Effects of Mutual Transits by Extrasolar Planet–Companion Systems on Light Curves

Abstract

Abstract We consider the effects of mutual transits by extrasolar planet–companion systems (in a true binary or a planet–satellite system) on light curves. We show that induced changes in light curves depend strongly on a ratio between a planet–companion’s orbital velocity around their host star and a planet–companion’s spin speed around their common center of mass. In both the slow and fast-spin cases (corresponding to long and short distances between them, respectively), a certain asymmetry appears in light curves. We show that, especially in the case of short distances, occultation of one faint object by the other, while the transit of the planet–companion system occurs in front of its parent star, causes an apparent increase in light curves, and characteristic fluctuations appear as important evidence of mutual transits. We show also that extrasolar mutual transits provide a complementary method for measuring the radii of two transiting objects, their separation and masses, and consequently for identifying the system as a true binary, a planet–satellite system, or others. Monitoring 10$^5$ stars for 3 yr with Kepler may lead to the discovery of a second Earth–Moon-like system if the fraction of such systems for an averaged star is larger than 0.05, or we may put an upper limit on the fraction as $f$$\lt$ 0.05.