Artist's impression of exoplanet HD 209458 b

The first planet discovered using the transit method was HD 209458 b in 1999. Interestingly, it was also the first planet whose atmosphere was studied. When a planet transits its star, part of the starlight is absorbed by particles in the atmosphere. The NASA/ESA Hubble Space Telescope became the first satellite to ever measure the composition of an atmosphere of an exoplanet. HD 209458 b is a popular target for space missions, being also revisited by ESA’s Cheops mission in 2020. Using data from this visit, scientists discovered that the planet has a dark outlook and no clouds in its atmosphere.