Surviving in the Neptune desert. The ultra-short period Neptune LTT 9779 b survived thanks to an unusually X-ray faint host star

I present XMM-Newton observations of the sun-like star LTT 9779 together with a study of the X-ray evaporation of its transiting planet, LTT 9779 b, the first ultra-hot Neptune in the Neptune desert. Its presence so close to its star is puzzling, as the intense XUV flux it receives from its star should have stripped it of its H/He-rich atmosphere. I find that only an X-ray faint host star is able to explain both our observations as well as the survival of the planet’s atmosphere under photoevaporation. LTT 9779 b is a super-Neptune (4.7 Earth radii) with a gaseous envelope that constitutes 8% of its mass. The planet, with an age of 2 Gyr, orbits its host star every 19 hours. This planet is one of the few that populate the Neptune desert, a region of parameter space at very short periods thought to be cleared out by stellar X-ray and extreme ultraviolet radiation (together, XUV), which drives substantial evaporation and can completely strip planets of their gaseous envelopes down to their rocky core. LTT 9779 b, however, is the only known ultra-short period planet in the Neptune desert that maintains a significant atmosphere. In order to find feasible evaporation pasts that can explain its current state, I simulated the star’s XUV  emission history by modelling its spin period evolution, as faster rotation produces higher XUV fluxes. I confirm that the XUV history expected for a solar-mass star should have already stripped LTT 9779 b of its envelope, ruling out this scenario. Moreover, I find that the planet’s atmosphere is only able to survive a faint stellar XUV history that matches the measured upper limits for both its spin period and its X-ray luminosity, which I estimated using XMM-Newton measurements.