Survival in the Neptune desert: LTT 9779 b kept its atmosphere thanks to an unusually X-ray faint host star

The Neptunian desert is a region in period-radius parameter space with very few Neptune-sized planets at short orbital periods. Amongst these, LTT 9779 b is the only known Neptune with a period shorter than 1 d to retain a significant H-He atmosphere. If the Neptune desert is the result of X-ray/EUV-driven photoevaporation, it is surprising that the atmosphere of LTT 9779 b survived the intense bombardment of high-energy photons from its young host star. However, the star has low measured rotational broadening, which points to the possibility of an anomalously slow spin period and hence a faint X-ray emission history that may have failed to evaporate the planet's atmosphere. We observed LTT 9779 with XMM-Newton and measured an upper limit for its X-ray luminosity that is a factor of 15 lower than expected for its age. We also simulated the evaporation past of LTT 9779 b and found that the survival of its atmosphere to the present day is consistent with an unusually faint XUV irradiation history that matches both the X-ray and rotation velocity measurements. We conclude that the anomalously low X-ray irradiation of the one Neptune seen to survive in Neptunian desert supports the interpretation of the desert as primarily a result of photoevaporation.