Osiris, a gas giant planet orbiting HD 209458, was the first exoplanet discovered orbiting a Sun-like star. It has been the subject of many astronomical milestones, including the first extrasolar atmosphere to be measured and the first direct light to be captured by an exoplanet. Osiris has a huge ellipsoidal envelope of hot gas surrounding it, and it experiences massive winds of thousands of miles per hour. Astronomers hope to learn more about the planet with better instruments.
The planet Osiris, officially known as HD 209458 b, is a gas giant planet orbiting a main-sequence star HD 209458, located 150 light-years away in the constellation Pegasus. The planet was discovered by spectroscopic studies on November 5, 1999, the first exoplanet discovered orbiting a Sun-like star, and its transit was observed shortly after, on November 9, 1999. This was also the first observation of an exoplanet transiting its star home. HD 209458 is a “Hot Jupiter” planet, with a mass 0.69 times that of Jupiter, a volume 146% greater than Jupiter, and an orbital radius of 0.045 AU (astronomical units), about one-eighth the distance between Mercury and the Sun.
In 2001, 2003, 2004, 2005, and 2007, observations of Osiris led to more milestones for exoplanet astronomy, including the first extrasolar atmosphere to be measured (November 27, 2001 by NASA), the first extrasolar atmosphere found to contain carbon and oxygen, the first direct (infrared) light to be captured by an exoplanet (March 22, 2005 by NASA), and one of the first two exoplanets to have their spectra observed (February 21, 2007 by NASA). In 2007 it was announced that, based on the application of new and theoretical models, Osiris’ atmosphere was found to contain water vapour, although a subsequent announcement questioned this possibility. The investigations are still ongoing.
Using the Spitzer Space Telescope, astronomers have detected a huge ellipsoidal envelope of hot gas surrounding the planet, which is left in a trail as it orbits its parent star at the rate of one orbit every 3.5 Earth days. This tail is superheated to 10,000 degrees K. The material in the tail is mostly hydrogen, fueled by Osiris at a rate of about half a million tons of hydrogen per second. This material loss is thought to be common to all planets that orbit their suns at distances less than 0.1 AU. Over his 5 billion year life, Osiris probably lost about 7% of his total mass due to this phenomenon.
Like other Hot Jupiter planets, Osiris has an extremely high temperature, is tidally tied to its parent star, and experiences massive winds of thousands of miles per hour. Its orbit is highly circularized, and it orbits close enough that its transit every few days by Earth can be observed, such as fluctuations in the energy produced by its parent star. Astronomers studying the planet are hoping to learn more, as much more interesting scientific information is likely to be revealed with better instruments.
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