We present Hubble Space Telescope ( HST ) optical transmission spectra of the transiting hot Jupiter WASP-12b , taken with the Space Telescope Imaging Spectrograph ( STIS ) instrument .
The resulting spectra cover the range 2900 to 10300 Å which we combined with archival WFC3 spectra and Spitzer photometry to cover the full optical to infrared wavelength regions .
With high spatial resolution , we are able to resolve WASP-12A ’ s stellar companion in both our images and spectra , revealing that the companion is in fact a close binary M0V pair , with the three stars forming a triple-star configuration .
We derive refined physical parameters of the WASP-12 system , including the orbital ephemeris , finding the exoplanet ’ s density is \sim 20 % lower than previously estimated .

From the transmission spectra , we are able to decisively rule out prominent absorption by TiO in the exoplanet ’ s atmosphere , as there are no signs of the molecule ’ s characteristic broad features nor individual bandheads .
Strong pressure-broadened Na and K absorption signatures are also excluded , as are significant metal-hydride features .
We compare our combined broadband spectrum to a wide variety of existing aerosol-free atmospheric models , though none are satisfactory fits .
However , we do find that the full transmission spectrum can be described by models which include significant opacity from aerosols : including Rayleigh scattering , Mie scattering , tholin haze , and settling dust profiles .
The transmission spectrum follows an effective extinction cross section with a power-law of index \alpha , with the slope of the transmission spectrum constraining the quantity \alpha T = -3528 \pm 660 K , where T is the atmospheric temperature .
Rayleigh scattering ( \alpha = -4 ) is among the best fitting models , though requires low terminator temperatures near 900 K. Sub-micron size aerosol particles can provide equally good fits to the entire transmission spectrum for a wide range of temperatures , and we explore corundum as a plausible dust aerosol .
The presence of atmospheric aerosols also helps to explain the modestly bright albedo implied by Spitzer observations , as well as the near black body nature of the emission spectrum .
Ti-bearing condensates on the cooler night-side is the most natural explanation for the overall lack of TiO signatures in WASP-12b , indicating the day/night cold-trap is an important effect for very hot Jupiters .
These finding indicate that aerosols can play a significant atmospheric role for the entire wide range of hot-Jupiter atmospheres , potentially affecting their overall spectrum and energy balance .