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Born on this day
Sir Charles Scott Sherrington
Charles Sherrington was an English neurophysiologist, histologist, bacteriologist, and a pathologist, Nobel laureate.
47th week in year
27 November 2022

Important eventsBack

A hydrogen atmosphere is discovered on the planet Osiris27.11.2001

Wikipedia (09 Jan 2014, 09:28)

HD 209458 b (Osiris) is an extrasolar planet that orbits the Solar analog star HD 209458 in the constellation Pegasus, some 150 light-years from Earth's solar system. The radius of the planet's orbit is 7 million kilometres, about 0.047 astronomical units, or one eighth the radius of Mercury's orbit. This small radius results in a year that is 3.5 Earth days long and an estimated surface temperature of about 1,000 °C (about 1,800 °F). Its mass is 220 times that of Earth (0.69 Jupiter masses) and its volume is some 2.5 times greater than that of Jupiter. The high mass and great volume of HD 209458 b indicate that it is a gas giant.

HD 209458 b represents a number of milestones in extraplanetary research. It was the first of many categories: a transiting extrasolar planet discovered, the first planet detected through more than one method, an extrasolar planet known to have an atmosphere, an extrasolar planet observed to have an evaporating hydrogen atmosphere, an extrasolar planet found to have an atmosphere containing oxygen and carbon, one of the first two extrasolar planets to be directly observed spectroscopically and the first extrasolar gas giant to have its superstorm measured, and the first planet to have its orbital speed measured, determining its mass directly. Based on the application of new, theoretical models, as of April 2007, it is alleged to be the first extrasolar planet found to have water vapor in its atmosphere.


On November 27, 2001 the Hubble Space Telescope detected sodium, the first planetary atmosphere outside our solar system to be measured. This detection was predicted by Sara Seager in late 2001. The core of the sodium line runs from pressures of 50 millibar to a microbar. This turns out to be about a third the amount of sodium at HD 189733 b.

In 2003–4, astronomers used the Hubble Space Telescope Imaging Spectrograph to discover an enormous ellipsoidal envelope of hydrogen, carbon and oxygen around the planet that reaches 10,000 K. The hydrogen exosphere extends to a distance RH=3.1 RJ, much larger than the planetary radius of 1.32 RJ. At this temperature and distance, the Maxwell–Boltzmann distribution of particle velocities gives rise to a significant 'tail' of atoms moving at speeds greater than the escape velocity. The planet is estimated to be losing about 100–500 million (1–5×108) kg of hydrogen per second. Analysis of the starlight passing through the envelope shows that the heavier carbon and oxygen atoms are being blown from the planet by the extreme "hydrodynamic drag" created by its evaporating hydrogen atmosphere. The hydrogen tail streaming from the planet is approximately 200,000 kilometres long, which is roughly equivalent to its diameter.

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. HD 209458 b will not evaporate entirely, although it may have lost up to about 7% of its mass over its estimated lifetime of 5 billion years. It may be possible that the planet's magnetic field may prevent this loss, as the exosphere would become ionized by the star, and the magnetic field would contain the ions from loss.

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