# Why do planets not stop revolving around the Sun? [duplicate]

Why do planets revolving around the Sun not stop revolving? Note I am not asking why planets do not collapse with Sun.

## marked as duplicate by John Rennie, ACuriousMind♦, Kyle Kanos, fibonatic, Brandon EnrightJan 27 '15 at 21:04

• Why should they? – ACuriousMind Jan 27 '15 at 18:04
• What would stop them moving? – HDE 226868 Jan 27 '15 at 18:07
• possible duplicate of Where do planets get energy to revolve around sun? – John Rennie Jan 27 '15 at 18:09
• Who says energy isn't being dissipated? – Kyle Kanos Jan 27 '15 at 18:34
• harshit, the answer can be summarized to something like: An object in nearly-uniform circular motion stays in nearly-uniform circular motion. – HDE 226868 Jan 27 '15 at 18:37

The angular momentum of any particular planet around the Sun (or the angular momentum of the planet-Sun system around its center of mass) is huge. To change the angular momentum of a system requires a torque exerted over some time interval: $$\frac{dL}{dt}=\tau \hspace{1in} \Delta{L}=\int \tau dt$$

There are simply no torques large enough to change the angular momentum appreciably in time frames meeting human existence. Even planet-to-planet gravitational interactions don't create appreciable torques as far as revolution around the Sun is concerned.

The Sun (ideally as a spherical, radially distributed mass) exerts no torque on a planet about any point along their adjoining line.

On the other hand, the existence of Neptune was inferred by wobbles in the motion of Uranus due to their mutual gravitational interaction.

There are apparently some very small torques within the solar system which result in the precession of rotational axes, but nothing large enough to cause revolution to change.

• I just told you. Nothing is changing their angular momentum. Do you understand what angular momentum is? – Bill N Jan 27 '15 at 18:36
• Do you mean that because of angular momentum planets will keep revolving around the sun. – harshit yagyasen Jan 27 '15 at 18:44
• Because nothing is changing the angular momentum of the planets. – Bill N Jan 27 '15 at 19:00
• Do Spinning top also have angular momentum? – harshit yagyasen Jan 27 '15 at 19:12
• See this link: physics.stackexchange.com/questions/271/… – Bill N Jan 27 '15 at 20:35