Most hot Jupiters are born beyond the water ice line ✛ ✎
Context
Hot Jupiters are gas giants that orbit very close to their stars, and their origin remains one of the big mysteries of planetary astrophysics. Since the discovery of the first one by Michel Mayor and Didier Queloz in 1995, astronomers have debated where exactly these giant planets form. In situ formation on close-in orbits is unlikely due to high temperatures and lack of material, so it's thought they are born farther out and then migrate. But the migration pathways—smooth disk-driven drift or resonant interactions with other planets—are still unclear. Hot Jupiters' atmospheric compositions may hold a memory of their birthplace, since volatiles like water and carbon dioxide condense at different temperatures in the protoplanetary disk, creating chemical boundaries known as snow lines.
Methods
To test this idea, the authors used numerical simulations with the ChemComp code, which tracks the evolution of the protoplanetary accretion disk and planet growth and migration. The model includes pebble and gas accretion, dust drift, and two new chemical processes: thermal decomposition of organic compounds and trapping of CO and CO₂ in amorphous water ice. Unlike previous work, it uses the individual chemical compositions of the host stars from the Hypatia catalog rather than standard solar abundances, allowing a more accurate estimate of the environment's metallicity. The simulations covered nine hot Jupiter systems, for which carbon (C/H) and oxygen (O/H) abundances in the atmospheres were measured via transit spectroscopy. By varying the initial position of the planetary embryo and the disk viscosity, the scientists searched for combinations that reproduce the observed atmospheric parameters.
Results
The simulations showed that a planet's atmospheric composition clearly depends on its birth location relative to the snow lines. Planets that formed inside the water ice line are oxygen-rich; those born beyond the CO₂ line have lower oxygen content. Out of the nine systems studied, at least six are consistent with formation beyond the water snow line, and some even beyond the CO₂ line. For instance, TrES-3 b likely formed in the inner disk, while WASP-77A b, WASP-121 b, and others required birth at large distances. Combining these results with observed orbital parameters—eccentricities and stellar obliquities—the authors concluded that many systems underwent dynamical scattering followed by tidal evolution. This explains why some hot Jupiters have elongated or tilted orbits even though they now sit close to their stars. Thus, planetary migration combined with dynamical interactions becomes key to understanding the observed diversity.
Implications
This work demonstrates for the first time that the carbon and oxygen content in hot Jupiter atmospheres can be used as a reliable proxy for their formation location. It connects the chemistry of exoplanet atmospheres to the dynamical evolution of planetary systems, helping to distinguish between scenarios of smooth disk migration and violent gravitational interactions. The findings align with the idea that gas giant formation often begins beyond the water snow line, where there is plenty of solid material for rapid core growth.
Future development
In the future, with new instruments like high-resolution telescopes, we'll be able to measure not only elemental abundances but also isotopic ratios, painting an even more detailed picture. Improved models that include multi-planet dynamics and more precise disk chemistry will help refine migration scenarios. It's also promising to explore how atmospheric composition relates to host star properties across large samples, revealing statistical trends in giant planet formation.
Impact
The results will impact planet formation theory, especially our understanding of the role of snow lines and migration. They are also crucial for interpreting data from exoplanet atmosphere missions like JWST and the upcoming Ariel.
Next steps
The next step will be to expand the sample of hot Jupiters with known atmospheric compositions and include effects like photodissociation and atmospheric escape in the models for a more accurate comparison with observations.
Key open problems
The research tackles a fundamental problem in planetary system formation and evolution: the mechanisms for rapid growth of giant cores, the role of disk turbulence, and the origin of orbital architectures, including hot Jupiters and super-Earths.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The authors modeled the formation and migration of nine hot Jupiters, tracking the chemical composition of their atmospheres as a function of the positions of snow lines. They used the ChemComp code, which accounts for the individual metallicity of parent stars and new chemical processes. The results show that most planets were born beyond the water ice line and then migrated inward, consistent with dynamical scenarios.
Where its strength is
A key strength is the combination of detailed chemical modeling with observational data on atmospheres and orbits, which allows pinpointing the birthplaces of planets and constraining migration mechanisms.
Where it could go next
- It might be interesting to see how tidal evolution after dynamical scattering affects the orbits and internal structure of planets, especially in the context of the eccentricities mentioned in the paper. In our archive, there is a detailed analysis of tidal interactions for Jupiters on eccentric orbits.based on: Tidal Swings of an Eccentric Jupiter arXiv:2607.16406v1
- It would be intriguing to compare the migration conclusions with a study where tidal waves sort planets by mass and orbit—this could provide an additional explanation for the observed distribution of hot Jupiters.based on: How Tidal Waves Sort Planets arXiv:2606.20789
- It would be interesting to account for water formation processes in planetary interiors, as in a study on the interaction between hydrogen and magma, to refine the interpretation of atmospheric oxygen abundances.based on: How Hydrogen and Magma Create Water Deep Inside Planets arXiv:2511.06507
Worth trying
- Test a joint analysis of chemical tracers and tidal evolution for systems with high eccentricity.
- Investigate how accounting for mass sorting of planets in tidal scenarios changes predictions about birthplaces.
- Incorporate endogenous water formation into the model to separate the contributions of primordial composition and internal processes.
Why this may matter
Refining the migration pathways and chemical evolution of hot Jupiters is important for understanding the architecture of planetary systems and the conditions under which giant planets can exist.
Nearby works in our archive
- Tidal Swings of an Eccentric Jupitersimilarity 0.69
- How Tidal Waves Sort Planetssimilarity 0.69
- How Hydrogen and Magma Create Water Deep Inside Planetssimilarity 0.65
- Worlds with Eternal Magma Oceanssimilarity 0.63
- Why Do Hot Jupiters Swell?similarity 0.62
- How Stars Lose Their Hydrogen Envelopesimilarity 0.62
The closest work in the archive sits at 0.69 by meaning — the usual level for our corpus.
Some hot Jupiters, like WASP-12b, are so close to their star that their atmospheres evaporate, forming comet-like tails of heavy elements. And on planet WASP-76b, it rains iron on the day side!
In Peter F. Hamilton's sci-fi novel 'Pandora's Star', gas giants with extreme weather reminiscent of hot Jupiters are described. And in the movie 'Interstellar', Miller's planet experiences colossal tidal forces similar to those acting on nearby gas giants.
Key numbers
- Number of studied hot Jupiters: 9
- Minimum number of planets born beyond the ice line: 6
- Typical protoplanetary disk viscosity: 10⁻⁴ – 10⁻³
- Protoplanetary disk mass: 0.07 M⋆
- Water ice crystallization temperature: 130 K