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 _ { \mathrm { DM } } \mathrel { \lower 2.58 pt \hbox { $ \buildrel \textstyle < \over { % \scriptstyle \sim } $ } } 3 { \times } 10 ^ { -16 } ~ { } \mathrm { 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 .