Strongly irradiated giant planets are observed to have radii larger than thermal evolution models predict. Although these inflated planets have been known for over fifteen years, it is unclear whether their inflation is caused by deposition of energy from the host star, or inhibited cooling of the planet. These processes can be distinguished if the planet becomes highly irradiated only when the host star evolves onto the red giant branch. We report the discovery of K2-97b, a 1.31 $pm$ 0.11 R$_mathrm{J}$, 1.10 $pm$ 0.11 M$_mathrm{J}$ planet orbiting a 4.20 $pm$ 0.14 R$_odot$, 1.16 $pm$ 0.12 M$_odot$ red giant star with an orbital period of 8.4 days. We precisely constrained stellar and planetary parameters by combining asteroseismology, spectroscopy, and granulation noise modeling along with transit and radial velocity measurements. The uncertainty in planet radius is dominated by systematic differences in transit depth, which we measure to be up to 30% between different lightcurve reduction methods. Our calculations indicate the incident flux on this planet was 170$^{+140}_{-60}$ times the incident flux on Earth while the star was on the main sequence. Previous studies suggest that this incident flux is insufficient to delay planetary cooling enough to explain the present planet radius. This system thus provides the first evidence that planets may be inflated directly by incident stellar radiation rather than by delayed loss of heat from formation. Further studies of planets around red giant branch stars will confirm or contradict this hypothesis, and may reveal a new class of re-inflated planets.
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