HD 209458 b

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HD 209458 b
Extrasolar planet List of extrasolar planets

Artist's conception of HD 209458 b transiting its star.
Parent star
Star HD 209458
Constellation Pegasus
Right ascension (α) 22h 03m 10.8s
Declination (δ) +18° 53′ 04″
Apparent magnitude (mV) 7.65
Distance 154 ly
(47.1 pc)
Spectral type G0V
Orbital elements
Semimajor axis (a) 0.045 AU
(6.732 Gm)
Periastron (q) ~0.04437 AU
(~6.637752 Gm)
Apastron (Q) ~0.04563 AU
(~6.826248 Gm)
Eccentricity (e) 0.014[1]
Orbital period (P) 3.52474541 ± 0.00000025 d
(0.00965 y)
Inclination (i) 86.1 ± 0.1°
Argument of
periastron		 (ω) 83°
Time of periastron (T0) 2,452,854.825415
± 0.00000025 JD
Semi-amplitude (K) 84.26 ± 0.81 m/s
Physical characteristics
Mass (m) 0.69 ± 0.05 MJ
(219.282 M⊕)
Radius (r) 1.35 ± 0.05 RJ
(15.112888 R⊕)
Density (ρ) 370 kg/m3
Surface gravity (g) 8.7279185 m/s² (0.89 g)
Temperature (T) 1,130 ± 150 K
Discovery information
Discovery date November 5, 1999
Discoverer(s) T. Brown
D. Latham
M. Mayor
G.W. Henry
G. Marcy
R.P. Butler
S.S. Vogt
D. Charbonneau
Detection method Transit and Radial velocity
Discovery site Lowell Observatory
Geneva Observatory
Discovery status Published
Other designations
Osiris, V376 Pegasi b
Database references
Extrasolar Planets
Encyclopaedia		 data
SIMBAD data

HD 209458 b, also unofficially named 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, with evidence of water vapor. [2] The radius of the planet's orbit is 7 million kilometers, 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 146% greater than that of Jupiter. The high mass and great volume of HD 209458 b lead to its classification as 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, 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, and one of the first two extrasolar planets to be directly observed spectroscopically. 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.[3][4][5]

HD 209458 b has been informally referred to as Osiris, though this name was not given by the astronomers credited with its discovery. The use of the Osiris nickname has faded among astronomers and the media.[6][7] HD 209458 is an 8th magnitude star, visible from Earth with binoculars.

Contents

[edit] Detection and discovery

[edit] Transits

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.

Soon after the discovery, separate teams, one led by David Charbonneau including Timothy Brown and others, and the other by Gregory W. Henry, were able to detect a transit of the planet across the surface of the star making it the first known transiting extrasolar planet. On September 9 and 16, 1999, Charbonneau's team measured a 1.7% drop in HD 209458's brightness, which was attributed to the passage of the planet across the star. On November 8, Henry's team observed a transit ingress.[8] Each transit lasts about three hours, during which the planet covers about 1.5% of the star's face.

The star had been observed many times by the Hipparcos satellite, which allowed astronomers to calculate the orbital period of HD 209458 b very accurately at 3.524736 days.[9]

[edit] Spectroscopic

Jupiter HD 209458 b

Spectroscopic analysis had shown that the planet had a mass about 0.69 times that of Jupiter.[10] 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.

[edit] Direct detection

On March 22, 2005, NASA released news that infrared light from the planet had been measured by the Spitzer Space Telescope, the first ever direct detection of light from an extrasolar planet. This was done by subtracting 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 750 °C (1300 °F). The circular orbit of HD 209458 b was also confirmed.

The transit of HD 209458 b.

[edit] Spectral observation

On February 21, 2007, NASA and Nature released news that HD 209458 b was one of the first two extrasolar planets to have their spectra directly observed, the other one being HD 189733 b.[11][12] This was long seen as the first mechanism by which extrasolar but non-sentient life forms could be searched for, by way of influence on a planet's atmosphere. A group of investigators led by Jeremy Richardson of NASA's Goddard Space Flight Center spectrally measured HD 209458 b's atmosphere in the range of 7.5 to 13.2 micrometers. The results defied theoretical expectations in several ways. The spectrum had been predicted to have a peak at 10 micrometers which would have indicated water vapor in the atmosphere, but such a peak was absent, indicating no detectable water vapor. Another unpredicted peak was observed at 9.65 micrometers, which the investigators attributed to clouds of silicate dust, a phenomenon not previously observed. Another unpredicted peak occurred at 7.78 micrometers, which the investigators did not have an explanation for. A separate team led by Mark Swain of the Jet Propulsion Laboratory reanalyzed the Richardson et al. data, and had not yet published their results when the Richardson et al. article came out, but made similar findings.

[edit] Rotation

As of August 2008, the most recent calculation of HD 209458 b's Rossiter-McLaughlin effect and so spin-orbit angle was that of Winn in 2005.[13] This is -4.4 +/- 1.4 degrees.[14]

Artist's depiction of the extrasolar planet HD 209458 b

[edit] Physical characteristics

It had been previously hypothesized that hot Jupiters particularly close to their parent star should exhibit this kind of inflation due to intense heating of their outer atmosphere. Tidal heating due to its orbit's eccentricity, which may have been more eccentric at formation, may also have played a role over the past billion years.[15]

[edit] Stratosphere and upper clouds

The atmosphere is at a pressure of one bar at an altitude of 1.29 Jupiter radii above the planet's center.[16]

Where the pressure is 33+/-5 millibars, the atmosphere is clear (probably hydrogen) and its Rayleigh effect is detectable. At that pressure the temperature is 2200+/-260 K.[16]

Observations by the orbiting Microvariability and Oscillations of STars telescope initially limited the planet's albedo (or reflectivity) below 30%, making it a surprisingly dark object. (The geometric albedo has since been measured to 3.8 ± 4.5%.[17]) In comparison, Jupiter has a much higher albedo of 52%. This would suggest that HD 209458 b's upper cloud deck is either made of less reflective material than is Jupiter's, or else has no clouds and Rayleigh-scatters incoming radiation like Earth's dark ocean.[18] Models since then have shown that between the top of its atmosphere and the hot, high pressure gas surrounding the mantle, exists a stratosphere of cooler gas.[19][20] This implies an outer shell of dark, opaque, hot cloud; usually thought to consist of vanadium and titanium oxides like M dwarf stars ("pM planets"), but other compounds like tholins cannot be ruled out as of yet.[20] The Rayleigh-scattering heated hydrogen rests at the top of the stratosphere; the absorptive portion of the cloud deck floats above it at 25 millibars.[21]

[edit] Exosphere

Surrounding that level, on November 27, 2001 the Hubble Space Telescope detected sodium, the first planetary atmosphere outside our solar system to be measured.[22] This detection was predicted by Sara Seager in late 2001.[23] The core of the sodium line runs from pressures of 50 millibar to a microbar.[24] This turns out to be about a third the amount of sodium at HD 189733 b.[25]

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. At this temperature, the Maxwell–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 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 kilometers 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.[26]

[edit] Presumed atmospheric water vapor

On April 10, 2007, Travis Barman of the Lowell Observatory announced evidence that the atmosphere of HD 209458 b contained water vapor. Using a combination of previously published Hubble Space Telescope measurements and new theoretical models, Barman found strong evidence for water absorption in the planet's atmosphere.[2][3][27] His method modeled light passing directly through the atmosphere from the planet's star as the planet passed in front of it. However, this hypothesis is still being investigated for confirmation.

Barman drew on data and measurements taken by Heather Knutson, a student at Harvard University, from the Hubble Space Telescope, and applied new theoretical models to demonstrate the likelihood of water absorption in the atmosphere of the planet. The planet orbits its parent star every three and a half days, and each time it passes in front of its parent star, the atmospheric contents can be analyzed by examining how the atmosphere absorbs light passing from the star directly through the atmosphere in the direction of Earth.

According to a summary of the research, atmospheric water absorption in such an exoplanet renders it larger in appearance across one part of the infrared spectrum, compared to wavelengths in the visible spectrum. Barman took Knutson's Hubble data on HD 209458 b, applied to his theoretical model, and allegedly identified water absorption in the planet's atmosphere.

On April 24, the astronomer David Charbonneau, who led the team that made the Hubble observations, cautioned that the telescope itself may have introduced variations that caused the theoretical model to suggest the presence of water. He hoped that further observations would clear the matter up in the following months.[28] As of April 2007, further investigation is being conducted.

[edit] See also

[edit] References

  1. ^ Jackson, Brian; Richard Greenberg, Rory Barnes (2008). "Tidal Heating of Extra-Solar Planets". Astrophysical Journal 681: 1631. doi:10.1086/587641. arΧiv:0803.0026. ; Gregory Laughlin et al. (2005). "ON THE ECCENTRICITY OF HD 209458b". The Astrophysical Journal 629: L121–L124. 
  2. ^ a b Barman (2007). "Identification of Absorption Features in an Extrasolar Planet Atmosphere". The Astrophysical Journal Letters 661 (2): L191–L194. doi:10.1086/518736. http://www.iop.org/EJ/abstract/1538-4357/661/2/L191/.  (web Preprint)
  3. ^ a b Water Found in Extrasolar Planet's Atmosphere - Space.com
  4. ^ Signs of water seen on planet outside solar system, by Will Dunham, Reuters, Tue Apr 10, 2007 8:44PM EDT
  5. ^ Water Identified in Extrasolar Planet Atmosphere, Lowell Observatory press release, April 10, 2007
  6. ^ "Dying Planet Leaks Carbon-Oxygen". NASA. 2004-02-19. http://www.nasa.gov/vision/universe/newworlds/Osiris_leaks.html. Retrieved on 2007-04-11. 
  7. ^ Vidal-Madjar, A.; et al. (2004). "Detection of Oxygen and Carbon in the Hydrodynamically Escaping Atmosphere of the Extrasolar Planet HD 209458b". The Astrophysical Journal Letters 604: L69–L72. doi:10.1086/383347. 
  8. ^ Henry et al. IAUC 7307: HD 209458; SAX J1752.3-3138 12 November 1999, reported a transit ingress on Nov. 8. David Charbonneau et al., Detection of Planetary Transits Across a Sun-like Star, November 19, reports full transit observations on September 9 and 16.
  9. ^ Castellano et al. (March 2000). "Detection of Planetary Transits of the Star HD 209458 in the Hipparcos Data Set". The Astrophysical Journal Letters (University of Chicago Press) 532 (1): L51–L53. doi:10.1086/312565. http://www.iop.org/EJ/article/1538-4357/532/1/L51/995926.html. 
  10. ^ Notes for star HD 209458
  11. ^ NASA's Spitzer First To Crack Open Light of Faraway Worlds
  12. ^ Richardson, L. Jeremy; et al. (2007). "A spectrum of an extrasolar planet". Nature 445 (7130): 892–895. doi:10.1038/nature05636. 
  13. ^ Winn, Joshua N. (2008). "Measuring accurate transit parameters". arΧiv: 0807.4929v2 [astro-ph]. 
  14. ^ Winn et al. (2005). "Measurement of Spin-Orbit Alignment in an Extrasolar Planetary System". The Astrophysical Journal 631 (2): 1215–1226. doi:10.1086/432571. http://www.iop.org/EJ/article/0004-637X/631/2/1215/62644.html. 
  15. ^ Jackson, Brian; Richard Greenberg, Rory Barnes (2008). "Tidal Heating of Extra-Solar Planets". ApJ 681: 1631. doi:10.1086/587641. arΧiv:0803.0026. 
  16. ^ a b A. Lecavelier des Etangs, A. Vidal-Madjar, J.-M. Désert and D. Sing (2008). "Rayleigh scattering by H in the extrasolar planet HD 209458b". Astronomy & Astrophysics 485: 865-869. doi:10.1051/0004-6361:200809704. 
  17. ^ Rowe, Jason F.; Matthews, Jaymie M.; Sara Seager; Dimitar Sasselov; Rainer Kuschnig; Guenther, David B.; Moffat, Anthony F. J.; Rucinski, Slavek M.; et al. (2008). "Towards the Albedo of an Exoplanet: MOST Satellite Observations of Bright Transiting Exoplanetary Systems". arΧiv: 0807.1928v1 [astro-ph]. 
  18. ^ Matthews J.,(2005), MOST SPACE TELESCOPE PLAYS `HIDE & SEEK' WITH AN EXOPLANET; LEARNS ABOUT ATMOSPHERE AND WEATHER OF A DISTANT WORLD
  19. ^ Ivan Hubeny; Adam Burrows (2008). "Spectrum and atmosphere models of irradiated transiting extrasolar giant planets". arΧiv: 0807.3588v1 [astro-ph]. 
  20. ^ a b Ian Dobbs-Dixon (2008). "Radiative Hydrodynamical Studies of Irradiated Atmospheres". arΧiv: 0807.4541v1 [astro-ph]. 
  21. ^ Sing, David K.; Vidal-Madjar, A.; Lecavelier des Etangs, A.; Desert, J. -M.; Ballester, G.; Ehrenreich, D. (2008). "Determining Atmospheric Conditions at the Terminator of the Hot-Jupiter HD209458b". arΧiv: 0803.1054v2 [astro-ph]. 
  22. ^ I. A. G. Snellen, S. Albrecht, E. J. W. de Mooij, and R. S. Le Poole (2008). "Ground-based detection of sodium in the transmission spectrum of exoplanet HD 209458b". Astronomy & Astrophysics 487: 357–362. doi:10.1051/0004-6361:200809762. http://www.aanda.org/index.php?option=article&access=standard&Itemid=129&url=/articles/aa/abs/2008/31/aa09762-08/aa09762-08.html. 
  23. ^ Seager et al. (2000). "Photometric Light Curves and Polarization of Close‐in Extrasolar Giant Planets". The Astrophysical Journal 540 (1): 504–520. doi:10.1086/309292. http://www.iop.org/EJ/article/0004-637X/540/1/504/50207.html. 
  24. ^ Sing, David K.; Vidal-Madjar, A.; Lecavelier des Etangs, A.; Desert, J. -M.; Ballester, G.; Ehrenreich, D. (2008). "Determining Atmospheric Conditions at the Terminator of the Hot-Jupiter HD209458b". arΧiv: 0803.1054v2 [astro-ph]. 
  25. ^ Seth Redfield, Michael Endl, William D. Cochran, and Lars Koesterke (20 January 2008). "Sodium Absorption from the Exoplanetary Atmosphere of HD 189733b Detected in the Optical Transmission Spectrum". The Astrophysical Journal Letters 673 (673): L87–L90. doi:10.1086/527475. 
  26. ^ 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 Étangs and Caroline Terquem.
  27. ^ First sign of water found on an alien world - space - 10 April 2007 - New Scientist Space
  28. ^ J.R. Minkle (April 24, 2007). "All Wet? Astronomers Claim Discovery of Earth-like Planet". Scientific American. http://www.sciam.com/article.cfm?articleID=25A261F0-E7F2-99DF-313249A4883E6A86&chanID=sa007. 

[edit] External links

Sister project Wikimedia Commons has media related to: HD 209458 b

Coordinates: Sky map 22h 03m 10.8s, +18° 53′ 04″

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