In article ,
Rob Dekker wrote:
There is nothing that constantly
travels back and forth between the Earth and the Sun to keep Earth in its
orbit, ...

This is an assumption.
I think the current theory postulates that 'gravitons' are constantly
exchanged, at speed of light.

No, that is precisely *NOT* what the current theory is, which was my point.
Gravitational waves, aka gravitons, convey only *changes* in the field.

This is in analogy to all other forces of nature, where some 'particle' is
associated with the force.

The analogy is false, or at least misleading. Gravity seems to be a
rather different kind of force. Physicists are still struggling to fit it
into the same sort of theory as the other forces.

But hard proof either way is still is not there...

On the contrary. It is easily proved that gravity is *not* the result of
the exchange of particles at the speed of light. If that were so, gravity
would show aberration, which would make planetary orbits unstable.

Light and other forms of electromagnetic radiation *do* show aberration:
a moving observer, looking to the side, sees stars slightly ahead of their
true positions, because a photon which to him seems to come straight in
from the side, in fact had to angle forward a bit to get down the barrel
of his moving telescope. That's a very loose explanation, but a more
rigorous treatment gives the same result. Astronomers noticed this in the
early 18th century.

Planetary orbits are stable only if gravity appears to come from precisely
the true position of the Sun. If gravity traveled at a finite speed and
thus showed aberration, the Earth wouldn't be here.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |