HD 209458 b

The designation HD 209458 b indicates that this is the first planet to be discovered around the star HD 209458, as per exoplanet naming convention. The host star's designation comes from the Henry Draper Catalogue. The planet has also been called "Osiris" after the Egyptian god. This name was first proposed in 2003 by A. Vidal-Madjar and A. Lecavelier des Etangs, who compared the planet's evaporating atmosphere to Osiris having lost part of his body in the myth of his death and resurrection. The name has subsequently seen some use by other astronomers, and has been acknowledged by the IAU, but as of 2025 it has not yet been approved as an official proper name. == Detection and discovery ==

Transits of planet HD 209458 b transiting the star, adapted from Brown et al. (2001) Spectroscopic Spectroscopic analysis had shown that the planet had a mass about 0.69 times that of Jupiter. 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 . The nearly circular orbit of HD 209458 b was also confirmed. 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 observed, the other one being HD 189733 b. The very high-precision observations done by ESO's Very Large Telescope and its powerful CRIRES spectrograph of carbon monoxide gas show that it is streaming at enormous speed from the extremely hot day side to the cooler night side of the planet. The observations also allow another exciting "first"—measuring the orbital speed of the exoplanet itself, providing a direct determination of its mass. or disequilibrium atmosphere chemistry. == Rotation ==

In August 2008, the measurement of HD 209458 b's Rossiter–McLaughlin effect and hence spin–orbit angle is −4.4 ± 1.4°. The study in 2012, updated the spin-orbit angle to −5°. == Physical characteristics ==

, ESA, and G. Bacon (STScI) 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.) In comparison, Jupiter has a much higher albedo of 0.52. 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. 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. Exosphere On November 27, 2001, astronomers announced that they had detected sodium in the atmosphere of the planet, using observations with the Hubble Space Telescope. This was the first planetary atmosphere outside the Solar System to be measured. The core of the sodium line runs from pressures of 50 millibar to a microbar. This turns out to be about a third the amount of sodium at HD 189733 b. The additional data did not confirm the presence of sodium in the atmosphere of HD 209458 b as in 2020. 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 . The hydrogen exosphere extends to a distance RH=3.1 RJ, much larger than the planetary radius of 1.32 RJ. At this temperature and distance, the Maxwell–Boltzmann distribution of particle velocities gives rise to a significant "tail" of atoms moving at speeds greater than the escape velocity. The planet is estimated to be losing about 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 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 . 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. Atmosphere composition 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. 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. As of April 2007, further investigation is being conducted. On October 20, 2009, researchers at JPL announced the discovery of water vapor, carbon dioxide, and methane in the atmosphere. The refined spectra obtained in 2021 has detected instead water vapor, carbon monoxide, hydrogen cyanide, methane, ammonia and acetylene, all consistent with the extremely high carbon to oxygen molar ratio of 1.0 (while Sun has C/O molar ratio of 0.55). If true, the HD 209458 b may be a prime example of the carbon planet. == Magnetic field ==

In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's. Since HD 209458 b orbits less than 0.1 AU from its host star, theorists hypothesized that it may cause stellar flaring synchronized to the orbital period of the exoplanet. A 2011 search for these magnetic star-planet interactions that would result in coronal radio emissions did not detect any signal. Similarly, no magnetospheric radio emissions were detected from the planet either. , WASP-6b, WASP-31b, WASP-39b, HD 189733b, HAT-P-12b, WASP-17b, WASP-19b, HAT-P-1b and HD 209458b. == See also ==