   #copyright

HD 209458 b

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   CAPTION: HD 209458 b

     Extrasolar planet          Lists of extrasolar planets

           Artist's conception of HD 209458 b transiting its star.

                            Parent star
   Star                   HD 209458
   Constellation          Pegasus
   Right ascension    (α) 22^h 03^m 10.8^s
   Declination        (δ) +18° 53′ 04″
   Spectral type          F8-G0V
                          Orbital elements
   Semimajor axis     (a) 0.045 AU
   Eccentricity       (e) 0.00
   Orbital period     (P) 3.52474541 ± 0.00000025 d
   Inclination        (i) 86.1 ± 0.1°
   Longitude of
   periastron         (ω) 83°
   Time of periastron (τ) 2,452,854.825415
                          ± 0.00000025 JD
                      Physical characteristics
   Mass               (m) 0.69 ± 0.05 M[J]
   Radius             (r) 1.32 ± 0.05 R[J]
   Density            (ρ) 370 kg/ m^3
   Temperature        (T) 1,130 ± 150 K
                       Discovery information
   Discovery date         1999
   Discoverer(s)          D. Charbonneau, T. Brown,
                          D. Latham, M. Mayor,
                          G.W. Henry, G. Marcy,
                          R.P. Butler, S.S. Vogt
   Detection method       Transit and Doppler Spectroscopy
   Discovery status       Confirmed
                         Other designations
   Osiris (unofficial nickname)

   HD 209458 b 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 from Earth
   with binoculars.

   The radius of the planet's orbit is 7 million kilometers, ~0.047
   astronomical units, 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 1000 K. Its mass is 220 times
   that of Earth's (0.7 Jupiter masses), which indicates that it is
   probably a gas giant.

   HD 209458 b was the first transiting extrasolar 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.

   HD 209458 b is informally known as Osiris.

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.

   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. 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 its orbital period very accurately at
   3.524736 days.

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

   The transit of HD 209458 b.
   Enlarge
   The transit of HD 209458 b.

   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 1-5×10^8 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. HD 209458b
   will not evaporate entirely, although it may have lost up to about 7%
   of its mass over its estimated lifetime of 5 billion years.

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 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 750 °C (1300 °F). 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
   is tidally locked, showing only one side to its star. This would result
   in uneven heating and therefore the possibility of very strong winds,
   which may be detectable.
   Retrieved from " http://en.wikipedia.org/wiki/HD_209458_b"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
