In this morning's astro-ph, we read about a topic of occasional
interest in this group. I find the second sentence and the last
sentence of particular interest.



astro-ph/0606197
Title: Dark matter vs. modifications of the gravitational
inverse-square law. Results from planetary motion in the solar
system
Authors: M. Sereno (Univ. Zuerich), Ph. Jetzer (Univ. Zuerich)
Comments: 7 pages, 4 figures, accepted for publication in MNRAS

Dark matter or modifications of the Newtonian inverse-square law in
the solar-system are studied with accurate planetary astrometric
data. From extra-perihelion precession and possible changes in the
third Kepler's law, we get an upper limit on the local dark matter
density, rho_{DM} 3*10^{-16} kg/m^3 at the 2-sigma confidence
level. Variations in the 1/r^2 behavior are considered in the form of
either a possible Yukawa-like interaction or a modification of gravity
of MOND type. Up to scales of 10^{11} m, scale-dependent deviations in
the gravitational acceleration are really small. We examined the MOND
interpolating function mu in the regime of strong gravity. Gradually
varying mu suggested by fits of rotation curves are excluded, whereas
the standard form mu(x)= x/(1+x^2)^{1/2} is still compatible with
data. In combination with constraints from galactic rotation curves
and theoretical considerations on the external field effect, the
absence of any significant deviation from inverse square attraction in
the solar system makes the range of acceptable interpolating functions
significantly narrow. Future radio ranging observations of outer
planets with an accuracy of few tenths of a meter could either give
positive evidence of dark matter or disprove modifications of gravity.


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