On the MOND External Field Effect in the Solar System

Abstract

In the framework of the MOdified Newtonian Dynamics (MOND), the internal dynamics of a gravitating system s embedded in a larger one S is affected by the external background field E of S even if it is constant and uniform, thus implying a violation of the Strong Equivalence Principle: it is the so-called External Field Effect (EFE). In the case of the solar system, E would be A_cen\approx 10^-10 m s^-2 because of its motion through the Milky Way: it is orders of magnitude smaller than the main Newtonian monopole terms for the planets. We address here the following questions in a purely phenomenological manner: are the Sun's planets affected by an EFE as large as 10^-10 m s^-2? Can it be assumed that its effect is negligible for them because of its relatively small size? Does $\bds E$ induce vanishing net orbital effects because of its constancy over typical solar system's planetary orbital periods? It turns out that a constant and uniform acceleration, treated perturbatively, does induce non-vanishing long-period orbital effects on the longitude of the pericenter of a test particle. In the case of the inner planets of the solar system and with E\approx 10^-10 m s^-2, they are 4-6 orders of magnitude larger than the present-day upper bounds on the non-standard perihelion precessions recently obtained with by E.V. Pitjeva with the EPM ephemerides in the Solar System Barycentric frame. The upper limits on the components of E are E_x <= 1 x 10^-15 m s^-2, E_y <= 2 x 10^-16 m s^-2, E_z <= 3 x 10^-14 m s^-2. This result is in agreement with the violation of the Strong Equivalence Principle by MOND.