# Directly imaged exoplanets in reflected starlight. The importance of knowing the planet radius

**Authors:** Óscar Carrión-González, Antonio García Muñoz, Juan Cabrera, Szilárd Csizmadia, Nuno C. Santos, Heike Rauer  
**arXiv:** [2006.08784](https://arxiv.org/abs/2006.08784)  
**Date:** 2020-06-15  
**Summary page:** https://www.summarizepaper.com/en/paper/2006.08784v1/

## Key points

- The study investigates the information content in reflected-starlight spectra of exoplanets
- Focuses on Barnard's Star b, a candidate super-Earth with an assumed radius 0.6 times that of Neptune, an atmosphere dominated by H$_2$-He, and a CH$_4$ volume mixing ratio of 5$\cdot$10$^{-3}$
- A model of the exoplanet is set up with seven parameters including its radius, atmospheric methane abundance, and basic properties of a cloud layer
- Synthetic spectra are generated at zero phase (full disk illumination) from 500 to 900 nm and spectral resolution R$\sim$125-225
- Using an MCMC-based retrieval methodology, they analyze which planet/atmosphere parameters can be inferred from the measured spectrum and the theoretical correlations amongst them
- If the planet radius is known, they can generally discriminate between cloud-free and cloudy atmospheres and constrain the methane abundance to within two orders of magnitude
- Without a radius determination it is challenging to discern whether the planet has clouds and estimates on methane abundance degrade significantly
- Even without precise knowledge of an exoplanet's radius they can still constrain it to within a factor of two for all cases explored

## Summary

In their study, "Directly imaged exoplanets in reflected starlight. The importance of knowing the planet radius," Óscar Carrión-González and colleagues investigate the information content in reflected-starlight spectra of exoplanets. They focus on Barnard's Star b, a candidate super-Earth with an assumed radius 0.6 times that of Neptune, an atmosphere dominated by H$_2$-He, and a CH$_4$ volume mixing ratio of 5$\cdot$10$^{-3}$. The authors set up a model of the exoplanet described by seven parameters including its radius, atmospheric methane abundance, and basic properties of a cloud layer. They generate synthetic spectra at zero phase (full disk illumination) from 500 to 900 nm and spectral resolution R$\sim$125-225. They simulate a measured spectrum with a simplified, wavelength-independent noise model at Signal-to-Noise ratio S/N=10. Using an MCMC-based retrieval methodology, they analyze which planet/atmosphere parameters can be inferred from the measured spectrum and the theoretical correlations amongst them. They consider limiting cases in which the planet radius is either known or completely unknown, as well as intermediate cases in which the planet radius is partly constrained. If the planet radius is known, they can generally discriminate between cloud-free and cloudy atmospheres and constrain the methane abundance to within two orders of magnitude. However, if the planet radius is unknown or only partly constrained new correlations between model parameters occur and accuracy of retrievals decreases. Without a radius determination it is challenging to discern whether the planet has clouds and estimates on methane abundance degrade significantly. Nevertheless Carrión-González et al find that even without precise knowledge of an exoplanet's radius they can still constrain it to within a factor of two for all cases explored.