HD 209458 b

HD 209458 b was first identified by astronomers in 1999 using the radial velocity method at the Keck Observatory in Hawaii. However, it gained massive historical significance later that year when it became the first exoplanet ever observed passing directly in front of its host star. This transit provided the first definitive proof of an exoplanet's physical existence.

HD 209458 b earned the nickname "comet planet" because observations by the Hubble Space Telescope revealed that extreme heat from its nearby star is blowing its atmosphere into space. A massive plume of hydrogen gas is trailing off the planet, much like the tail of a comet. It was the first planet found to be actively losing its atmosphere.

HD 209458 b is located approximately 159 light-years away from Earth in the constellation Pegasus. It is a massive gas giant, often compared to Jupiter, but it orbits its star at a distance of just 0.05 Astronomical Units. This incredibly close orbit makes it a textbook example of a "hot Jupiter."

Yes, in 2007, a team using the Spitzer Space Telescope detected water vapor in the atmosphere of HD 209458 b. This marked the first time water had ever been confirmed in the atmosphere of a planet outside our solar system. It paved the way for modern atmospheric studies of distant exoplanets.