# Absolute densities in exoplanetary systems: photodynamical modelling of Kepler-138

**Authors:** J. M. Almenara, R. F. Díaz, C. Dorn, X. Bonfils, S. Udry  
**arXiv:** [1805.12520](https://arxiv.org/abs/1805.12520)  
**Date:** 2018-05-31  
**Summary page:** https://www.summarizepaper.com/en/paper/1805.12520v1/

## Key points

- Study focuses on determining density of transiting planets in multiple systems using photometry data alone
- Photodynamical model used to simulate individual photometric observations and analyze Kepler-138 planetary system
- Approach provides more precise results compared to previous determinations
- Discrepancy found in stellar bulk density compared to previous study, leading to inconsistencies in determination of masses and radii for star and planets
- Kepler-138b identified as having a size between Mars and Earth
- Generalized Bayesian inference model used to further characterize planetary interiors and quantify interior degeneracy
- Analysis reveals significantly thick volatile layers for both Kepler-138b and Kepler-138d, with gas layer of Kepler-138b likely enriched
- Kepler-138c appears to be predominantly rocky
- Study demonstrates how photodynamical modeling can provide valuable insights into exoplanetary systems' densities and interior compositions
- Contributes to understanding of planetary formation and evolution processes in multiple systems like Kepler 138 by allowing better characterization of interiors with greater precision than before

## Summary

The study focuses on determining the density of transiting planets in multiple systems using photometry data alone. By applying a photodynamical model, which simulates individual photometric observations, the researchers analyze the Kepler-138 planetary system. They find that this approach provides more precise results compared to previous determinations. Additionally, they identify a discrepancy in the stellar bulk density when compared to a previous study, leading to inconsistencies in the determination of masses and radii for both the star and the planets. One significant finding is that Kepler-138b, an interior planet in the system, has a size between Mars and Earth. To further characterize the planetary interiors and quantify interior degeneracy, the researchers utilize a generalized Bayesian inference model. This model allows them to calculate confidence regions for parameters such as core thickness, mantle thickness, and ocean and gas layers. The analysis reveals that both Kepler-138b and Kepler-138d have significantly thick volatile layers. Furthermore, it suggests that the gas layer of Kepler-138b is likely enriched. On the other hand, Kepler-138c appears to be predominantly rocky. Overall, this study demonstrates how photodynamical modeling can provide valuable insights into exoplanetary systems' densities and interior compositions. It contributes to our understanding of planetary formation and evolution processes in multiple systems like Kepler-138 by allowing us to better characterize their interiors with greater precision than before.