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It is well known that planets with short orbital periods (< 10 days) are common around stars like the Sun. Stars expand as they evolve, and thus we expect their close planetary companions to be engulfed. However, this phase has never been directly observed. Here, we present the discovery of ZTF SLRN-2020, a short-lived optical outburst in the Galactic disk accompanied by bright and long-lived infrared emission. The resulting light curve and spectra share striking similarities with those of red novae -- a class of eruptions now confirmed to arise from mergers of binary stars. Its exceptionally low optical luminosity (10^{35} erg/s) and radiated energy (6.5 X 10^{41} erg) point to the engulfment of a planet (of 1 - 10 Jupiter masses) by its Sun-like host star. We estimate the Galactic rate of such Sub-luminous Red Novae (SLRNe) to be $\sim 0.1 - few per year. Future Galactic plane surveys are well-poised to routinely identify them, revealing the demographics of planetary engulfment and the ultimate fate of planets in the inner Solar System.