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The Neptunian desert is a region in period-radius parameter space with veryfew Neptune-sized planets at short orbital periods. Amongst these, LTT 9779 bis the only known Neptune with a period shorter than one day to retain asignificant H-He atmosphere. If the Neptune desert is the result ofX-ray/EUV-driven photoevaporation, it is surprising that the atmosphere of LTT9779 b survived the intense bombardment of high energy photons from its younghost star. However, the star has low measured rotational broadening, whichpoints to the possibility of an anomalously slow spin period and hence a faintX-ray emission history that may have failed to evaporate the planet'satmosphere. We observed LTT 9779 with XMM-Newton and measured an upper limitfor its X-ray luminosity that is a factor of fifteen lower than expected forits age. We also simulated the evaporation past of LTT 9779 b and found thatthe survival of its atmosphere to the present day is consistent with anunusually faint XUV irradiation history that matches both the X-ray androtation velocity measurements. We conclude that the anomalously low X-rayirradiation of the one Neptune seen to survive in Neptunian desert supports theinterpretation of the desert as primarily a result of photoevaporation.

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