The Effect of the Transit of Venus on ACRIM’s Total Solar Irradiance Measurements: Implications for Transit Studies of Extrasolar Planets

We used the 8 June 2004 transit of Venus (ToV) as a surrogate to testobserving methods, strategies and techniques that are being contemplated forfuture space missions to detect and characterize extrasolar terrestrial planets(ETPs) as they transit their host stars, notably NASA's Kepler mission plannedfor 2008. As an analog to "Kepler-like" photometric transit observations, weobtained (spatially unresolved) radiometric observations with the ACRIM 3instrument on ACRIMSAT to follow the effect of the ToV on the total solarirradiance (TSI). Contemporaneous high-resolution broadband imagery with NASA'sTRACE spacecraft provided, directly, measures of the stellar (solar)astrophysical noise that can intrinsically limit such transit observations.During the ~ 5.5 h transit, the planet's angular diameter was approximately1/32 the solar diameter, thus covering ~ 0.1 of the stellar surface. With ourACRIM 3 data, we measure temporal changes in TSI with a 1 sigma per sample(unbinned) uncertainty of approximately 100 mW m^-2 (0.007%). A diminution inTSI of ~ 1.4 W m^-2 (~ 0.1%, closely corresponding to the geometricallyocculted area of the photosphere) was measured at mid-transit compared with amean pre/post transit TSI of ~ 1365.9 W m^-2. These observations serve as asurrogate to future photometric observations of ETPs such as Kepler willdeliver. Detailed analysis of the ToV, a rare event within our own solarsystem, with time-resolved radiometry augmented with high-resolution imageryprovides a useful analogue for investigating the detectability andcharacterization of ETPs from observations that are anticipated in the nearfuture.Comment: Accepted to ApJ 8 Dec 2005; 14 pages of text, 8 figures, 1 tabl