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We present an optical eclipse observation of the hot Jupiter WASP-12b usingthe Space Telescope Imaging Spectrograph on board the Hubble Space Telescope.These spectra allow us to place an upper limit of $A_g < 0.064$ (97.5%confidence level) on the planet's white light geometric albedo across 290--570nm. Using six wavelength bins across the same wavelength range also producesstringent limits on the geometric albedo for all bins. However, ouruncertainties in eclipse depth are $\sim$40% greater than the Poisson limit andmay be limited by the intrinsic variability of the Sun-like host star --- thesolar luminosity is known to vary at the $10^{-4}$ level on a timescale ofminutes. We use our eclipse depth limits to test two previously suggestedatmospheric models for this planet: Mie scattering from an aluminum-oxide hazeor cloud-free Rayleigh scattering. Our stringent nondetection rules out bothmodels and is consistent with thermal emission plus weak Rayleigh scatteringfrom atomic hydrogen and helium. Our results are in stark contrast with thosefor the much cooler HD 189733b, the only other hot Jupiter with spectrallyresolved reflected light observations; those data showed an increase in albedowith decreasing wavelength. The fact that the first two exoplanets with opticalalbedo spectra exhibit significant differences demonstrates the importance ofspectrally resolved reflected light observations and highlights the greatdiversity among hot Jupiters.

The Very Low Albedo of WASP-12b From Spectral Eclipse Observations with $\textit{Hubble}$