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Detailed characterization of exoplanets has begun to yield measurements oftheir atmospheric properties that constrain the planets' origins and evolution.For example, past observations of the dayside emission spectrum of the hotJupiter WASP-12b indicated that its atmosphere has a high carbon-to-oxygenratio (C/O $>$ 1), suggesting it had a different formation pathway than iscommonly assumed for giant planets. Here we report a precise near-infraredtransmission spectrum for WASP-12b based on six transit observations with theHubble Space Telescope/Wide Field Camera 3. We bin the data in 13spectrophotometric light curves from 0.84 - 1.67 $\mu$m and measure the transitdepths to a median precision of 51 ppm. We retrieve the atmospheric propertiesusing the transmission spectrum and find strong evidence for water absorption(7$\sigma$ confidence). This detection marks the first high-confidence,spectroscopic identification of a molecule in the atmosphere of WASP-12b. Theretrieved 1$\sigma$ water volume mixing ratio is between $10^{-5}-10^{-2}$,which is consistent with C/O $>$ 1 to within 2$\sigma$. However, we alsointroduce a new retrieval parameterization that fits for C/O and metallicityunder the assumption of chemical equilibrium. With this approach, we constrainC/O to $0.5^{+0.2}_{-0.3}$ at $1\,\sigma$ and rule out a carbon-rich atmospherecomposition (C/O$>1$) at $>3\sigma$ confidence. Further observations andmodeling of the planet's global thermal structure and dynamics would aid inresolving the tension between our inferred C/O and previous constraints. Ourfindings highlight the importance of obtaining high-precision data withmultiple observing techniques in order to obtain robust constraints on thechemistry and physics of exoplanet atmospheres.

A Detection of Water in the Transmission Spectrum of the Hot Jupiter WASP-12b and Implications for its Atmospheric Composition