One of the first tests of Einstein’s theory of general relativity was to explain the precession of the perihelion of Mercury, which had long bamboozled astronomers. Newton’s law of gravity simply cannot account for it. But relativity does.
Now it’s Saturn’s turn to flummox astrophysicists. The Russian astronomer Elean Pitjeva, who heads the Laboratory of Ephemeris Astronomy at the Institute of Applied Astronomy in St Petersburg, has analysed a huge data set of planetary observations dating back to 1913, including 3D observations of the Cassini spacecraft now orbiting Saturn.
She says that the precession of Saturn’s perihileon, as predicted by general relativity, needs to be corrected to fit the data. The correction is tiny: -0.006 arcseconds per century.
That’s an astonishing claim but perhaps not surprising given the growing body of evidence that some kind of correction to gravity is needed to explain various puzzling phenomena such as the Pioneer and Flyby anomalies.
Obviously Pitjeva’s work needs to be independently verified but already the astronomy-mill is hard at work guessing what might cause the deviation from Einsteinian physics.
It’s possible that known physics will do the trick: for example, our knowledge of trans-neptunian objects may have enough uncertainty to allow for this kind of correction.
Lorenzo Iorio at the National Institute of Nuclear Physics in Pisa Italy, outlines various explanations of known physics:
Our knowledge of trans-neptunian objects may have enough uncertainty to allow for this kind of correction but this turns out to generate a prograde precession no the retrograde precession found by Pitjeva
The Lense-Thirring effect generates a force that is four orders of magnitude too small to account for the difference
Mutual cancellations among unmodelled or mismodelled effects may have conspired to cause the effect but Iorio says this looks exceedingly unlikely
Neither do various exotic modifications of gravity or the DGP braneworld model explain the figures, says Iorio
So what’s left? A magnificent conundrum for astronomers to puzzle over until they get better data and/or a new theory of gravity that explains all.
Ref: arxiv.org/abs/0811.0756: On the Recently Determined Anomalous Perihelion Precession of Saturn