Far-Ultraviolet Emissions of the Sun in Time: Probing Solar Magnetic Activity and Effects on Evolution of Paleoplanetary Atmospheres

Using a full frequency-dependent atmosphere code that can incorporate irradiation by a central primary star, we calculate selfconsistent boundary conditions for the evolution of the radius of the transiting planet HD 209458b. Using a well-tested extrasolar giant planet evolutionary code, we then calculate the behavior of this planetʹs radius with age. The measured radius is in fact a transit radius that resides high in HD 209458bʹs inflated atmosphere. Using our derived atmospheric and interior structures, we find that irradiation plus the proper interpretation of the transit radius can yield a theoretical radius that is within the measured error bars. We conclude that if HD 209458bʹs true transit radius is at the lower end of the measured range, an extra source of core heating power is not necessary to explain the transit observations.