GIANT PLANETS AT SMALL ORBITAL DISTANCES

Using Doppler spectroscopy to detect the reflex motion of the nearby s tar, 51 Pegasi, Mayor & Queloz (1995) claim to have discovered a giant planet in a 0.05 AU, 4.23 day orbit. They estimate its mass to be in the range 0.5-2 Jupiter masses, but are not able to determine its natu re or origin. Including the effects of the severe stellar insolation i mplied, we extend the theory of giant planets we have recently develop ed to encompass those at very small orbital distances. Our calculation s 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 giant planets for a variety of compositions (H/H e, He, H2O, and olivine), the evolutionary tracks for solar-compositio n gas giants, and the geometry of the Hayashi forbidden zone in the ga s-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. In addition, we demonstrate that for t he mass range quoted, such planets are well within their Roche lobes. We show that the strong composition dependence of the model radii and the distinctive spectral signatures provide clear diagnostics that mig ht reveal 51 Peg B's nature, should interferometric or adaptive-optics techniques ever succeed in photometrically separating planet from sta r.