  The metastable helium line at 1083 nm can be used to probe the extended upper
atmospheres of close-in exoplanets and thus provide insight into their
atmospheric mass loss, which is likely to be significant in sculpting their
population. We used an ultranarrowband filter centered on this line to observe
two transits of the low-density gas giant HAT-P-18b, using the 200" Hale
Telescope at Palomar Observatory, and report the detection of its extended
upper atmosphere. We constrain the excess absorption to be $0.46\pm0.12\%$ in
our 0.635 nm bandpass, exceeding the transit depth from the Transiting
Exoplanet Survey Satellite (TESS) by $3.9\sigma$. If we fit this signal with a
1D Parker wind model, we find that it corresponds to an atmospheric mass loss
rate between $8.3^{+2.8}_{-1.9} \times 10^{-5}$ $M_\mathrm{J}$/Gyr and
$2.63^{+0.46}_{-0.64} \times 10^{-3}$ $M_\mathrm{J}$/Gyr for thermosphere
temperatures ranging from 4000 K to 13000 K, respectively. With a J magnitude
of 10.8, this is the faintest system for which such a measurement has been made
to date, demonstrating the effectiveness of this approach for surveying mass
loss on a diverse sample of close-in gas giant planets.
