Introduction

PLATON (PLanetary Atmospheric Transmission for Observer Noobs) is a fast and easy to use forward modelling and retrieval tool for exoplanet atmospheres. It is based on ExoTransmit by Eliza Kempton. The two main modules are:

  1. TransitDepthCalculator: computes a transit spectrum for an exoplanet
  2. Retriever: retrieves atmospheric properties of an exoplanet, given the observed transit spectrum. The properties that can be retrieved are metallicity, C/O ratio, cloudtop pressure, scattering strength, and scattering slope

The transit spectrum is calculating from 300 nm to 30 um, taking into account gas absorption, collisionally induced gas absorption, and Rayleigh scattering. TransitDepthCalculator is written entirely in Python and is designed for performance. By default, it calculates transit depths on a fine wavelength grid (λ/Δλ = 1000 with 4616 wavelength points), which takes ~170 milliseconds on a midrange consumer computer. The user can instead specify bins which are directly relevant to matching observational data, in which case the code avoids computing depths for irrelevant wavelengths and is many times faster.

Retriever uses TransitDepthCalculator as a forward model, and can retrieve atmospheric properties using either MCMC or nested sampling. Typically, nestled sampling finishes in < 10 min. MCMC relies on the user to specify the number of iterations, but typically reaches convergence in less than an hour.