Orbits · Learning guide

Why distant planets take longer

A planet’s year follows a simple pattern: larger orbits take disproportionately longer to complete.

The relationship

For a small body orbiting a star, period squared is proportional to the cube of the semi-major axis. With years, AU and solar masses, P = √(a³/M).

A useful comparison

At 1 AU around one solar mass, the period is approximately one year. At 4 AU the period is approximately eight years, not four. Distance and period do not grow in a straight line.

What the model leaves out

Real orbits are affected by other bodies, non-spherical mass distributions and relativistic corrections. The explorer illustrates the two-body relationship rather than predicting a spacecraft trajectory.

Read the orbit carefully

Semi-major axis describes the size of an ellipse. It is not generally equal to the planet’s instantaneous distance from the star.

Further reading: Reference source ↗. Written for this starter collection. Check primary references for specialised use.