We present a transmission spectrum for the Neptune-sized exoplanet HD 106315 c from optical to infrared wavelengths based on transit observations from the Hubble Space Telescope/Wide Field Camera 3 , K2 , and Spitzer .
The spectrum shows tentative evidence for a water absorption feature in the 1.1 - 1.7 \mu m wavelength range with a small amplitude of 30 ppm ( corresponding to just 0.8 \pm 0.04 atmospheric scale heights ) .
Based on an atmospheric retrieval analysis , the presence of water vapor is tentatively favored with a Bayes factor of 1.7 - 2.6 ( depending on prior assumptions ) .
The spectrum is most consistent with either enhanced metallicity , high altitude condensates , or both .
Cloud-free solar composition atmospheres are ruled out at > 5 \sigma confidence .
We compare the spectrum to grids of cloudy and hazy forward models and find that the spectrum is fit well by models with moderate cloud lofting or haze formation efficiency , over a wide range of metallicities ( 1 - 100 \times solar ) .
We combine the constraints on the envelope composition with an interior structure model and estimate that the core mass fraction is \gtrsim 0.3 .
With a bulk composition reminiscent of that of Neptune and an orbital distance of 0.15 AU , HD 106315 c hints that planets may form out of broadly similar material and arrive at vastly different orbits later in their evolution .