The ratio of the squares of the periods of any two planets is equal to the ratio of the cubes of their average distance from the sun. So, answer is "DISTANCE".
The laws describing the motions of the planets within the system are termed as Kepler's Laws.
Kepler's three laws of planetary motion are often stated as follows :
Kepler's 1st law : “The orbits of the planets are ellipses, with the Sun at one focus of the ellipse”. The Sun isn't at the middle of the ellipse, but is instead at one focus (generally there's nothing at the opposite focus of the ellipse).
Kepler's 2nd law : “The line joining the world to the Sun sweeps out equal areas in equal times because the planet travels round the ellipse”. Thus, a planet executes elliptical motion with constantly changing angular speed because it moves about its orbit. The purpose of nearest approach of the world to the Sun is termed perihelion; the purpose of greatest separation is termed ‘aphelion’. Hence, by Kepler's second law, the earth moves fastest when it's near perihelion and slowest when it's near aphelion.
Kepler's 3rd law : “The ratio of the squares of the revolutionary periods for 2 planets is capable the ratio of the cubes of their semi–major axes”. Kepler's Third Law implies that the amount for a planet to orbit the Sun increases rapidly with the radius of its orbit. Thus, we discover that Mercury, the innermost planet, takes only 88 days to orbit the Sun.
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