[Alan Schein]

Alan Schein asked the Naked Scientists:
   
Hi Nakeds,
 
Is it true that the orbits of the planets must decay over time, as the gravitational attraction of the Sun will eventually overcomes the centrifugal force imparted to the planets from the spinning proto-solar disk?
 
Cheers and thanks,
 
Alan Schein

What do you think?

[Soul Surfer]

Yes they will decay over the very very long term but not for the reason that you suggest.  The gravitational attraction of the sun is totally balanced by the orbital angular momentum of the planet.  However as it orbits the sun the planet has to push its way through the solar wind and other debris.  The orbital motion also emits gravitational radiation.   Both of these effects slowly reduce this angular momentum which contrary to intuition causes the planet to accelerate as it orbits closer to the sun.

[syhprum]

I do not think the 100 watts or so that the Earth radiates as gravitational waves is a very big factor.

[Soul Surfer]

Yes it is if you wait long enough.

[Bill S]

What about tidal effects?  For example the moon is moving away from the Earth, which,I believe, is due to tidal effects.

[Soul Surfer]

Tidal effects are more powerful but only act to slow down the rotation of a moon or planet and the planet or star that it is orbiting.  This has happened with our moon and the moon is busy slowing down the earth,s rotation  Also these effects always work to increase the orbital energy (as you mention) rather than decrease it as we are discussing here.