Giant Planets at Small Orbital Distances

Using Doppler spectroscopy to detect the reflex motion of the nearby star, 51 Pegasi, Mayor \& Queloz (1995) claim to have discovered a giant planet in a 0.05-A.U., 4.23-day orbit. They estimate its mass to be in the range 0.5 \mj to 2 \mj, but are not able to determine its nature or origin. Including the effects of the severe stellar insolation implied, we extend the theory of giant planets we have recently developed to encompass those at very small orbital distances. Our calculations can be used to help formulate search strategies for luminous planets in tight orbits around other nearby stars. We calculate the radii and luminosities of such giants for a variety of compositions (H/He, He, H$_2$O, and olivine), evolutionary tracks for solar- composition gas giants, and the geometry of the Hayashi forbidden zone in the gas-giant mass regime. We show that such planets are stable and estimate the magnitude of classical Jeans evaporation and of photodissociation and loss due to EUV radiation. Even over the lifetime of the primary, the present companion would not have lost a large fraction of its mass. In addition, we demonstrate that for the mass range quoted, such planets are well within their Roche lobes. We show that the strong composition-dependence of the model radii and distinctive spectral signatures provide clear diagnostics that might reveal 51 Peg B's nature, should interferometric or adaptive-optics techniques ever succeed in photometrically separating planet from star.