Skip to Content

The Puzzle Over Saturn’s Orbit (cont’d)

If modified theories of gravity are correct, we ought to see the effects in the orbit of Saturn. But nobody is quite sure whether we do or not.

Many astronomers think our universe is filled with mysterious dark stuff that exerts a gravitational pull on big things like galaxies. In fact, most galaxies spin so fast that they would fly apart unless there were a substantial amount of this dark gloop holding them together.

But if dark matter does fill our galaxy, we ought to see it in our Solar System. There’s no shortage of dark matter detectors looking for the stuff. Most have drawn a blank and those that do claim to have seen it have been ridiculed.

There is an alternative hypothesis, however. This is the idea that Newton’s equations of motion work in a different way at the very low accelerations that stars experience as they orbit a galaxy.

The equations that describe this so-called Modified Newtonian Dynamics or MOND are non-linear and so lead to other predictions. “An important consequence of the non-linearity is that the gravitational dynamics of a system is influenced by the external gravitational environment in which the system is embedded,” say Luc Blanchet at the Universite Pierre et Marie Curie and Jerome Novak at the Universite Denis Diderot, both in Paris.

This is called the external field effect and it ought to have a measurable influence on the Solar System, particularly on the precession of the perihelion of the planets.

Today, Blanchet and Novak calculate the size of this effect and compare it to the best data we have of planetary motion.

It turns out that the planets most effected are the distant gas giants: Saturn, Uranus and Neptune. And the best monitored of these is Saturn, since astronomers have been able to follow the motion of the Cassini spacecraft as it orbits the ringed giant.

Blanchet and Novak say that the accuracy of these measurements can be used to rule out some formulations of MOND. “We find that the precession effect is rather large for outer gaseous planets, and in the case of Saturn is comparable to, and in some cases marginally excluded by published residuals of precession permitted by the best planetary ephemerides.”

But the story doesn’t end there. One of the best sets of data about Saturn’s motion has been compiled by the Russian astronomer Elena Pitjeva, who heads the Laboratory of Ephemeris Astronomy at the Institute of Applied Astronomy in St Petersburg.

In 2005, she published a comprehensive set of data on Saturn’s motion. It’s this that Blanchet and Novak used to compare their calculations against.

But back in 2008, rumours began to circulate that Pitjeva had found something strange in more recent data. These were outlined in a paper by Lorenzo Iorio at the National Institute of Nuclear Physics in Italy and covered by the Physics arXiv Blog at the time.

The bottom line was that Pitjeva had reportedly discovered that the precession of Saturn’s perihelion, as predicted by general relativity, needed to be corrected to fit the most recent data from Cassini.

Pitjeva doesn’t appear to have published these data, even now almost three years later. And Iorio hasn’t updated his paper either.

But it raises an intriguing question. Could the data from Cassini be telling us something interesting about MOND?

Perhaps Blanchet and Novak could politely enquire about the status of Pitjeva’s result and compare it with their calculations. Just to settle the matter for curious souls.

Ref: Testing MOND in the Solar System

Deep Dive


Five poems about the mind

DREAM VENDING MACHINE I feed it coins and watch the spring coil back,the clunk of a vacuum-packed, foil-wrappeddream dropping into the tray. It dispenses all kinds of dreams—bad dreams, good dreams,short nightmares to stave off worse ones, recurring dreams with a teacake marshmallow center.Hardboiled caramel dreams to tuck in your cheek,a bag of orange dreams…

Work reinvented: Tech will drive the office evolution

As organizations navigate a new world of hybrid work, tech innovation will be crucial for employee connection and collaboration.

lucid dreaming concept
lucid dreaming concept

I taught myself to lucid dream. You can too.

We still don’t know much about the experience of being aware that you’re dreaming—but a few researchers think it could help us find out more about how the brain works.

panpsychism concept
panpsychism concept

Is everything in the world a little bit conscious?

The idea that consciousness is widespread is attractive to many for intellectual and, perhaps, also emotional
reasons. But can it be tested? Surprisingly, perhaps it can.

Stay connected

Illustration by Rose WongIllustration by Rose Wong

Get the latest updates from
MIT Technology Review

Discover special offers, top stories, upcoming events, and more.

Thank you for submitting your email!

Explore more newsletters

It looks like something went wrong.

We’re having trouble saving your preferences. Try refreshing this page and updating them one more time. If you continue to get this message, reach out to us at with a list of newsletters you’d like to receive.