We obtained spectra of the pre-main sequence star AU Microscopii during a transit of its Neptune-sized planet to investigate its orbit and atmosphere .
We used the high-dispersion near-infrared spectrograph IRD on the Subaru telescope to detect the Doppler “ shadow ” from the planet and constrain the projected stellar obliquity ; we find that the stellar spin and orbit are aligned ( \lambda = -4.7 _ { -6.4 } ^ { +6.8 } degrees ) , suggesting that the planet formed and possibly migrated within the protoplanetary disk .
We use both the IRD data and spectra obtained with NIRSPEC on Keck-2 to search for absorption in the 1083 nm line of metastable triplet He I by the planet ’ s atmosphere and place an upper limit for the equivalent width of 3.7 mÅ at 99 % confidence .
From this limit , we constrain the mass loss rate of the planetary atmosphere to < 0.15 - 0.45 M _ { \oplus } Gyr ^ { -1 } , comparable to the rate predicted by an energy-limited escape calculation and in the range required to test models of XUV-driven atmospheric escape from close-in young “ Neptunes ” .