The Effect of Adiabatic Index on Radius Evolution and the Mass Loss
- 1University of California
Abstract
Models that track the size evolution of exoplanets often assume a prescribed initial thermal state or a single adiabatic index to describe the planetary interior structure, the latter of which is taken to be γ ∼ 1.4, which is likely appropriate for evolved planets (≳1 Gyr). Extrapolating this high γ to earlier ages (down to ∼million years old) is problematic since, according to thermodynamics, the adiabatic index of young planets is ∼1.2, which is low enough to drastically change how interior mass is distributed. We quantify the effect of varying the adiabatic index from 1.2 to 1.4 on the expected radius of the exoplanet over time. We find that envelopes of larger adiabatic indices start puffier with all else equal and undergo faster radius contraction with accelerated mass loss. The assumption of high γ can therefore overestimate the effect of mass loss in shaping the exoplanetary population, especially when young planets are considered. We highlight the need for a more careful consideration of the initial thermal condition and internal mass distribution of planets in evolutionary models to properly interpret the radii measurements of exoplanets.
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BibTeX
@article{collander2026effect,
title = {{The Effect of Adiabatic Index on Radius Evolution and the Mass Loss}},
author = {Arick Collander and Eve J. Lee},
journal = {The Astrophysical Journal},
year = {2026},
month = nov,
doi = {10.3847/1538-4357/aeb2eb},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/aeb2eb},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - The Effect of Adiabatic Index on Radius Evolution and the Mass Loss
AU - Arick Collander
AU - Eve J. Lee
PY - 2026
DA - 2026/11/04
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/aeb2eb
UR - https://iopscience.iop.org/article/10.3847/1538-4357/aeb2eb
AB - Models that track the size evolution of exoplanets often assume a prescribed initial thermal state or a single adiabatic index to describe the planetary interior structure, the latter of which is taken to be γ ∼ 1.4, which is likely appropriate for evolved planets (≳1 Gyr). Extrapolating this high γ to earlier ages (down to ∼million years old) is problematic since, according to thermodynamics, the adiabatic index of young planets is ∼1.2, which is low enough to drastically change how interior mass is distributed. We quantify the effect of varying the adiabatic index from 1.2 to 1.4 on the expected radius of the exoplanet over time. We find that envelopes of larger adiabatic indices start puffier with all else equal and undergo faster radius contraction with accelerated mass loss. The assumption of high γ can therefore overestimate the effect of mass loss in shaping the exoplanetary population, especially when young planets are considered. We highlight the need for a more careful consideration of the initial thermal condition and internal mass distribution of planets in evolutionary models to properly interpret the radii measurements of exoplanets.
SN - 1538-4357
SN - 0004-637X
ER -