Free Planet Orbital Period Calculator
Find the orbital period of a planet using Kepler's third law in a simplified form: T = sqrt(a^3) years, where a is the semi-major axis in AU.
AU
Orbital Period
1.000000 years
Period (days)365.25 days
Orbital Period vs Semi-Major Axis
Planet Orbital Period from Kepler's Law
When the semi-major axis is given in astronomical units and the central star has approximately one solar mass, the period in years is simply the 3/2 power of the axis.
Formula
T = a^(3/2) years
This is the elegant form of Kepler's third law with the Sun's mass baked in.
Example Calculation
Earth at 1 AU from the Sun.
- 01T = a^(3/2)
- 02T = 1^(3/2) = 1 year
- 03T = 365.25 days