---
name: "planets"
section: "1"
platform: "linux"
package: "planets"
version: "0.1.13-23+b3"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/planets"
---

# planets(1)

Gravitational simulation of planetary bodies

## NAME

planets - Gravitational simulation of planetary bodies

## DESCRIPTION

**Planets** is a simple interactive program for playing with simulations of planetary systems. It is great teaching tool for understanding how gravitation works on a planetary level.

The user interface is aimed at being simple enough for a fairly young kid can get some joy of it.  There's also a special kid-mode aimed at very young children which grabs the focus and converts key banging into lots of random planets.

## KEYBINDINGS

### Universe definition

- a

  Add Planet

- j

  Place random orbital planet

- r

  Place random planet

- u

  Undo (undoes last planet insertion)

- e

  Reset to empty universe

- g

  Go Back (goes back to just after last planet insertion)

- Mouse

  Click on a planet to delete it

### Physics

- b

  Toggle bounce (experimental)

### Display control

- Cursor keys

  Panning

- c, Space

  Move display to center of mass

- x

  Initiate center of mass tracking

- \=

  Zoom in

- \-

  Zoom out

- p

  Toggle Pause

- o

  Change all colors randomly

- t

  Toggle Trace

- d

  Double Trace Length

- h

  Halve Trace Length

- Mouse

  Drag a box around a set of planets to follow the center of mass of those planets

### Program control

- H

  Display help dialog

- k

  Display option dialog

- Ctrl-Shift-k

  Toggle kid-mode.  Kid mode locks the keyboard and mouse, so the only way to get out is to toggle kid-mode again to get out.

- l

  Load Universe After pressing l, press any other character to load the universe with that name. Universes are stored in ~/.planets/ .

- s

  Save Universe After pressing s, press any other character to save the universe with that name. Universes are saved in ~/.planets/ .

- q, Esc

  Quit

## TECHNICAL DETAILS

**Planets** uses a fourth-order runge-kutta approximation for the simulation itself.  Planet bouncing is achieved by adding a repulsive force to planets at close quarters.   **Planets** is fairly flexible: you can change the gravitational constant, the time-slice of the simulation, and even the exponent used in the gravitational law.  Universes are saved in the ~/.planets directory, and are simple human readable and editable files.

## BUGS

Currently bouncing doesn't work very well unless you make the time-slice quite small.  Ideally, it would be nice to have a billiard-style bounce system, but it's not clear how to do this accurately in the presence of a strong gravitational field.

## AUTHOR

**Planets** was written by Yaron M. Minsky \<yminsky@cs.cornell.edu\> as a gift for his nephew, Eyal Minsky-Fenick.

This manpage was contributed originally by Martin Pitt \<martin@piware.de\> for the Debian GNU/Linux system (but may be used by others).