HD 209458 b
HD 209458 b is 150 light years distant from Earth exoplanet around the star HD 209458, which is unofficially named by its discoverers after the ancient Egyptian fertility and death god Osiris.
Description
The planet HD 209458 b has a lower mass than Jupiter by one-third, but its diameter is 1.3 times larger than that of Jupiter because of its proximity to the central star heated atmosphere. This results in a very low density of about 0.3 g / cc, so that the planet can not have a particularly extensive core. HD 209458 b orbits its sun -like central star HD 209458 at a distance of 6.92 million kilometers, ie the orbit is one-eighth as large as that of Mercury. The orbital period is only 3 ½ days, the mean orbital speed around 140 km / s, the eccentricity of the orbit is 0.07. Due to the close orbit and the large mass is expected Osiris the class of " hot Jupiters " to. Its surface temperature is estimated to be at least 1000K. Spectroscopic observations of the transit in front of the central star with the Keck telescope indicate a retrograde orbit due to the Rossiter - McLaughlin effect. In its atmosphere water vapor was detected.
Discovery history
The Earth is only 3 degrees above the orbital plane of Osiris, so that HD 209458 in 1999 was the first star, and with the "Sun Pass" ( Transit) was observable of an exoplanet. Caused by the passing of the planet, the apparent brightness of the central star every 3 ½ days decreased for 2 ½ hours by 1.6 %. The albedo of the planet itself, which was measured from the MOST satellite is, with 30 % surprisingly low and is well below the value of Jupiter (52%)
In 2003, one could observe with the Hubble Space Telescope, that the atmosphere of Osiris because of its proximity to the central star " evaporated " and a tail forms, similar to the case for a comet. In the planet's atmosphere is oxygen and carbon in the form of carbon dioxide and methane were detected.
In April 2007, the Hubble Space Telescope found significant amounts of water vapor fixed in the planet's atmosphere. For HD 209458b is the first planet outside our solar system, has been demonstrated on the water. In subsequent studies, however, the water vapor spectra could not be confirmed. The planet has a predominantly consisting of hydrogen atmospheric " tail " on which to about 200,000 km away (around its diameter ) extends behind him. The strong heating of the atmosphere leads to the fact that HD 209458 b loses 100-500000 tons of hydrogen per second to the surrounding space. This phenomenon was also detected in the extra-solar planet HD 189733 b. It is estimated that the planet has lost about 7 % of its mass during its approximately 5 billion years long existence in this way. However, the exact mechanism of Wasserstoffevaporation is quite complex and possibly the loss rate is also much lower than previously thought.
Using the Spitzer Space Telescope was able in 2005 to measure the infrared radiation from the planet during a transit.