The eagerly awaited detection of an Earth-mass planet using the radial-velocity technique has been announced last week (Dumusque et al. 2012). This planet, Alpha Cen Bb orbits our close neighbor alpha Cen B and has the potential to become a true Rosetta stone in exoplanet science, if its transiting nature were revealed. HST/STIS is the only facility able to detect a transit from such a small planet at a high confidence level. We propose to quasi-continuously monitor the expected transit window of Alpha Cen B b during 26 hours (16 orbits) to encompass the orbital solutions allowed by the published radial-velocity dataset, that we reanalyzed for this purpose. If alpha Cen Bb is transiting, our proposed observations will not only confirm the existence of the planet but also yield for the first time the density of an Earth-mass exoplanet and constrain its interior composition. Our observations using the G750M grating will provide the first spectrum of a terrestrial exoplanet, with possibly detectable water features. In the case of an exosphere tail, detection of the sodium doublet would be achievable, depending on the tail extension. Finally, the transit light curve shape will constrain alpha Cen Bb's orbital eccentricity. This would be an important constraint for planetary formation and evolution models in binary systems. If no transit is detected, our observations will still be important: detection of p-mode oscillations and granulation is expected for alpha Cen A and B, which will constrain convection and radiative transport modeling in the atmosphere of cool main-sequence stars. This program aims at searching for a transit of the Earth-mass planet detected around Alpha Centauri Bb. The transit could occur at the inferior conjunction, which ephemeris can be derived from the radial velocities, albeit with a 1-sigma uncertainty of ~4h. Therefore, we will use a CVZ opportunity to quasi-continuously monitor the expected transit window of Alpha Cen B b during 26 hours (16 orbits) to encompass the possible orbital solutions. Not only the CVZ window shall contain a transit, but it shall also allow for a specific roll angle of the telescope, in order to exclude Alpha Centauri A from the long slit. These conditions are met during the 2013 July CVZ opportunity. We propose to do the acquisition of Alpha Cen B in the same way as for programs #12374, 12758, or 13060, where an ACQ exposure is first performed on Alpha Cen A before shifting the pointing to Alpha Cen B, which is defined here as an offset target. ACQ and ACQ/PEAK exposures are then performed on Alpha Cen B, using the narrowest long slit (52"x0.05") in order to obtain the best possible centering of the star in the largest slit (52"x2") that will be used for the science exposures. This choice is made to minimize slit losses while hiding Alpha Cen A (which is also why the telescope roll angle matters). For the science exposures, we will use STIS/CCD with G750M (6094) and an exposure time of 6s. Alpha Cen B *will be saturated* and we estimate that the bleeding should extent over no more than +/-30 pixels from the spectral trace (and would not reach the diffraction spikes from Alpha Cen A for the selected roll angle). EXTRA-SOLAR PLANETS 0 false false 0.0 0.0 false false false AVAILABLE STAR G V-IV The star coordinates are copied from Proposal 13060 by T. Ayres and were directly measured from high-precision Chandra HRC-I images. They differ minimally from those cited in Simbad. HR-5459 HD128620 J2000 STAR K V-IV These offset are calculated from a position angle (theta) and separation (rho) between Alpha Cen A and Alpha Cen B, taking Alpha Cen A as the reference: rho = 4.57" theta = 271.3 deg From there, we have: RA_offset* = -rho x COS(theta) = -4.57" = -0.305s RA_offset = RA_offset* / COS(DEC) = -9.34" = -0.625s DEC_offset = rho * SIN(theta) = 0.104". HR-5460 HD128621 STIS/CCD Spectroscopic No No G750M 6094 The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. This CVZ observation is to be done from July 07 17:18 UT to July 08 19:06 UT. The visit is to schedule from July 07 16:58 UT to July 08 19:52 UT (Start of GSACQ to end of HOME). 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This exposure will be deliberately satured (time to saturation = 0.3s). This exposure will be deliberately satured (time to saturation = 0.3s). This exposure will be deliberately satured (time to saturation = 0.3s). This exposure will be deliberately satured (time to saturation = 0.3s). This exposure will be deliberately satured (time to saturation = 0.3s). This exposure will be deliberately satured (time to saturation = 0.3s). This exposure will be deliberately satured (time to saturation = 0.3s). This exposure will be deliberately satured (time to saturation = 0.3s). This exposure will be deliberately satured (time to saturation = 0.3s). This exposure will be deliberately satured (time to saturation = 0.3s). This exposure will be deliberately satured (time to saturation = 0.3s). 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