Transit Detection Probability of Habitable Planets as a Function of Stellar Spectral Type

Abstract

Thousands of exoplanets have now been detected. Many of these discoveries occurred using the transit method: the planet’s existence is deduced from a small dimming of the host star’s light every time the planet crosses in front of star’s face as seen from the Earth. Habitable planets (where water could exist on their surfaces) occur within the “Goldilocks zone:” the planet is not too close (i.e. hot) or far away (i.e. cold) for life to exist on their surfaces. To examine how likely we are to detect habitable planets around a given star, we explore planetary conditions around a large variety of standard stellar classes with different temperatures and radii. This analysis led to the interesting result that the detection probability of a habitable planet is only a function of stellar surface temperature and is independent of stellar radius. Inhabitants of a habitable planet around a cool star would see an equally sized red disk in the sky regardless of if they were orbiting a dwarf star or a supergiant star. Red stars occult a wider band of stars (viewed from a habitable planet) during their motion along the ecliptic than blue stars, making the detection probability greater for red stars.