The Heavy-element Content Trend of Planets: A Tracer of Their Formation Sites

Yasuhiro Hasegawa1, Bradley M. S. Hansen2, and Gautam Vasisht1
Astrophysical Journal Letters 876, L32 Link to Article [DOI: 10.3847/2041-8213/ab1b5a]
1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
2Mani L. Bhaumik Institute for Theoretical Physics, Department of Physics & Astronomy, University of California Los Angeles, Los Angeles, CA 90095, USA

Identification of the main planet formation site is fundamental to understanding how planets form and migrate to their current locations. We consider the heavy-element content trend of observed exoplanets, derived from improved measurements of mass and radius, and explore how this trend can be used as a tracer of their formation sites. Using gas accretion recipes obtained from hydrodynamical simulations, we confirm that the disk-limited gas accretion regime is most important for reproducing the trend. Given that such a regime is specified by two characteristic masses of planets, we compute these masses as a function of the distance (r) from the central star, and then examine how the regime appears in the mass–semimajor axis diagram. Our results show that a plausible solid accretion region emerges at r sime 0.6 au and expands with increasing r, using the conventional disk model. Given that exoplanets that possess the heavy-element content trend distribute currently near their central stars, our results imply the importance of planetary migration that would occur after solid accretion onto planets might be nearly completed at r ≥ 0.6 au. Self-consistent simulations would be needed to verify the predictions herein.

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