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The Incredible Dark Matter Mystery: Why Astronomers Say it is Missing in Action

Astrophysicists believe that our galaxy must be filled with more dark matter than ordinary matter. Now astronomers say they can find no evidence of dark matter’s gravitational influence on the planets. What gives?

Astronomers have a problem. Whenever they study the large scale structure of the universe, it soon becomes clear that the amount of visible matter cannot possibly generate enough gravity to hold together the structures they can see. Things like galaxy clusters and even galaxies themselves ought to fly apart given the amount of ordinary matter they contain.

Something else must be holding these things together. So astronomers have dreamt up the idea of dark matter—mysterious, invisible and non-interacting stuff that fills the universe, generating the gravity necessary to hold everything together.

This isn’t a small problem requiring a tiny amount of extra mass. The problem is huge. According to the latest picture of the large-scale structure of the Universe from the Planck space mission, ordinary visible matter makes up just 5 per cent of the total mass/energy of the Universe whereas dark matter makes up 27 per cent (the rest is the even more mysterious dark energy).

To make the numbers work, astrophysicists tell us that our galaxy ought to be at least 80 per cent dark matter.

That means our Solar System ought to be swimming in the stuff. Indeed, physicists have calculated that particles of dark matter ought to slam into each human on the planet at a rate of 100,000 times a year, as we saw last year.

But that raises an important question. If we’re ploughing through a thick sea of dark matter as astrophysicists suggest, why don’t we see evidence of it?

Most dark matter detectors work by looking for evidence of the collisions that dark matter must make with ordinary matter. A few of these experiments say they have found tentative evidence of these collisions.

But there is another way to look for dark matter—by its gravitational effects on the Solar System itself. If the Sun is surrounded by a thick soup of dark matter, we ought to be able to see its gravitational influence on the orbits of the planets, moons and asteroids.

Today, Nikolay Pitjev at St. Petersburg State University and Elena Pitjeva at the Institute of Applied Astronomy in St Petersburg, both in Russia, have used the most detailed set of measurements of planetary orbits ever made to study this question. Their conclusion is that the gravitational effect of dark matter on the solar system is negligible.

Pitjev and Pitjeva have compiled an impressive data set consisting of some 677,000 measurements of planetary positions taken since 1910. These include optical measurements from observatories on Earth, ranging measurements from various spacecraft such as Cassini at Saturn and the Mars and Venus Express missions plus various Russian radar measurements of planetary positions taken between 1961 and 1995.

This data has become increasingly accurate in recent years. For example, the data from Cassini gives its distance at Saturn to within a metre or so.

Astrophysicists have used these measurements to model the behaviour of the solar system, taking into account the perturbations caused by the major planets, the Moon, the 301 largest asteroids, the other asteroids modelled as a uniform ring, the 21 largest trans-Neptunian objects and so on.

Having taken all this into account, Pitjev and Pitjeva looked for anomalous gravitational effects that might be the result of dark matter. “If dark matter is present in the Solar system, then it should lead to some additional gravitational influence on all bodies,” they say.

The puzzling news is that Pitjev and Pitjeva find no evidence of this stuff in their analysis. If it is there, its effect must be smaller than the errors in the data.

Indeed, to satisfy this limit, they calculate that the amount of dark matter within the orbit of Saturn must be tiny. “The dark matter mass in the sphere within Saturn’s orbit should be less than 1.7 10^−10M⊙,” they say. That’s about the mass of a large asteroid.

So astronomers are left scratching their heads. On the one hand, they say dark matter must hold our galaxy together with a vice-like gravitational grip. On the other, its gravitational effect on the Solar System is negligible. Something has to give.

This problem of the contradictory effects of dark matter on different scales is fast turning into the most fascinating and urgent problem in physics and astronomy,

Researchers are currently spending big bucks to design, build and run giant experiments looking for dark matter in our vicinity. And yet the evidence already gathered from other sources, such as this analysis by Pitjev and Pitjeva, suggest that this investment may produce a very poor return.

That won’t stop them looking and nor should it. But the dark matter problem is likely to generate significant controversy in the coming months and years.

Ref: Constraints on Dark Matter in the Solar System

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