Expert: Philip Stahl - 3/10/2004

Question
What keeps a planet in its place relative to other neighboring planets and what causes a planet to spin? Also, what determines the direction of a planet's spin?

Answer
Hello.

Your answers in turn:

1) Planets are not all necessarily "kept in place" relative to neighboring planets. For example, right now - Pluto is closer to the Sun than Neptune- which is now the most distant planet. This is because Pluto's highly elliptical orbit sometimes takes it 'within' Neptune's orbit.

Other than that, one may say it is the force of centripetal acceleration acting on each planet (originating from the Sun) which keeps each planet 'almost' in place!

2) Each planet probably initiated its spin when it collapsed from the primitive solar nebula. Whatever spin tendencies it now has were probably acquired at that time.

What KEEPS a planet spinning, is its own inertia. Inertia is the property whereby a body continues in motion - inthis case rotational motion, unless a large outside force acts to stop it.In the Earth's case the inertia can be estimated by assuming a rigid sphere and calculating I = 2MR^2/5 where M is the mass (6 x 10^24 kg) and R isthe radius (6.4 x 10^6 m). Note these are approximate values. From this,the rotational kinetic energy can be obtained, and it is quite large.

3) The bottom line is we aren't certain what determines a planet's direction of spin. It is probably a mixture of factors, to do with how the planet formed as angular momentum was transferred from the proto-Sun in the earliest phase of the Solar System.