# Tidal Distortions as a Bottleneck on Constraining Exoplanet Compositions

**Authors:** David Berardo, Julien de Wit  
**arXiv:** [2301.08755](https://arxiv.org/abs/2301.08755)  
**Date:** 2023-01-20  
**Summary page:** https://www.summarizepaper.com/en/paper/2301.08755v2/

## Key points

- Advancements in the study of exoplanets have led to an increase in confirmed planets and improved precision of observations.
- Subtle effects, such as the distortion of a short period planet by its host star, can bias interpretations of exoplanet observations.
- A team led by David Berardo and Julien de Wit extended previous work on gravitational potential formulation to nearly 200 planets with periods less than 3 days.
- Five planets exhibited density variations greater than 10%, and up to 20 planets had deviations greater than 5% due to tidal effects.
- An analytic approximation for deviation was derived as a function of the orbital period, transit depth, and mass ratio between the planet and host star.
- Upgraded observational precision will cause shape deviations to become a bottleneck with regards to analysis of exoplanet compositions.
- Transit depth and RV amplitude contribute more often towards larger amounts of uncertainty compared to model-dependent variables such as stellar mass and radius.

## Summary

The study of exoplanets has seen significant advancements in recent years, with improvements in the precision of observations and an increase in the number of confirmed planets. However, these advancements have also highlighted the need to better understand subtle effects that may bias interpretations of exoplanet observations. One such effect is the distortion of a short period planet by its host star, which can affect its derived density and therefore impact our understanding of its composition. To address this issue, a team led by David Berardo and Julien de Wit extended previous work on gravitational potential formulation to a sample of nearly 200 planets with periods less than 3 days. They found that five planets exhibited density variations greater than 10%, and as many as 20 planets had deviations greater than 5%. To allow for rapid determination of these tidal effects, they derived an analytic approximation for deviation as a function of the orbital period, transit depth, and mass ratio between the planet and host star. The researchers discovered that current density error bars are typically larger than tidal deviations. However, reducing uncertainty on transit depth and radial velocity amplitude by a factor of three causes tidal effects to dominate density errors in more than 40% of planets in their sample. This implies that upgraded observational precision will cause shape deviations to become a bottleneck with regards to analysis of exoplanet compositions. The two parameters were found to dominate uncertainties compared to errors on stellar mass and radius. The team identified eight planets (including WASP-19 b, HAT-P-7 b, and WASP-12 b) for which current density uncertainties are up to four times smaller than the potential shift due to tides. This suggests a possible four-sigma bias on their density estimates. Furthermore, the researchers quantified the relative contribution each variable makes towards overall uncertainty using equation (12). They found that measurement-dependent variables such as transit depth and RV amplitude contribute more often towards larger amounts of uncertainty compared to model-dependent variables such as stellar mass and radius. The RV amplitude alone contributes at least 60% of the relative uncertainty for approximately 20% of the planets in their sample. In conclusion, this study highlights the need to consider tidal effects when interpreting exoplanet observations and suggests that upgraded observational precision will cause shape deviations to become a bottleneck with regards to analysis of exoplanet compositions.