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Osiris (planet))
HD 209458b, unofficially 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. HD 209458 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.
HD 209458b was 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 the first extrasolar planet found to have an atmosphere containing oxygen and carbon.
Discovery
Spectroscopic studies first revealed the presence of a planet around HD 209458 on November 5 1999. Astronomers had made careful photometric measurements of several stars known to be orbited by planets, in the hope that they might observe a dip in brightness caused by the transit of the planet across the star's face. This would require the planet's orbit to be inclined such that it would pass between the Earth and the star, and previously no transits had been detected.
On November 7, though, a 1.7% drop in HD 209458's brightness was measured, which was later confirmed as being due to a transit. Each transit lasts about three hours, and about 1.5% of the star's face is covered by the planet during the transit.
The star had been observed many times by the Hipparcos satellite, which allowed astronomers to calculate its orbital period very accurately at 3.524736 days. The planet was unofficially named "Osiris" (Ausare in Egyptian), after the Egyptian god who lost part of his body after having been killed and cut into pieces by his brother Set to prevent his return to life.
Physical parameters
Spectroscopic analysis had shown that the planet had a mass about 0.6 times that of Jupiter. The occurrence of transits allowed astronomers to calculate the planet's radius, which had not been possible for any previously known exoplanet, and it turned out to have a radius some 35% larger than Jupiter's. It had been previously hypothesised that hot Jupiters particularly close to their parent star should exhibit this kind of inflation due to intense heating of their outer atmosphere.
Detection of the atmosphere
On November 27 2001 the Hubble Space Telescope detected sodium in the planet's atmosphere, the first planetary atmosphere outside our solar system to be measured. 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°C. At this temperature, the Boltzmann distribution of particle velocities gives rise to a significant 'tail' of atoms moving at speeds greater than the escape velocity, and the planet is estimated to be losing about 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 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 Chthonian planets, has been proposed. HD 209458b itself will not become Chthonian, although it may have lost up to about 7% of its mass over its estimated lifetime of 5 billion years.
Direct detection
Illustration of the hydrodynamic drag
Image courtesy NASA/ESA/CNRS
On March 22, 2005. NASA released news that this was one of two planets whose infrared light was measured by the Spitzer Space Telescope, the first ever to do so. This was done by subtracting out the parent star's constant light and noting the difference as the planet transited in front of the star and was eclipsed behind it, providing a measure of the light from the planet itself. New measurements from this observation determined the planet's temperature as at least 1300 degrees F (750 degrees C). The circular orbit of HD 209458b was also confirmed.
It is thought that HD 209458b's days are the same length as its years, that is it rotates about its axis every time it rotates around its star, similar to how the moon orbits the earth. This means HD 209458b shows only one side to its star, resulting in uneven heating and therefore the possibility of very strong winds. Such strong winds may be detectable.
References
- Castellano T., Jenkins J., Trilling D.E., Doyle L., Koch D. (2000), Detection of Planetary Transits of the Star HD 209458 in the Hipparcos Data Set, Astrophysical Journal, Volume 532, Issue 1, p. L51-L53
- Charbonneau, D. (2003), HD 209458 and the Power of the Dark Side, Scientific Frontiers in Research on Extrasolar Planets, ASP Conference Series, Vol 294, Edited by Drake Deming and Sara Seager. (San Francisco: ASP) ISBN 1-58381-141-9, 2003, p. 449-456
- Fortney J.J., Sudarsky D., Hubeny I., Cooper C.S., Hubbard W.B., Burrows A., Lunine J.I. (2003), On the Indirect Detection of Sodium in the Atmosphere of the Planetary Companion to HD 209458, Astrophysical Journal, Volume 589, p. 615-622
- Hébrard G., Lecavelier Des Étangs A., Vidal-Madjar A., Désert J.-M., Ferlet R. (2003), Evaporation Rate of Hot Jupiters and Formation of Chthonian Planets, Extrasolar Planets: Today and Tomorrow, ASP Conference Proceedings, Vol. 321, held 30 June - 4 July 2003, Institut D'Astrophysique de Paris, France. Edited by Jean-Philippe Beaulieu, Alain Lecavelier des Etangs and Caroline Terquem
External links
Last updated: 05-13-2005 07:56:04
