Discovery of HD 209458 b

HD 209458 b is an exoplanet orbiting a Sun-like star in the constellation Pegasus, about a hundred and fifty light years from Earth. It is a hot Jupiter, a gas giant that circles very close to its star, with a year lasting just over three days. In 1999 astronomers detected it through the slight wobble of the star and then, in a landmark observation, saw it pass in front of the star, the first time a transit of a planet outside the solar system was observed. This allowed its size and mass to be measured. It was later the first exoplanet found to have an atmosphere, in which sodium was detected, and an evaporating cloud of hydrogen around it.

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