2018AJ....155..223C

We present a new Spitzer transit observation of K2-28b, a sub-Neptune (R_p = 2.45 ± 0.28 R_⊕) orbiting a relatively bright (V_mag = 16.06, K_mag = 10.75) metal-rich M4 dwarf (EPIC 206318379). This star is one of only seven with masses less than 0.2 M_☉ known to host transiting planets, and the planet appears to be a slightly smaller analogue of GJ 1214b (2.85±0.20R_⊕). Our new Spitzer observations were taken two years after the original K2 discovery data and have a significantly higher cadence, allowing us to derive improved estimates for this planet's radius, semimajor axis, and orbital period, which greatly reduce the uncertainty in the prediction of near future transit times for the James Webb Space Telescope (JWST) observations. We also evaluate the system's suitability for atmospheric characterization with JWST and find that it is currently the only small (< 3R_⊕) and cool (<600 K) planet aside from GJ 1214b with a potentially detectable secondary eclipse. We also note that this system is a favorable target for near-infrared radial velocity instruments on larger telescopes (e.g., the Habitable Planet Finder on the Hobby-Eberly Telescope), making it one of only a handful of small, cool planets accessible with this technique. Finally, we compare our results with the simulated catalog of the Transiting Exoplanet Survey Satellite (TESS) and find K2-28b to be representative of the kind of mid-M systems that should be detectable in the TESS sample.