# Changes in planet orbits as a star (eg. the Sun) decreases is mass.

by Jus
Tags: decreases, mass, orbits, planet, star
 P: 17 Hi, I'm actually a Design Engineer but I find the topic of Relativity extremely interesting and I was wondering if somebody could give me some guidance on this. If Planets and Stars (due to their mass) 'warp' Space-Time then this means that our Sun distorts the fabric of space just like a heavy ball on a trampoline... Have I understood that right? Ok, presuming that the above is correct, the orbits of the planets around the sun follow this distortion just like a ball rolling around a sink bowl... Does that sound right? Again assuming I've understood the above points correctly, if the Sun is decreasing in mass all of the time then clearly the amount of distortion caused by the mass of the Sun on the fabric of space is decreasing... Does that sound right? If all of the above is true, then wouldn't this mean that the planets following the path of the distortion (the balls rolling around the sink) would actually be moving away from the Sun as the 'sink' is effectively becoming slightly flatter due to the reduction in mass of the Sun? Sorry if this sounds like a stupid question but it's been bothering me for the last few days! Thanks in advance. Jus
 Mentor P: 13,603 Yes, but you don't need relativity to arrive at this result. Newtonian mechanics says the same thing will happen.
 P: 171 Yes planets are moving their orbits as sun losses mass. But numbers are almost insignificant. When you add both solar winds and mass-energy conversion of sun, I think that you get about one earth's own radius increase in earth's orbit over entire life of sun so far.
P: 17

## Changes in planet orbits as a star (eg. the Sun) decreases is mass.

 Quote by S.Vasojevic Yes planets are moving their orbits as sun losses mass. But numbers are almost insignificant. When you add both solar winds and mass-energy conversion of sun, I think that you get about one earth's own radius increase in earth's orbit over entire life of sun so far.

Thanks for the answer. Sorry if it sounded like a stupid question, I didn't realise that the change in Orbit would be so insignificant.
 P: 253 I use to think the same as you do, but then, how to understand the gravitational radiation orbital decay. Beeing GR or Newtonian seems irrelevant. Can someone explain, pls.
 HW Helper Sci Advisor P: 8,962 The sun losses about 10^-13 of it's mass/year This increases the Earth-Sun distance at the same rate. But since the Earth-Sun distance is 150million km this is only about 1cm/year
 P: 253 On the contrary ! Everyone believes that the orbit radius must increase (even I do !) How to explain the Orbital decay from gravitational radiation and here -Pag. 15-16 ? The orbit must vary in the same way as the conditions are similar (loss of mass/energy and non relativistic velocities). We say: radius increase. Other say: radius decrease. Why ?
Mentor
P: 5,883
 Quote by heldervelez On the contrary ! Everyone believes that the orbit radius must increase (even I do !) How to explain the Orbital decay from gravitational radiation and here -Pag. 15-16 ? The orbit must vary in the same way as the conditions are similar (loss of mass/energy and non relativistic velocities). We say: radius increase. Other say: radius decrease. Why ?
Consider the Earth orbiting the Sun. What would happen to the distance between the Earth and the Sun if the Earth's kinetic energy were gradually siphoned off?
P: 253
 Quote by George Jones Consider the Earth orbiting the Sun. What would happen to the distance between the Earth and the Sun if the Earth's kinetic energy were gradually siphoned off?
I say : radius increase (and We say as seen above).

How should I understand the 'assumed' orbit decay due to radiation of gravitacional waves (masss loss) ?
Mentor
P: 5,883
 Quote by heldervelez I say : radius increase (and We say as seen above).
No Newtonian gravity, or, equivalently, Kepler's third law, can be used to show that the distance decreases. I would do this, but, right now, my three-year-old is demanding that I read to her.
 P: 253 I agree with you George Jones and I have no need to see a 'proof'. My surprise is that under GR we can not have such a different result. Newtonian is an approximation of GR then I am prepared to sentences like this: the rate of increase of radius on GR is more (or less, not important to the discusssion) than under Newtonian gravitation. but never like this: under Newtonian: increase of radius ; under GR decrease of radius. I can not understand such a different outcome.

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