In 1999, David Charbonneau et al., and then Gregory Henry and collaborators, observed independently HD 209458 b (discovered with the radial velocity method), and showed that it was occulting its star at each rotation.
They deduced, from the weak decrease of the star's brightness during the event, that the planet has 1,3 times Jupiter's radius and a density of 0.4, thus demonstrating that HD 209458 b can only be a gazeous giant planet.
In 2000, the same observations were made with the Hubble Space telescope. The excellent photometric precision of the measurements allowed us to highlight the presence of sodium in the atmosphere of the planet.
In 2001, other observations with the HST led Alfred Vidal-Madjar and his collaborators to conclude that hydrogen was escaping from HD 209458 b, thus forming a kind of cometary tail. In 2003, carbon and oxygen were also detected.
In 2005, Drake Deming et al. observed in infrared the passage of HD 209458 b behind its star ("secondary" transit) with the space telescope Spitzer. Thus, they could deduce the temperature of the planet to be 1130K.