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Three planets have been directly imaged around the young star HR 8799. Theplanets are 5-13 Mjup and orbit the star at projected separations of 24-68 AU.While the initial detection occurred in 2007, two of the planets were recoveredin a re-analysis of data obtained in 2004. Here we present a detection of thefurthest planet of that system, HR 8799 b, in archival HST/NICMOS data from1998. The detection was made using the locally-optimized combination of imagesalgorithm to construct, from a large set of HST/NICMOS images of differentstars taken from the archive, an optimized reference point-spread functionimage used to subtract the light of the primary star from the images of HR8799. This new approach improves the sensitivity to planets at smallseparations by a factor of ~10 compared to traditional roll deconvolution. Thenew detection provides an astrometry point 10 years before the most recentobservations, and is consistent with a Keplerian circular orbit with a~70 AUand low orbital inclination. The new photometry point, in the F160W filter, isin good agreement with an atmosphere model with intermediate clouds andvertical stratification, and thus suggests the presence of significant waterabsorption in the planet's atmosphere. The success of the new approach usedhere highlights a path for the search and characterization of exoplanets withfuture space telescopes, such as the James Webb Space Telescope or aTerrestrial Planet Finder.

HST/NICMOS detection of HR 8799 b in 1998