description HD 209458 b Overview
HD 209458 b orbits a Sun-like star about 153 light-years away and belongs to the hot-Jupiter class of giant planets. Its close orbit subjects its atmosphere to intense stellar radiation. In 1999 it became the first exoplanet observed transiting its star, meaning astronomers measured the slight dimming caused by the planet crossing in front of it. It had already been identified through its effect on the star's motion.
Observations also revealed escaping hydrogen, providing early evidence that a strongly irradiated giant planet can lose atmospheric gas. That makes the planet valuable for studying atmospheric dynamics and planetary evolution. Astronomers can compare information from its transit with measurements of starlight affected by its atmosphere. Its significance lies in connecting several ways of examining a distant world, rather than simply being the first planet discovered by transit.
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When was the exoplanet HD 209458 b discovered?
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.
Why is HD 209458 b called a "comet planet"?
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.
How far away is HD 209458 b from Earth?
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."
Was HD 209458 b the first exoplanet to have water detected?
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.
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