Which statement best explains why planets closer to the Sun move faster?

• A. Gravity is stronger nearer the Sun
• B. The Sun's radiation accelerates inner planets
• C. Inner planets have more gravity
• D. They travel a shorter distance

Answer: (A)

Planets stay in orbit because gravity from the Sun provides the inward pull that must be balanced by the planet’s motion along the path. Closer to the Sun, gravity is stronger, so a higher orbital speed is required to keep the planet from being pulled in or flung outward. In mathematical terms, for a circular orbit the speed is v = sqrt(GM_sun / r); as r decreases, v increases. That makes the statement about stronger gravity nearer the Sun the best explanation for why inner planets move faster.

Radiation doesn’t set the orbital speed, the Sun’s radiation doesn’t push planets along their orbits in a sustained way. Inner planets don’t have more gravity of their own that changes their orbital speed around the Sun; the Sun’s gravity dominates. And although the inner orbit has a shorter distance to travel each revolution, the speed is determined by the gravitational balance, not merely by the path length.