  We present observations of the 1.35+/-0.07 Earth-radius planet L 98-59 c
using Wide Field Camera~3 on the Hubble Space Telescope. L 98-59 is a nearby
(10.6 pc), bright (H=7.4 mag), M3V star that harbors three small, transiting
planets. As one of the closest known transiting multi-planet systems, L 98-59
offers one of the best opportunities to probe and compare the atmospheres of
rocky planets that formed in the same stellar environment. We measured the
transmission spectrum of L 98-59 c during a single transit, with the extracted
spectrum showing marginal evidence for wavelength-dependent transit depth
variations which would indicate the presence of an atmosphere. Forward modeling
was used to constrain possible atmospheric compositions of the planet based on
the shape of the transmission spectrum. Although L 98-59 is a fairly quiet
star, we have seen evidence for stellar activity, and therefore we cannot rule
out a scenario where the source of the signal originates with inhomogeneities
on the host-star surface. While intriguing, our results are inconclusive and
additional data is needed to verify any atmospheric signal. Fortunately,
additional data will soon be collected from both HST and JWST. Should this
result be confirmed with additional data, L 98-59 c would be the first planet
smaller than 2 Earth-radii with a detected atmosphere, and among the first
small planets with a known atmosphere to be studied in detail by the JWST.

Earth and Planetary Astrophysics