A Link Between the Semi-Major Axis of Extrasolar Gas Giant Planets and Stellar Metallicity

Abstract

The fact that most extrasolar planets found to date are orbiting metal-rich stars lends credence to the core accretion mechanism of gas giant planet formation over its competitor, the disc instability mechanism. However, the core accretion mechanism is not refined to the point of explaining orbital parameters such as their unexpected semi-major axes and eccentricities. We propose a model, which correlates the metallicity of the host star with the original semi-major axis of its most massive planet, prior to migration, considering that the core accretion scenario governs giant gas planet formation. The model predicts that the optimum regions for planetary formation shift inward as stellar metallicity decreases, providing an explanation for the observed absence of long period planets in metal-poor stars. We compare our predictions with the available data on extrasolar planets for stars with masses similar to the mass of the Sun. A fitting procedure produces an estimate of what we define as the Zero Age Planetary Orbit (ZAPO) curve as a function of the metallicity of the star. The model also hints that the lack of planets circling metal-poor stars may be partly caused by an enhanced destruction probability during the migration process, since the planets lie initially closer to the central stars.