The Worldbuilder Project

Understanding even gross features of planets is surprisingly complicated. Conditions on the surface of a planet depend crucially on temperature - whether the surface is warm or cold determines whether liquids exist, whether a gaseous atmosphere can exist or whether everything is frozen solid. Temperature in turn depends on the radiation budget, the distribution of albedo and the properties of the atmospere. Weather and climate are nothing but manifestations of atmosphere heat transport driven by imbalances in the radiation budget. Daily and yearly variation of temperature and the existence of climate zones is linked to the variation of the radiation budget along an orbit.

There's significant feedback loops involved, for instance a cooling planet with liquid water might see more snow, which increases albedo, which in turn leads to a runaway glaciation. A heating planet increases the water vapour content in the atmosphere, which in turn strengthens the Greenhouse effect. Trying to understand and anticipate the interaction between such feedback loops without a simulation is difficult however.

Have you sometimes wondered how climate zones and seasons might be different if Earth were significantly more tilted? How weather patterns might be altered as a result? Or, how the seaons would be like on a planet that orbits a double star system and has two suns in the sky? How weather would be on a world in which the troposphere is not capped by a low tropopause because there is no UV component in the stellar radiation heating a stratosphere? These problems are what the world-builder is intended to address.

General Scope

The idea of the worldbuilder is to develop an easy-to-use simulation code for a planet that can be subsequently refined. Via a configuration file, a setting can be defined, the simulation can be instructed to run to a certain state, and then various quantities and distributions can be requested as plots. Currently available features include:

  • Configuration file parser
  • Numerical n-body orbital dynamics solver including General Relativity corrections
  • Radiative heat balance, both global and position-differential
  • Simple Greenhouse effect model
  • Plot output in gnuplot or plotmtv format
Some features under development are:
  • detailed simulation of radiation propagation through the atmosphere
  • dynamical position-differential albedo changes
  • atmosphere vertical structure
  • stability constraints - atmosphere evaporation, tidal breakup, orbital perturbations

Orbital simulations

The simulation code comes with a numerical planar n-body solver, i.e. the simulation is not restricted to stable orbits and can be used to investigate mid-term stability problems or impact danger from asteroids. General relativity effects on the trajectory are taken into account (i.e. there is periapsis precession and a last stable orbit around a Black Hole), but the time delay in a strong gravity field is not simulated.

Thermal simulation

The code solves the surface heat budget based on incoming radiation, outgoing infrared radiation, IR retention by the atmosphere, thermal inertia of the ground and gross heat transport by the atmosphere for a user-defined collection of surface elements. The surface can be characterized on average or in detail by specifying a material map, such that e.g. continents and oceans can be drawn. Quantities of interest include current temperature distribution, daily temperature variations at a position, yearly averaged heat budget to study climate zones or global average temperatures as function of the current orbital state.

Radiation propagation

Based on parametrized absorption cross sections as a function of wavelength for different gases, the code can compute transmission coefficients for different types of radiation given the composition and surface pressure of an atmosphere. Such coefficients can e.g. be utilized to determine the Greenhouse effect in different atmospheres, or to get an idea of the hard radiation levels on the surface of a world.

Case studies

The simulation code is benchmarked by computing problems in out own solar system as well as tested by running some fictional test cases. Below is a list of what kind of problems the worldbuilder has been applied to so far.


The source code of the simulation is available under the GNU General Public License 2.0+ - in short, you may use, re-distribute and modify the software freely, but you are required to provide the source code and license any additions/changes also GPL if you re-distribute.

The tarball extracts into its own subfolder, the code is inside the src/ folder, the examples/ folder a few sample configuration files. A few tutorials on how to use the software are found below. There is no Linux executable provided.

Download Worldbuilder V0.3


Setting up a star
Setting up a planetary orbit
Irradiation part I
Irradiation part II
Orbital stability
Planets orbiting binary stars
Measuring days
Time series

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Created by Thorsten Renk 2016-2021 - see the disclaimer, privacy statement and contact information.