  An exosphere was recently detected around the exoplanet WASP-107b, a
low-density, warm Neptune, based on an absorption feature from metastable
helium (which has a vacuum wavelength of 10833 \AA). Inspired by the WASP-107b
detection, we reanalyzed archival HST observations of another evaporating
exoplanet, WASP-12b, to search for signs of helium in its exosphere. We find no
significant increase in transit depth at 10833 \AA. We compare this result to
theoretical predictions from a 1D model, and find that the expected helium
feature amplitude is small, in agreement with the observed non-detection. We
discuss possible explanations for why the helium feature is weaker for WASP-12b
than WASP-107b, and conclude that the amplitude of the signal is highly
sensitive to the stellar spectrum and the geometry of the evaporating gas
cloud. These considerations should be taken into account in the design of
future searches for helium exospheres.
