This paper sets out a mathematical model for determining the apparent visual magnitude of an extrasolar planet situated within the habitable zone of its parent star, as seen from Earth, and makes the case that such bodies can be detected with ground based instruments during asteroidal occultations of bright nearby stars, such as Alpha Centauri A/B, Sirius A, Procyon A, Altair, Vega and Fomalhaut. The Alpha Centauri system is focused upon as a target case, where apparent magnitudes of hypothetical exoplanets are projected, possessing either Earth or Jupiter-like albedo (reflectivity) properties, a range of physical sizes and changing phase illuminations as they revolve about their parent stars.