Interesting paper in the Astrophysical Journal Letters from Welbanks et al. 2019. This is a fairly small sample so you have to suspend a bit of judgement on the results - but, its a start. Their key discussion point is as follows:
"The overall low H2O abundances across the sample contrasts with solar system predictions. Besides the carbon enhancements seen in the solar system, other elements such as nitrogen, sulfur, phosphorous, and noble-gases are also enhanced in Jupiter (Atreya et al.2018); the oxygen abundance is unknown as H2O condenses at the low temperatures of solar system giants. Considering that oxygen is the most cosmically abundant element after H and He, it is expected to be even more enhanced than carbon in giant planets, according to solar system predictions (Mousis et al.2012). Therefore, the consistent depletion of H2O abundances in our sample suggest different formation pathways for these close-in exoplanets compared to the long-period solar system giants."
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"The overall low H2O abundances across the sample contrasts with solar system predictions. Besides the carbon enhancements seen in the solar system, other elements such as nitrogen, sulfur, phosphorous, and noble-gases are also enhanced in Jupiter (Atreya et al.2018); the oxygen abundance is unknown as H2O condenses at the low temperatures of solar system giants. Considering that oxygen is the most cosmically abundant element after H and He, it is expected to be even more enhanced than carbon in giant planets, according to solar system predictions (Mousis et al.2012). Therefore, the consistent depletion of H2O abundances in our sample suggest different formation pathways for these close-in exoplanets compared to the long-period solar system giants."
