Hyades Member K2-136c: The Smallest Planet in an Open Cluster with a Precisely Measured Mass

K2-136 is a late-K dwarf ($0.742\pm0.039$ M$_\odot$) in the Hyades open cluster with three known, transiting planets and an age of $650\pm70$ Myr. Analyzing K2 photometry, we found that K2-136b, c, d have periods $8.0$, $17.3$, $25.6$ days radii $1.014\pm0.050$ R$_\oplus$, $3.00\pm0.13$ $1.565\pm0.077$ respectively. We collected 93 radial velocity measurements (RVs) HARPS-N spectrograph (TNG) 22 RVs ESPRESSO (VLT). data jointly, K2-136c induced semi-amplitude $5.49\pm0.53$ m s$^{-1}$, corresponding to mass $18.1\pm1.9$ M$_\oplus$. also placed $95$% upper limits on K2-136b $4.3$ $3.0$ M$_\oplus$, Further, analyzed HST XMM-Newton observations establish planetary high-energy environment investigate possible atmospheric loss. now smallest planet measured one youngest ever measurement. has $\sim$75% radius Neptune but similar mass, yielding density $3.69^{+0.67}_{-0.56}$ g cm$^{-3}$ ($\sim$2-3 times denser than Neptune). Mass estimates for (and possibly d) may be feasible more RV observations, insights into all planets' atmospheres through transmission spectroscopy would challenging potentially fruitful. This research future young are critical investigating compositions characteristics small exoplanets at very early stages their lives providing how evolve time.