Three planets have been directly imaged around the young star HR 8799 . The planets are 5–13 M _ { Jup } and orbit the star at projected separations of 24–68 AU . While the initial detection occurred in 2007 , two of the planets were recovered in a re-analysis of data obtained in 2004 . Here we present a detection of the furthest planet of that system , HR 8799 b , in archival HST/NICMOS data from 1998 . The detection was made using the locally-optimized combination of images algorithm to construct , from a large set of HST/NICMOS images of different stars taken from the archive , an optimized reference point-spread function image used to subtract the light of the primary star from the images of HR 8799 . This new approach improves the sensitivity to planets at small separations by a factor of \sim 10 compared to traditional roll deconvolution . The new detection provides an astrometry point 10 years before the most recent observations , and is consistent with a Keplerian circular orbit with a \sim 70 AU and low orbital inclination . The new photometry point , in the F160W filter , is in good agreement with an atmosphere model with intermediate clouds and vertical stratification , and thus suggests the presence of significant water absorption in the planet ’ s atmosphere . The success of the new approach used here highlights a path for the search and characterization of exoplanets with future space telescopes , such as the James Webb Space Telescope or a Terrestrial Planet Finder .