Polar stellar-spots and grazing planetary transits

Possible explanation for the low number of discovered grazing planets

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
e-mail: moshagh@astro.up.pt
² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal

Received: 29 August 2015
Accepted: 1 October 2015

Abstract

We assess whether there is a physically feasible explanation for the few discovered (nearly) grazing planetary transits through all ground- and space-based transit surveys. We performed simulations to generate a synthetic distribution of detectable planets based on their impact parameter, and found that more (nearly) grazing planets should have been detected than have been detected. Our explanation for the insufficient number of (nearly) grazing planets is based on the simple assumption that many (nearly) grazing planets transit host stars that harbor a dark giant polar spot, and thus the transit light-curve vanishes due to the occultation of the grazing planet and the polar spot. We conclude by evaluating the properties required of polar spots to cause the grazing transit light-curve to disappear and find that the spot properties are compatible with the expected properties from observations.