27 nov 2001 - The first atmosphere detected on an extrasolar planet

Description:

The first confirmed exoplanet discovery occurred in 1992, with two terrestrial-mass planets found orbiting the pulsar PSR B1257+12. The first exoplanet orbiting a main-sequence star followed three years later, in 1995, when astronomers announced a "hot Jupiter" orbiting the G-type star 51 Pegasi.

Another milestone in exoplanet studies occurred on 27th November 2001, when astronomers using the Hubble Space Telescope reported the discovery of sodium in the atmosphere of Osiris, also known as HD 209458 b. This planet, located 159 light years away, was found to have a radius about 1.35 times that of Jupiter and a mass of 0.69 Jupiters, orbiting the Sun-like star HD 209458.

Its very close orbit around the star, just 3.5 Earth-days long, gave Osiris an estimated temperature of 1,200 K (927 °C; 1,700 °F) and a "transition zone" in the outer atmosphere with much higher temperatures of 5,500 K (5,227 °C; 9,440 °F). This intense heat was shown to be expanding the atmosphere and causing it to produce a comet-like tail of hydrogen, carbon, and oxygen, with even higher temperatures of up to 15,000 K.

Astronomers estimated the planet to be losing between 100 and 500 million kg (0.2–1 billion lb) of hydrogen per second, and predicted a 7% loss of mass (equivalent to about 15 Earth masses) over its lifetime of 5 billion years

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