 Illustration of the hydrodynamic drag Image courtesy NASA/ESA/CNRSHD 209458b, provisionally dubbed Osiris by its discoverers after the Egyptian god Osiris, is an extrasolar planet that orbits the Sun-like star HD 209458 in the constellation Pegasus, some 150 light years from Earth's solar system. HD209458 is a 7th magnitude star, visible on Earth with binoculars. The radius of the planet's orbit is only 7 million kilometers, ~0.047 astronomical units, one-eighth the radius of Mercury's orbit. This small radius results in a year only 3.5 Earth days long and an estimated surface temperature of about 1000°C. Its mass is 220 times that of Earth's (0.7 Jupiter masses), suggesting that it is probably a gas giant.
Overview
On November 27 2001 the Hubble Space Telescope detected hydrogen in the planet's atmosphere, the first planetary atmosphere outside our solar system to be measured. In autumn of 2003 Hubble also detected oxygen and carbon. These are not considered to be signs of life, however; the carbon and oxygen are in atomic form, ionized by the star's heat. Astronomers used the Hubble Space Telescope Imaging Spectrograph to discover an enormous ellipsoidal envelope of tenuous atmosphere around the planet that reaches 10,000°C. At this temperature hydrogen atoms easily reach escape velocity and the planet loses about 10,000 tonnes of hydrogen per second to this process. Analysis of the starlight passing through the envelope shows that the heavier carbon and oxygen atoms are being blown off of the planet by the extreme "hydrodynamic drag" created by its evaporating hydrogen atmosphere. The hydrogen tail streaming off of the planet is 200,000 kilometers long.
It is thought that this type of atmosphere loss may be common to all planets orbiting Sun-like stars closer than around 0.1 AU. The evaporation mechanism is so distinctive that a whole new class of extrasolar planets consisting of the cores of evaporated gas giants, called Cthonian planets, has been proposed. None have yet been detected.
"Osiris", the Egyptian god, lost part of his body like HD 209458b after having been killed and cut into pieces by his brother Set to prevent his return to life.
HD 209458b is the first transiting planet discovered, the first extrasolar planet known to have an atmosphere, the first extrasolar planet observed to have an evaporating hydrogen atmosphere, and now the first extrasolar planet found to have an atmosphere containing oxygen and carbon.
Other facts about HD 209458b
- Diameter: 1.3 times that of Jupiter.
- Belongs to a type of extrasolar planet known as hot Jupiters—giant, gaseous planets in low orbits.
- Transit: Every 3.5 days, 3 hours in duration. Eclipses 1.5% of the face of the parent star.
Atmosphere
- Complex: sodium in the lower atmosphere, evaporating hydrogen detected in upper atmosphere, oxygen and carbon also in the upper atmosphere.
- Extended: During the eclipses the upper atmosphere covers 15% of the face of the parent star.
History of HD 209458b
- 1999: Discovery. Entered the astronomical Hall of Fame when the extrasolar planet passed in front of its parent star and partly eclipsed it.
- 2001: Hubble detected the element sodium in the lower part of HD 209458bs atmosphere.
- 2003: Hydrogen in upper atmosphere detected with Hubble. Signs of evaporation.
- 2004: Oxygen and carbon detected in upper atmosphere with Hubble. Evaporation mechanism so distinctive that a whole new class of extrasolar planets, the chthonian planets, being the cores of evaporated gas giants, is proposed.
Source: http://spacetelescope.org/bin/news.pl?string=heic0403
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