* Astronomy

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RE: HD209458b

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Title: Exoplanet HD209458b: inflated hydrogen atmosphere but no sign of evaporation
Authors: Lotfi Ben-Jaffel
(Version v2)

Many extrasolar planets orbit closely to their parent star. Their existence raises the fundamental problem of loss and gain in their mass. For exoplanet HD209458b, reports on an unusually extended hydrogen corona and a hot layer in the lower atmosphere seem to support the scenario of atmospheric inflation by the strong stellar irradiation. However, difficulties in reconciling evaporation models with observations call for a reassessment of the problem. Here, we use HST archive data to report a new absorption rate of ~8.9% ± 2.1% by atomic hydrogen during the HD209458b transit, and show that no sign of evaporation could be detected for the exoplanet. We also report evidence of time variability in the HD209458 Lyman-a flux, a variability that was not accounted for in previous studies, which corrupted their diagnostics. Mass loss rates thus far proposed in the literature in the range 5x(10^{10}-10^{11} g s^{-1}) must induce a spectral signature in the Lyman-a line profile of HD209458 that cannot be found in the present analysis. Either an unknown compensation effect is hiding the expected spectral feature or else the mass loss rate of neutrals from HD209458 is modest.

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Title: Exoplanet HD209458b: inflated hydrogen atmosphere but no sign of evaporation
Authors: Lotfi Ben-Jaffel

Many extrasolar planets orbit closely to their parent star. Their existence raises the fundamental problem of loss and gain in their mass. For exoplanet HD209458b, reports on an unusually extended hydrogen corona and a hot layer in the lower atmosphere seem to support the scenario of atmospheric inflation by the strong stellar irradiation. However, difficulties in reconciling evaporation models with observations call for a reassessment of the problem. Here, we use HST archive data to report a new absorption rate of ~ 8.9% ± 2.1% by atomic hydrogen during the HD209458b transit, and show that no sign of evaporation could be detected for the exoplanet. We also report evidence of time variability in the HD209458 Ly-a flux, a variability that was not accounted for in previous studies, which corrupted their diagnostics. Mass loss rates thus far proposed in the literature in the range 5 x 10^{10}-10^{11} {
m g s^{-1}} must induce a spectral signature in the Lyman-\alpha line profile of HD209458 that cannot be found in the present analysis. Either an unknown compensation effect is hiding the expected spectral feature or else the mass loss rate of neutrals from HD209458 is modest.

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Evaporating Exoplanet hd209458b

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Position (J2000): RA: 22 03 10.0 Dec: +18 53 04
Width: 14'1

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Astronomers have detected water in the atmosphere of a planet outside our solar system for the first time.
The finding, to be detailed in an upcoming issue of Astrophysical Journal, confirms previous theories that say water vapour should be present in the atmospheres of nearly all the known extrasolar planets. Even hot Jupiters, gaseous planets that orbit closer to their stars than Mercury to our Sun, are thought to have water.
The discovery, announced today, means one of the most crucial elements for life as we know it can exist around planets orbiting other stars.

We know that water vapour exists in the atmospheres of one extrasolar planet and there is good reason to believe that other extrasolar planets contain water vapour - Travis Barman, an astronomer at the Lowell Observatory in Arizona who made the discovery.

HD209458b is a world well-known among planet hunters. In 1999, it became the first planet discovered around a normal star outside our solar system and, a few years later, was the first exoplanet confirmed to have oxygen and carbon in its atmosphere.
HD209458b is separated from its star by only about 7 million kilometresabout 100 times closer than Jupiter is to our Sunand is so hot scientists think about it is losing about 10,000 tons of material every second as vented gas.
Thus, despite the discovery of water in its atmosphere, it is unlikely that any life exists on or in the gaseous planet.

Source

Title: Identification of Absorption Features in an Extrasolar Planet Atmosphere
Authors: T. S. Barman

Water absorption is identified in the atmosphere of HD209458b by comparing models for the planet's transmitted spectrum to recent, multi-wavelength, eclipse-depth measurements (from 0.3 to 1 microns) published by Knutson et al. (2007). A cloud-free model which includes solar abundances, rainout of condensates, and photoionisation of sodium and potassium is in good agreement with the entire set of eclipse-depth measurements from the ultraviolet to near-infrared. Constraints are placed on condensate removal by gravitational settling, the bulk metallicity, and the redistribution of absorbed stellar flux. Comparisons are also made to the Charbonneau et al. (2002) sodium measurements.

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Earth's inhabitants are used to temperatures that vary, sometimes greatly, between day and night. New measurements for three planets outside our solar system indicate their temperatures remain fairly constant -- and blazing hot -- from day to night, even though it is likely one side of each planet always faces its sun and the other is in permanent darkness.
The reason apparently is supersonic winds, perhaps as strong as 9,000 miles an hour, that constantly churn the planets' atmospheres and keep temperatures on the dark side from plunging.
The planets, gas giants similar in size to Jupiter, were discovered in the last decade orbiting stars about the same size as our sun and less than 150 light years from Earth. All of them orbit within about 5 million miles of their stars, far less than Mercury's distance from our sun.
The three planets are 51 Pegasi, about 50 light years from our sun, HD179949b about 100 light years distant, and HD209458b about 147 light years away. A light year is about 5.88 trillion miles.
Astronomers have wondered whether planets orbiting so close to their stars but with one side in constant daylight and the other permanently dark would exhibit sharp temperature differences between the day side and the night side. For the three planets in this study, the temperatures appear to be constant, likely because of the strong winds that mix the atmosphere planetwide, said Eric Agol, a University of Washington assistant professor of astronomy and co-author of a poster presenting the findings today at the American Astronomical Society national meeting in Seattle.

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A Spitzer Infrared Radius for the Transiting Extrasolar Planet HD209458b
Authors: L. Jeremy Richardson, Joseph Harrington, Sara Seager, Drake Deming

Researchers have measured the infrared transit of the extrasolar planet HD209458b using the Spitzer Space Telescope.
They observed two primary eclipse events (one partial and one complete transit) using the 24 micron array of the Multiband Imaging Photometer for Spitzer (MIPS), and analysed a total of 2392 individual images (10-second integrations) of the planetary system, recorded before, during, and after transit.
The researchers performed optimal photometry on the images and use the local zodiacal light as a short-term flux reference. At this long wavelength, the transit curve has a simple box-like shape, allowing robust solutions for the stellar and planetary radii independent of stellar limb darkening, which is negligible at 24 microns.
The researchers derive a stellar radius of R = 1.06 ±0.07 solar radii, a planetary radius of R = 1.26 ±0.08 Jupiter radii, and a stellar mass of 1.17 solar masses. Within the errors, their results agree with the measurements at visible wavelengths. The 24-micron radius of the planet therefore does not differ significantly compared to the visible result. They point out the potential for deriving extrasolar transiting planet radii to high accuracy using transit photometry at slightly shorter IR wavelengths where greater photometric precision is possible.

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Physical parameters of the upper atmosphere of the extrasolar planet
HD209458b
Every 3.5 days, the transits of the gaseous planet orbiting the star HD209458 offer the unique opportunity to investigate the spectral features of an extra-solar planetary atmosphere.
The star HD 209458 is 47 parsecs (153 light years) away in the constellation of Pegasus, and is about the same age, color and size as our own Sun. It is very near the star, 51 Pegasi, around which the first extrasolar planet was discovered in 1995.

Using the Hubble Space Telescope researchers first discovered the extended upper atmosphere of HD209458b through the detection of a 15% HI Lyman alpha absorption. They concluded that the hydrogen must be escaping the planet with a lower limit rate of 10^10 g/s.
Additional observations, subsequently allowed them to detect OI and CII in the upper atmosphere implying that this atmosphere is hydrodynamically escaping (blowing off).

Further study of the upper atmosphere will better constrain the "blow off" state by directly estimating the physical conditions and the flow characteristics. In particular the researchers hope to determine the temperature and density at the base of the upper
atmosphere (the thermosphere), and the density distribution and ionization state just below that level.
Comparison between the optical and ultraviolet occultation light curves will provide useful information on the molecular/haze content of the lower atmosphere, and for the first time a detailed probe of the atmosphere of an "evaporating" extra-solar planet.

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