petitRADTRANS 3: most notable changes


petitRADTRANS 3 comes with a lot of quality-of-life features, optimization, code rationalization, more standard opacities, but also with breaking changes. For more details, see the detailed changelog.

Hereafter is a short summary of the most notables changes.

Instructions to convert pRT2 opacities in the pRT3 format are given in the conversion section.


  • Full integration of SpectralModel: a convenient and modular way to manage your models and run retrievals, both for low resolution and high resolution observations.

  • Full integration of Planet: the latest NASA exoplanet archive data accessible as easily as Planet.get("<planet name>")!

  • Automatic opacity files download: missing a file? Let petitRADTRANS download it for you! As a plus, now downloading the 12 GB default input_data folder at installation is no longer necessary!

  • Support for HDF5 opacity files: load opacities faster than ever!

  • More CIA and cloud opacities, see Available opacities species.

  • Helpful error and warning messages for the most common issues.

  • Possibility to retrieve or optimize uncertainties (use with caution).

  • Data preparation of high-resolution observations: remove telluric contaminations with SysRem or polynomial fitting.

  • Simple transit light loss modelling (ingress, egress) for SpectralModel.

  • Useful built-in functions: convert from Bayes factor to sigma significance, calculate uncertainties, orbital phases, and more, with easy-to-use functions.


  • Functions, arguments and attributes now have clearer names (e.g., calc_flux() was changed to calculate_flux()).

  • Spectral functions of Radtrans (calculate_flux and calculate_transit_radii) now return wavelengths, spectrum, and a dictionary containing additional outputs, instead of nothing.

  • Function Radtrans.calculate_flux now output by default wavelengths in cm (instead of frequencies in Hz) and flux in instead of Setting the argument frequencies_to_wavelengths=False restores the previous behaviour.

  • Function Radtrans.calculate_transit_radii now output by default wavelengths in cm (instead of frequencies in Hz). Setting the argument frequencies_to_wavelengths=False restores the previous behaviour.

  • Object Radtrans is now imported using from petitRADTRANS.radtrans import Radtrans (was from petitRADTRANS import Radtrans) for more stable installation.

  • Improved petitRADTRANS memory usage and performances.

  • Input data path is now stored in a config file within the folder <HOME>/.petitRADTRANS, generated when installing the package or using it for the first time.


  • Multiple Radtrans attributes, some are now function outputs.

Converting pRT2 opacities to pRT3 format


The conversion is necessary because we switched from Fortran binary tables to HDF5. Correlated-k opacity files you downloaded from ExoMol do not have to be converted, since we adopted their format. You still need to place them in differently called folders, however, see the instructions on “Importing opacity tables from the ExoMol website” here.

  • If you ever added custom opacities to pRT before pRT3 was released (May 2024) you need to convert them to pRT3 (HDF5) format before you can use them, this is explained below.

  • If you have a pRT2 input_data folder full of standard pRT2 opacities (those that we supplied through Keeper) you can convert those all in one go, this is also explained below. Note that this will only work for pRT’s standard opacities, not for species you added yourself.


We recommend starting with the conversion of your custom opacity species, because you need an input_data folder in the old pRT2 format for doing this (i.e., containing the opa_input_files directory). If you run convert_all(clean=True), old pRT2 opacity files will be removed after conversion, including the input_data/opa_input_files directory.


If you convert all standard pRT2 opacities with convert_all() (see below) the input_data structure will be modified and can no longer be used with pRT2. If you still want to use pRT2, it is better to make a copy of the input_data directory before conversion.

Below are examples for how to do the conversion.

Correlated-k opacities

To manually convert correlated-k opacities, you can use the following example:

from petitRADTRANS.__file_conversion import _correlated_k_opacities_dat2h5_external_species
from petitRADTRANS.chemistry.prt_molmass import get_species_molar_mass

    longname='12C-1H4__HITEMP.R1000_0.1-250mu',  # see c-k file naming convention
    contributor='Your name',
    description="Using HITRAN's air broadening prescription.",


Argument longname must be a valid pRT file name, otherwise it will be rejected. The correlated-k file naming is available in the available opacities section.

The function needs to following input parameters:

  • path_to_species_opacity_folder: string that gives the absolute path of the folder that contains the correlated-k opacity files in the old pRT2 format (in the example above we are converting 'CH4_hargreaves'.

  • path_prt2_input_data: absolute path of the pRT2 input data folder.

  • longname: The species (unique) longname following the pRT3/Exomol format, which will also be the name of the HDF5 file (leave out the '.h5' extension).

  • doi: DOI of the reference that describes the line list ('10.3847/1538-4365/ab7a1a' points to Hargreaves et al. 2020 in our case). Can be left empty for internal use.

  • contributor: in case you want to share your HDF5 file with us (please :) ), this is the contributor name we will mention in the available opacities section.

  • description: any additional information you think is useful to know for a user.

  • molmass: the mass of the absorber in atomic mass units.

After conversion the new HDF5 file will be placed into your pRT2 input data folder, in the above example in '/Users/molliere/pRTv2/input_data/opacities/lines/corr_k/'. You then need to move the file 12C-1H4__HITEMP.R1000_0.1-250mu.ktable.petitRADTRANS.h5 from there into the pRT3 folder, following the folder structure described for adding Exomol opacities. In our example here, the new path of the file is is /Users/molliere/pRT3/input_data/opacities/lines/correlated_k/CH4/12C-1H4/. Note the change in the path to the input folder of pRT3. Also do not forget to adapt your absolute paths accordingly (very likely you do not have a folder called molliere, for example).

Line-by-line opacities

To manually convert line-by-line opacities, you can proceed as follow:

First, move the folder containing your pRT2-formatted opacities of the species you want to convert to the pRT3 input data folder, using the folder structure described for adding Exomol opacities. For \(\rm CH_4\)’s main isotopologue, this would correspond to /path/to/input_data/opacities/lines/line_by_line/CH4/12C-1H4/pRT2_CH4_directory (here pRT2_CH4_directory is the directory you moved, you don’t need to change its name). Then, execute the following in a Python console:

from petitRADTRANS.__file_conversion import line_by_line_opacities_dat2h5
from petitRADTRANS.chemistry.prt_molmass import get_species_molar_mass

    directory='/path/to/input_data/opacities/lines/line_by_line/species/isotopologue/old_directory',  # change accordingly
    path_input_data='/path/to/old/pRT2/input_data', # path to old pRT input data folder
    output_name='pRT_valid_opacity_filename',  # e.g., '12C-1H4__HITEMP.R1e6_0.3-28mu', see lbl file naming convention
    molmass=get_species_molar_mass('SpeciesChemicalFormula'),  # change accordingly (e.g. '12C-1H4')
    doi='doi of the opacity source',  # change accordingly, can be left empty for personal use
    contributor='Your name',  # change accordingly, can be left empty for personal use
    clean=True  # if True, automatically remove the old pRT2 opacity files stored in "directory"


Argument output_name must be a valid pRT file name, otherwise it will be rejected. The line-by-line file naming convention is available in the available opacities section.

If you have put your old directory at the correct place, the resulting file should already be in the correct position (here, '/path/to/input_data/opacities/lines/line_by_line/species/isotopologue/pRT_valid_opacity_filename.xsec.petitRADTRANS.h5').

Automatic conversion of the pRT2 input_data folder

Once you have set the path to your input_data folder (see “Getting started”) The simplest way to convert you pRT2 opacities into the pRT3 format is to use the provided convert_all function:

from petitRADTRANS.__file_conversion import convert_all

convert_all(clean=True)  # to not remove the old files automatically, set clean to False


If you want to keep the pRT2-formatted files, you should use clean=False. Note that some of these files will be displaced, and hence will no longer be usable as is by pRT2. Running clean=True will minimize the impact of the conversion on your storage.

Note that this will only convert the pRT2 default opacities. Custom-made opacities need to be converted manually (see above sections).