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. EclipseDepthCalculator: computes an eclipse spectrum
  3. 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
  4. CombinedRetriever: can retrieve atmospheric properties for eclipse depths, or a combination of transit and eclipse depths

The transit spectrum is calculated from 300 nm to 30 um, taking into account gas absorption, collisionally induced gas absorption, clouds, and 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 ~65 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. The user can also download higher resolution data (R=2000 or R=10,000) from here and drop them into PLATON’s data folder; the runtime is roughly proportional to the resolution.

The eclipse spectrum is calculated with the same physics included, but it does not include scattering as a source of emission; scattering is only included as a source of absorption.

The retrievers use TransitDepthCalculator/EclipseDepthCalculator as a forward model, and can retrieve atmospheric properties using either MCMC or nested sampling. Typically, nestled sampling with only transit depths finishes in < 10 min. MCMC relies on the user to specify the number of iterations, but typically reaches convergence in less than an hour. Eclipse depths typically take longer to calculate by a factor of a few, resulting in longer retrievals.