  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.
