The motions of the planets have been used to make the best estimate yet of the upper limit of the mass of the graviton – a hypothetical particle that is a quantum of the gravitational field. That is the claim of Leo Bernus at the Paris Observatory and colleagues, who used over a century’s worth of data in their calculations.
In theories that try to provide a quantum description of gravity, the graviton mediates the gravitational force between massive objects. It can be thought of as a gravitational version of the photon, which mediates the electromagnetic force between charged objects. A correct theory of quantum gravity has yet to be developed, but it is possible to test some aspects of nascent theories including their predictions of whether the graviton has a mass.
If gravitational fields have an infinite range – as Einstein’s general theory of relativity dictates – gravitons must be massless and travel at the speed of light. However, some theories of quantum gravity suggest that the graviton could have an extremely small mass. If this were true, it would limit the range of the gravitational force and impose a subluminal speed limit on the graviton.