  We present a search for helium in the upper atmospheres of three sub-Neptune
size planets to investigate the origins of these ubiquitous objects. The
detection of helium for a low density planet would be strong evidence for the
presence of a primary atmosphere accreted from the protoplanetary nebula
because large amounts of helium are not expected in the secondary atmospheres
of rocky planets. We used Keck+NIRSPEC to obtain high-resolution transit
spectroscopy of the planets GJ1214b, GJ9827d, and HD97658b around the 10,833
Ang He triplet feature. We did not detect helium absorption for any of the
planets despite achieving a high level of sensitivity. We used the
non-detections to set limits on the planets' thermosphere temperatures and
atmospheric loss rates by comparing grids of 1D models to the data. We also
performed coupled interior structure and atmospheric loss calculations, which
suggest that the bulk atmospheres (winds) of the planets would be at most
modestly enhanced (depleted) in helium relative to their primordial
composition. Our lack of detections of the helium triplet for GJ1214b and
GJ9827d are highly inconsistent with the predictions of models for the present
day mass loss on these planets. Higher signal-to-noise data would be needed to
detect the helium feature predicted for HD97658b. We identify uncertainties in
the EUV fluxes of the host stars and the lack of detailed mass loss models
specifically for cool and metal-enhanced atmospheres as the main limitations to
the interpretation of our results. Ultimately, our results suggest that the
upper atmospheres of sub-Neptune planets are fundamentally different than those
of gas giant planets.
