  Understanding the effects of high-energy radiation and stellar winds on
planetary atmospheres is vital for explaining the observed properties of
close-in exoplanets. Observations of transiting exoplanets in the triplet of
metastable helium lines at 10830 A allow extended atmospheres and escape
processes to be studied for individual planets. We observed one transit of
WASP-107b with NIRSPEC on Keck at 10830 A. Our observations, for the first
time, had significant post-transit phase coverage, and we detected excess
absorption for over an hour after fourth contact. The data can be explained by
a comet-like tail extending out to ~7 planet radii, which corresponds to
roughly twice the Roche lobe radius of the planet. Planetary tails are expected
based on 3D simulations of escaping exoplanet atmospheres, particularly those
including the interaction between the escaped material and strong stellar
winds, and have been previously observed at 10830 A, in at least one other
exoplanet. With both the largest mid-transit absorption signal and the most
extended tail observed at 10830 A, WASP-107b remains a keystone exoplanet for
atmospheric escape studies.
