A link between the semimajor axis of extrasolar gas giant planets and stellar metallicity

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 the unexpected semimajor axes and eccentricities. We propose a model that correlates the metallicity of the host star with the original semimajor axis of its most massive planet, prior to migration, assuming that the core accretion scenario governs giant gas planet formation. The model predicts that the optimum regions for planetary formation shift inwards 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 hints that the lack of planets circling metal-poor stars may be partly caused by an enhanced destruction probability during the migration process, because the planets lie initially closer to their central star.