What is Planet Osiris?

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  • Written By: Michael Anissimov
  • Edited By: Bronwyn Harris
  • Last Modified Date: 19 January 2020
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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 of Pegasus. The planet was discovered by spectroscopic studies on 5 November 1999, the first extrasolar planet discovered orbiting a Sun-like star, and its transit was observed shortly afterward, on 9 November 1999. This was also the first observation of an extrasolar planet transiting its home star. HD 209458 is a "Hot Jupiter" planet, with a mass 0.69 times that of Jupiter, a volume 146% greater than Jupiter, and a orbital radius of 0.045 AU (astronomical units), about an eighth of the distance between Mercury and the Sun.

In 2001, 2003, 2004, 2005 and 2007, observation of Osiris led to more milestones for the astronomy of extrasolar planets, including the first extrasolar atmosphere to be measured (27 November 2001 by NASA), the first extrasolar atmosphere found to contain carbon and oxygen, the first direct light (infrared) to be captured from an extrasolar planet (22 March 2005 by NASA), and one of the first two extrasolar planets to have their spectra observed (21 February 2007 by NASA). In 2007, it was announced that, based on the application of new and theoretical models, it was found that Osiris' atmosphere contained water vapor, though a subsequent announcement cast doubt on this possibility. Investigation is still ongoing.


Using the Spitzer Space Telescope, astronomers have detected a huge ellipsoidal envelope of hot gases surrounding the planet, being left in a trail as it orbits its home star at a rate of one orbit every 3.5 Earth days. This tail is superheated to 10,000 degrees K. The tail material is primarily hydrogen, fed from Osiris at a rate of approximately half a million tonnes of hydrogen per second. This material loss is thought to be common among all planets orbiting their suns at distances of less than 0.1 AU. Over its lifetime of 5 billion years, Osiris has probably lost about 7% of its total mass to this phenomenon.

Like other Hot Jupiter planets, Osiris has an extremely high temperature, is tidally locked to its home star, and experiences massive winds of thousands of miles per hour. Its orbit is highly circularized, and it orbits closely enough that its transit can be observed every few days from the Earth, as fluctuations in the energy output from its home star. Astronomers studying the planet hope to learn more about it, as much more interesting scientific information is likely to be revealed with better instruments.


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