I have encountered some ambiguity regarding the applicability of Newton's third law of action-reaction to the force of gravitation. For two masses m1 and m2, separated by a distance r, the law of gravitation says force on 2 due to 1 is F21=Gm1m2/r2 along the direction from 1 to 2.

Similarly F12=Gm1m2/r2 is along the direction from 2 to 1.

It is seen that F12 = -F21.

But the above relation can directly be obtained by the seeing the respective expressions. Is it really necessary to invoke Newton's third law to validate this?

There are many (almost all) webpages that I got from goggle mention that Newton's third law is indeed applicable to the force of gravitation. Some webpages of university also mention this, such as, 

http://theory.uwinnipeg.ca/physics/circ/node7.html

and a Wikipedia page too, https://en.wikipedia.org/wiki/Reaction_(physics)#Gravitational_forces

But I am not in agreement with the above. Arguments in my favor are:

1) On the one hand above wikipedia page advocates gravitational forces as an example of Newton's third law, on the other hand another page supports my view.

https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion#Relationship_to_the_conservation_laws

It mentions " Newton stated the third law within a world-view that assumed instantaneous action at a distance between material particles. However, he was prepared for philosophical criticism of this action at a distance, and it was in this context that he stated the famous phrase "I feign no hypotheses". In modern physics, action at a distance has been completely eliminated, except for subtle effects involving quantum entanglement."

2) Another excerpt from the same page is

"Newton's Scholium (explanatory comment) to this law:

Whatever draws or presses another is as much drawn or pressed by that other. If you press a stone with your finger, the finger is also pressed by the stone."

I guess Newton is talking about bodies when they are in contact with one another.

3) The classical book of Resnick Halliday adds "The force on A by B and the force on B by A act at the same instant."

Now let us reconsider the famous though experiment in which solar system has just two bodies Sun and earth. Even if Sun is taken out of the solar system, earth still follows its regular trajectory for next 8 and half minutes as if the Sun was still there.

If it actually obeyed Newton's third law then earth should fly away tangentially at the instant when the Sun was taken out.

My final view is:

Newton's third law is applicable when bodies are in close contact with each other, so that force from one body to another and vice versa are exerted simultaneously and at the same instant.There is no time delay in the transmission of the forces.

Gravitational forces would obey third law only if they could act instantly, but from the discovery of gravitational waves we know that they travel with the same speed as the electromagnetic waves. So, the third law is not applicable to gravitational forces, and even in cases when the two bodies are not in direct contact of each other.

Please throw some light on it with conclusion, if any.

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