Discovering the Growth Histories of Exoplanets: The Saturn Analog HD 149026b

The transiting “hot Saturn” HD 149026b, which has the highest mean density of any confirmed planet in the Neptune-Jupiter mass range, has challenged theories of planet formation since its discovery in 2005. Previous investigations could not explain the origin of the planet’s 45-110 Earth-mass solid core without invoking catastrophes such as gas giant collisions or heavy planetesimal bombardment launched by neighboring planets. Here we show that HD 149026b’s large core can be successfully explained by the standard core accretion theory of planet formation. The keys to our reconstruction of HD 149026b are (1) applying a model of the solar nebula to describe the protoplanet nursery; (2) placing the planet initially on a long-period orbit at Saturn’s heliocentric distance of 9.5 AU; and (3) adjusting the solid mass in the HD 149026 disk to twice that of the solar nebula in accordance with the star’s heavy element enrichment. We show that the planet’s migration into its current orbit at 0.042 AU is consistent with our formation model. Our study of HD 149026b demonstrates that it is possible to discover the growth history of any planet with a well-defined core mass that orbits a solar-type star.

💡 Research Summary

HD 149026b, discovered in 2005, is a transiting “hot Saturn” that stands out because of its exceptionally high mean density for a planet in the Neptune‑Jupiter mass range. Precise measurements of its mass (≈0.36 M_J) and radius (≈0.73 R_J) imply a solid core of roughly 45–110 M_⊕, far larger than predicted by standard core‑accretion models. Earlier explanations invoked catastrophic events such as giant‑planet collisions or an intense bombardment of planetesimals triggered by neighboring bodies, but these scenarios are statistically unlikely and require fine‑tuned conditions.

The present study demonstrates that the observed properties of HD 149026b can be reproduced within the conventional core‑accretion framework by adjusting three key ingredients: (1) the protoplanetary disk model, (2) the planet’s initial orbital distance, and (3) the solid‑mass budget of the disk. The authors adopt a Minimum‑Mass Solar Nebula (MMSN) surface‑density profile, Σ(r)=Σ_0 (r/1 AU)^{‑3/2}, as a baseline for the gas component. Spectroscopic analysis of the host star shows a metallicity of