# 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. CombinedRetriever: can retrieve atmospheric properties for transit depths, eclipse depths, or a combination of the two.

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=10,000 or R=375,000) 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. The speed of these retrievals is highly dependent on the wavelength range, data precision, prior ranges, opacity resolution, and number of live points (nested sampling) or iterations/walkers (MCMC). A very rough guideline is that a retrieval with 200 live points and R=1000 (suitable for exploratory work) for STIS + WFC3 + IRAC 3.6 um + IRAC 4.5 um data takes <1 hour, while a retrieval with 1000 live points and R=10,000 (suitable for the final version) takes 1-2 days. There are a variety of ways to speed up the retrieval, as described in our PLATON II paper. These include using correlated-k instead of opacity sampling with R=10,000, or removing the opacity data files of unimportant molecules (thereby zeroing their opacities).