The Student Room Group


Calculate the orbital time period in years for a planet going round the Sun in an orbit of radius twice that of Earth. Give your answer to 3 significant figures.
You may use Kepler’s Third Law for this question. T^2 = (4π^2/GM)r^3
You then substitute the mass of the Sun and the diameter of the Earth into the equation to find T, the period.
For objects orbiting the sun there are some very handy natural units.

Orbital radius - AU (1 AU is the average sun earth distance)

Orbital period - Year (time for earth to orbit the sun)
Mass = mass of the sun

Then Kepler 3 is just T^2/ R^3 = 1

And if you do your working out this way the answer will pop out in units of earth year without any further conversion.

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