Implemented transit models

PyTransit implements a set of transit models that all share a common interface that is described in more detail in Transit Models.

Road Runner model

RoadRunner (pytransit.RoadRunnerModel) is a fast and flexible transit model presented in Parviainen (accepted to MNRAS 2020). I’ll write a proper documentation soon, but these example notebooks should help you up to speed until then

Uniform model

The uniform model (pytransit.UniformModel and pytransit.UniformModelCL) reproduces an exoplanet transit over a uniform disc. This model is useful when modelling secondary eclipses, or when the effects from the stellar limb darkening can be ignored.

Quadratic model

The quadratic transit model (pytransit.QuadraticModel and pytransit.QuadraticModelCL) reproduces an exoplanet transit over a stellar disk with the limb darkening modelled by a quadratic limb darkening model, as presented in Mandel & Agol (ApJ 580, 2001).

Oblate star model


Power-2 model

Power-2 model (pytransit.QPower2Model and pytransit.QPower2ModelCL) implements the transit model with a power-2 law limb darkening profile presented by Maxted & Gill (A&A 622, A33 2019). The model is fast to evaluate and aims to model the limb darkening accurately for cool stars.


  • Accurate limb darkening model for cool stars.
  • Fast to evaluate.

General model

The general model (pytransit.GeneralModel) implements the flexible transit model presented by Giménez (A&A 450, 2006). The stellar limb darkening follows a “general” limb darkening model, and the accuracy of limb darkening can be increased as needed.

The model is calculated using a polynomial series and both the number of polynomials npoly and the number of limb darkening coefficients nldc can be set in the initialisation. Higher npoly leads to a more accurate transit model, but also increases computation time. Increasing the number of limb darkening coefficients doesn’t significantly increase computation time, but


  • A flexible model that can model limb darkening accurately.
  • Somewhat slower to evaluate than the specialized models.
  • PyTransit implements a special “transmission spectroscopy mode” for the general model that accelerates the transit model evaluation significantly for transmission spectroscopy where the light curves are computed from a spectroscopic time series.
  • The four-coefficient model presented in Mandel & Agol (ApJ 580, 2001) is not implemented in PyTransit since the Giménez model offers the same functionality with higher flexibility.

Chromosphere model

Optically thin shell model (pytransit.ChromosphereModel and pytransit.ChromosphereModelCL) by Schlawin et al. (ApJL 722, 2010) to model a transit over a chromosphere.