On June 19th in the world of astronomy, one of the most significant events occurred in 1999: the discovery of the first known "hot Jupiter" exoplanet, HD 209458 b, also nicknamed "Osiris."

This groundbreaking discovery was made by a team of astronomers led by Geoffrey Marcy and Paul Butler using the radial velocity method. What made this find particularly exciting was that it was the first exoplanet to be observed transiting its star, which allowed astronomers to directly measure its size and density.

HD 209458 b is located about 159 light-years away from Earth in the constellation Pegasus. It's a gas giant about 60% more massive than Jupiter but with a radius 35% larger, making it less dense than our own solar system's largest planet.

The most fascinating aspect of this planet is its extremely close orbit around its star. It completes an orbit in just 3.5 Earth days and is so close to its star that its atmosphere is being slowly boiled away into space! Imagine a world where a year lasts less than a week, and the sky is constantly filled with the blinding light of a nearby sun.

This discovery opened up a whole new field of exoplanet research. It allowed astronomers to study the composition of exoplanet atmospheres for the first time, as they could analyze the starlight filtering through the planet's atmosphere during transits.

So, on this day in 1999, astronomers essentially found the cosmic equivalent of a steamy sauna world, forever changing our understanding of planetary systems and paving the way for future exoplanet discoveries. It's like they stumbled upon a cosmic hot tub party, but instead of bubbles, they found a world of scientific possibilities!