELT-11b: A Highly Inflated Sub-Saturn Exoplanet Transiting the V = 8 Subgiant HD 93396

“…For example, detailed studies of KELT-11b will allow its chemical composition to be determined, which in turn will constrain parameters involving its formation and evolution of planets (e.g., Madhusudhan et al 2014). Furthermore, the source of inflation in hot Jupiters can be investigated in the extreme case of KELT-11b.…”

Observing transiting planets with JWST

“…The reason for this is the clouds' decreased optical depth when not being viewed in transit geometry (Fortney 2005). Moreover, the analysis of the resulting atmospheric composition and abundance ratios may allow placement of constraints on planets' formation locations, although the exact interpretation of the atmospheric abundances depends on the assumptions and the degree of complexity of the model being used to describe the planet formation and evolution (Öberg et al 2011;Ali-Dib et al 2014;Thiabaud et al 2014;Helling et al 2014;Marboeuf et al 2014b,a;Madhusudhan et al 2014aMadhusudhan et al , 2016Mordasini et al 2016;Öberg & Bergin 2016;Cridland et al 2016).…”

“…The observational evidence of C/O > 1 planets is debated (Madhusudhan et al 2011b;Crossfield et al 2012;Swain et al 2013;Stevenson et al 2014;Line et al 2014;Kreidberg et al 2015;Benneke 2015) and there have been numerous studies trying to theoretically assess whether the formation of C/O > 1 or C/O → 1 planets is possible (Öberg et al 2011;Ali-Dib et al 2014;Thiabaud et al 2014Thiabaud et al , 2015Helling et al 2014;Marboeuf et al 2014b,a;Madhusudhan et al 2014aMadhusudhan et al , 2016Öberg & Bergin 2016), while one of the most recent works on this topic indicates that hot Jupiters (which usually have masses 3M ) and planets of lower mass may never have C/O > 1 .…”