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A&A, 712, L7 (2026)https://doi.org/10.1051/0004-6361/202661133c ? The Authors 2026Astronomy&AstrophysicsL ETTER TO THE E DITORThe majority of hot Jupiters formed beyond the water ice lineYaxing He 1,2,? , Bertram Bitsch 2 , Adrien Houge 3 , Joe Williams 4 , and Masahiro Ogihara 1,51State Key Laboratory of Dark Matter Physics, Tsung-Dao Lee Institute, Shanghai Jiao Tong University, 1 Lisuo Road,Shanghai 201210, China2Department of Physics, University College Cork, Cork, T12 R229, Ireland3Center for Star and Planet Formation, Globe Institute, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen, Denmark4School of Physics and Astronomy, University of Exeter, Stocker Road, Exeter EX4 4QL, United Kingdom5School of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, ChinaReceived 26 May 2026 / Accepted 15 July 2026ABSTRACTAtmospheric compositions of giant exoplanets can retain information about their formation environments, as volatile species condenseat dif f erent temperatures in protoplanetary discs. We investigated whether the atmospheric compositions of hot Jupiters can constraintheir formation locations. We performed planet formation simulations using the ChemComp code, including pebble drift, pebble and gasaccretion, planet migration, stellar abundances, and two additional chemical processes: thermal decomposition of refractory organicsand CO/CO 2 trapping in water ice. We applied this framework to nine observed hot Jupiter systems and compared the resultingatmospheric metallicities (C/H and O/H) with observational constraints. We found that the observed atmospheric abundances of thenine hot Jupiter systems can be reproduced by planets forming at dif f erent locations relative to the H 2 O and CO 2 snowlines. Ourresults suggest that at least six of the nine systems are consistent with formation beyond the H 2 O snowline. Combined with theobserved orbital separations, eccentricities, and spin–orbit obliquities, these inferred formation locations indicate that many systemslikely experienced dynamical scattering followed by tidal evolution. Atmospheric abundances, in combination with detailed orbitalparameters, can provide a powerful diagnostic of the formation and migration histories of hot Jupiter systems, opening up avenues tounderstand the origin of giant planets in general.Key words. planets and satellites: atmospheres –planets and satellites: composition –planets and satellites: formation1. IntroductionHot Jupiters are gas giant planets with orbital periods of upto a few days, and their origins remain a key challenge inplanet formation theory. Because such massive planets areunlikely to form in situ at their present orbital separations,they are generally thought to originate at larger separationsin protoplanetary discs and subsequently migrate inwards.Several formation pathways have been proposed, includingdisc-driven migration (e.g. Lin et al. 1996; Ida & Lin 2008;Mordasini et al. 2009; Bitsch et al. 2015) and high-eccentricitymigration followed by tidal circularisation (e.g. Rasio & Ford1996; Fabrycky & Tremaine 2007; Beaugé & Nesvorný 2012;Buchhave et al. 2018; Dawson & Johnson 2018). Distinguishingbetween these scenarios requires observational constraints thatconnect the present-day properties of hot Jupiters to their forma-tion histories.The atmospheric compositions of exoplanets provide animportant probe of their formation environments. Because majorvolatile species such as H 2 O, CO 2 , CH 4 , and CO condenseat dif f erent temperatures in protoplanetary discs, their snow-lines separate regions with distinct gas and solid composi-tions (Öberg et al. 2011; Pontoppidan et al. 2014; Booth et al.2017; Eistrup et al. 2018; Mollière et al. 2022). The evolu-tion and radial transport of gas and dust further redistributethese volatiles, producing time-dependent chemical structures?Corresponding author: yaxinghe@sjtu.edu.cnin the disc (Piso et al. 2015; Booth et al. 2017; Krijt et al. 2018;Schneider & Bitsch 2021; Mah et al. 2023).Planets forming at dif f erent radial locations relative tothese snowlines are expected to accrete material with dif f er-ent elemental abundances, which can leave observable sig-natures in their atmospheres (Öberg et al. 2011; Booth et al.2017; Madhusudhan et al. 2017; Schneider & Bitsch 2021;Bitsch et al. 2022). As a result, the elemental abundances ofplanetary atmospheres, in particular the C/O ratio as well asthe elemental C/H and O/H ratios, may retain information aboutthe location where a planet accreted most of its gas and solids(Cridland et al. 2017; Turrini et al. 2021). Atmospheric compo-sitions can be used to constrain the formation locations of giantplanets in protoplanetary discs.Recent planet formation models that couple disc evo-lution with pebble accretion, gas accretion, and planetarymigration have demonstrated that the disc composition canstrongly inf l uence the atmospheric compositions of giant plan-ets (Schneider & Bitsch 2021; Bitsch et al. 2022; Danti et al.2023; Savvidou & Bitsch 2023; Penzlin et al. 2024). In partic-ular, radial drift and evaporation of icy pebbles can enrich thegas phase in volatile elements, producing a wide range of atmo-spheric abundances depending on the formation location andmigration history of the planet (Booth et al. 2017; Bitsch et al.2022; Guzmán Franco et al. 2026; O’Donovan & Bitsch 2026).However, most previous studies have assumed solar elementalabundances for the host star and have only explored a limitedOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0),which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.This article is published in open access under the Subscribe to Open model. Subscribe to A&A to support open access publication.L7, page 1 of 8