Mature sub-Neptune-sized planets with hydrogen/helium atmospheres (from 2.5 - 4 R_Earth) are common but remain enigmatic due to predominantly cloudy, featureless near-infrared spectra. With this JWST Cycle 3 program, we will investigate young sub-Neptunes (< 200 Myr), which so far have shown preferentially clear atmospheres favorable for transmission spectroscopy. Within this large program we will:

• Determine if sub-Neptune atmospheric metallicity if primordial. Sub-Neptunes could form with high atmospheric metallicities or they may evolve to be metal-rich through photoevaporation, photochemistry, and planetary bombardment.
• Determine whether sub-Neptunes are water enriched or depleted. Depending on their formation location, they could be either "gas dwarfs" with water-depleted atmospheres and rocky cores, or "water worlds" with water-enriched atmospheres and ice/rock cores.
• Analyze the homogeneity of multi-sub-Neptune systems. Depending on formation histories, planets in the same system may be all water worlds, gas dwarfs, or some combination of both.

To achieve our science goals, we will observe seven sub-Neptunes in three systems to fully characterize all known 3+ multi-sub-Neptune systems orbiting young solar analogues. Our sample will answer questions about intra- and extra-system diversity for the first time. All planets will be observed with NIRISS/SOSS and NIRSpec/G395H to obtain high signal-to-noise broad infrared spectra for our sample. You can learn more about our planets here and the status of our program here.