HD 209458 b

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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) 22h 03m 10.8s
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
(ω) 83°
Time of periastron (τ) 2,452,854.825415
± 0.00000025 JD
Physical characteristics
Mass (m) 0.69 ± 0.05 MJ
Radius (r) 1.32 ± 0.05 RJ
Density (ρ) 370 kg/ m3
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.


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.
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 1-5×108 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. 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.

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