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HD 209458 b, which is also nicknamed Osiris after the Egyptian god, is an
exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
that orbits the
solar analog Solar-type star, solar analogs (also analogues), and solar twins are stars that are particularly similar to the Sun. The stellar classification is a hierarchy with solar twin being most like the Sun followed by solar analog and then solar-type ...
HD 209458 in the constellation
Pegasus Pegasus ( grc-gre, Πήγασος, Pḗgasos; la, Pegasus, Pegasos) is one of the best known creatures in Greek mythology. He is a winged divine stallion usually depicted as pure white in color. He was sired by Poseidon, in his role as hor ...
, some from the
Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
. The radius of the planet's orbit is , 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 . Its mass is 220 times that of Earth (0.69
Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth th ...
masses) and its volume is some 2.5 times greater than that of Jupiter. The high mass and volume of HD 209458 b indicate that it is a
gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...
. HD 209458 b represents a number of milestones in extraplanetary research. It was the first of many categories: * a transiting extrasolar planet * The first planet detected through more than one method * an extrasolar planet known to have an
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
* an extrasolar planet observed to have an evaporating
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxi ...
atmosphere * an extrasolar planet found to have an atmosphere containing the elements
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well ...
and carbon * one of the first two extrasolar planets to be directly observed spectroscopically * The first extrasolar gas giant to have its superstorm measured * the first planet to have its orbital speed measured, determining its mass directly. Based on the application of newer
theoretical A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may be ...
models, as of April 2007, it is thought to be the first extrasolar planet found to have
water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous phase ...
in its atmosphere.Water Found in Extrasolar Planet's Atmosphere
Space.com Space.com is an online publication focused on space exploration, astronomy, skywatching and entertainment, with editorial teams based in the United States and United Kingdom. The website offers live coverage of space missions, astronomical discov ...
In July 2014, NASA announced finding very dry atmospheres on HD 209458 b and two other exoplanets ( HD 189733 b and WASP-12b) orbiting Sun-like stars.


Detection and discovery


Transits

Spectroscopic Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wav ...
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 Transit may refer to: Arts and entertainment Film * ''Transit'' (1979 film), a 1979 Israeli film * ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countries in the world * ''Transit'' (2006 film), a 2006 ...
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 David Brian Charbonneau is a professor of Astronomy at Harvard University. His research focuses on the development of novel techniques for the detection and characterization of exoplanets orbiting nearby, Sun-like stars. Early life and educati ...
including Timothy Brown and others, and the other by Gregory W. Henry, were able to detect a
transit Transit may refer to: Arts and entertainment Film * ''Transit'' (1979 film), a 1979 Israeli film * ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countries in the world * ''Transit'' (2006 film), a 2006 ...
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 partial transit, seeing only the ingress.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.

November 19, reports full transit observations on September 9 and 16.
Initially unsure of their results, the Henry group decided to rush their results to publication after overhearing rumors that Charbonneau had successfully seen an entire transit in September. Papers from both teams were published simultaneously in the same issue of the
Astrophysical Journal ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery Hale and Jame ...
. 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 ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial obj ...
satellite, which allowed astronomers to calculate the orbital period of HD 209458 b very accurately at 3.524736 days.


Spectroscopic

Spectroscopic analysis had shown that the planet had a mass about 0.69 times that of
Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth th ...
. The occurrence of transits allowed astronomers to calculate the planet's radius, which had not been possible for any previously known
exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
, and it turned out to have a radius some 35% larger than Jupiter's. It had been previously hypothesized that
hot Jupiter Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (). The close proximity to their stars and high surface-atmosphere temp ...
s particularly close to their parent star should exhibit this kind of inflation due to intense heating of their outer atmosphere.
Tidal heating Tidal heating (also known as tidal working or tidal flexing) occurs through the tidal friction processes: orbital and rotational energy is dissipated as heat in either (or both) the surface ocean or interior of a planet or satellite. When an objec ...
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.


Direct detection

On March 22, 2005, NASA released news that
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
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 Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are p ...
'' 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 HD 189733 b is an exoplanet approximately away from the Solar System in the constellation of Vulpecula. Astronomers in France discovered the planet orbiting the star HD 189733 on October 5, 2005, by observing its transit across the star's fac ...
. 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 The Goddard Space Flight Center (GSFC) is a major NASA space research laboratory located approximately northeast of Washington, D.C. in Greenbelt, Maryland, United States. Established on May 1, 1959 as NASA's first space flight center, GSFC emp ...
spectrally measured HD 209458 b's atmosphere in the range of 7.5 to 13.2
micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...
s. The results defied theoretical expectations in several ways. The spectrum had been predicted to have a peak at 10 micrometres, 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 micrometres, which the investigators attributed to clouds of silicate dust, a phenomenon not previously observed. Another unpredicted peak occurred at 7.78 micrometres, for which the investigators did not have an explanation. 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. On 23 June 2010, astronomers announced they have measured a superstorm (with windspeeds of up to ) for the first time in the atmosphere of HD 209458 b. The very high-precision observations done by
ESO The European Organisation for Astronomical Research in the Southern Hemisphere, commonly referred to as the European Southern Observatory (ESO), is an intergovernmental research organisation made up of 16 member states for ground-based ast ...
’s
Very Large Telescope The Very Large Telescope (VLT) is a telescope facility operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each with a primary mirror 8.2 m across ...
and its powerful CRIRES spectrograph of
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...
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. As of 2021, the spectra of planetary atmosphere taken by different instruments remains highly inconsistent, indicating either metal-poor atmosphere, temperatures below blackbody equilibrium or disequilibrium atmosphere chemistry.


Rotation

In August 2008, the measurement of HD 209458 b's
Rossiter–McLaughlin effect The Rossiter–McLaughlin effect is a spectroscopic phenomenon observed when an object moves across the face of a star. Description The Rossiter–McLaughlin effect is a spectroscopic phenomenon observed when either an eclipsing binary's second ...
and hence spin–orbit angle is −4.4 ± 1.4°. The study in 2012, updated the spin-orbit angle to -5°.


Physical characteristics


Stratosphere and upper clouds

The atmosphere is at a
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and ...
of one bar at an altitude of 1.29 Jupiter radii above the planet's center. 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 . Observations by the orbiting
Microvariability and Oscillations of STars telescope The Microvariability and Oscillations of Stars/Microvariabilité et Oscillations STellaire (MOST), was Canada's first space telescope. Up until nearly 10 years after its launch it was also the smallest space telescope in orbit (for which its cre ...
initially limited the planet's
albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
(or reflectivity) below 0.3, making it a surprisingly dark object. (The geometric albedo has since been measured to be 0.038 ± 0.045.) 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.Matthews J., (2005)

MOST Space Telescope Play "Hide & Seek' With an Exoplanet; Learns About Atmosphere and Weather of a Distant World'']
Models since then have shown that between the top of its atmosphere and the hot, high pressure gas surrounding the mantle, there exists a
stratosphere The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air h ...
of cooler gas. This implies an outer shell of dark, opaque, hot clouds; usually thought to consist of
vanadium Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an oxide layer ( pass ...
and
titanium oxide Titanium oxide may refer to: * Titanium dioxide (titanium(IV) oxide), TiO2 * Titanium(II) oxide (titanium monoxide), TiO, a non-stoichiometric oxide * Titanium(III) oxide (dititanium trioxide), Ti2O3 * Ti3O * Ti2O * δ-TiOx (x= 0.68–0.75) * Ti ...
s, but other compounds like
tholin Tholins (after the Greek (') "hazy" or "muddy"; from the ancient Greek word meaning "sepia ink") are a wide variety of organic compounds formed by solar ultraviolet or cosmic ray irradiation of simple carbon-containing compounds such as carb ...
s cannot be ruled out yet. A 2016 study indicates the high-altitude cloud cover is patchy with about 57 percent coverage. The Rayleigh-scattering heated hydrogen rests at the top of the
stratosphere The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air h ...
; 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 Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable is ...
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 HD 189733 b is an exoplanet approximately away from the Solar System in the constellation of Vulpecula. Astronomers in France discovered the planet orbiting the star HD 189733 on October 5, 2005, by observing its transit across the star's fac ...
. 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
ellipsoid An ellipsoid is a surface that may be obtained from a sphere by deforming it by means of directional scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface;  that is, a surface that may be defined as the z ...
al envelope of
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxi ...
, carbon and
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well ...
around the planet that reaches . The hydrogen
exosphere The exosphere ( grc, ἔξω "outside, external, beyond", grc, σφαῖρα "sphere") is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the densit ...
extends to a distance ''R''H=3.1 ''R''J, much larger than the planetary radius of 1.32 RJ. At this temperature and distance, the
Maxwell–Boltzmann distribution In physics (in particular in statistical mechanics), the Maxwell–Boltzmann distribution, or Maxwell(ian) distribution, is a particular probability distribution named after James Clerk Maxwell and Ludwig Boltzmann. It was first defined and used ...
of particle velocities gives rise to a significant "tail" of atoms moving at speeds greater than the
escape velocity In celestial mechanics, escape velocity or escape speed is the minimum speed needed for a free, non- propelled object to escape from the gravitational influence of a primary body, thus reaching an infinite distance from it. It is typically s ...
. 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 In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) and ...
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. It may be possible that the planet's
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
may prevent this loss, because the exosphere would become
ionized Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...
by the star, and the magnetic field would contain the
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conv ...
s from loss.


Atmosphere composition

On April 10, 2007, Travis Barman of the
Lowell Observatory Lowell Observatory is an astronomical observatory in Flagstaff, Arizona, United States. Lowell Observatory was established in 1894, placing it among the oldest observatories in the United States, and was designated a National Historic Landmark ...
announced evidence that the atmosphere of HD 209458 b contained
water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous phase ...
. Using a combination of previously published Hubble Space Telescope measurements and new
theoretical A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may be ...
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 Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of higher le ...
, from the
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...
, and applied new
theoretical A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may be ...
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 Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
spectrum, compared to wavelengths in the
visible spectrum The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' or simply light. A typical human eye will respond to wavele ...
. Barman took Knutson's Hubble data on HD 209458 b, applied to his
theoretical A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may be ...
model, and allegedly identified water absorption in the planet's atmosphere. On April 24, the astronomer
David Charbonneau David Brian Charbonneau is a professor of Astronomy at Harvard University. His research focuses on the development of novel techniques for the detection and characterization of exoplanets orbiting nearby, Sun-like stars. Early life and educati ...
, 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 (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous phase ...
, carbon dioxide, and
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on ...
in the atmosphere. The refined spectra obtained in 2021 has detected instead
water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous phase ...
,
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...
,
hydrogen cyanide Hydrogen cyanide, sometimes called prussic acid, is a chemical compound with the formula HCN and structure . It is a colorless, extremely poisonous, and flammable liquid that boils slightly above room temperature, at . HCN is produced on an in ...
,
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on ...
,
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...
and
acetylene Acetylene (systematic name: ethyne) is the chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pure f ...
, 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 A carbon planet is a theoretical type of planet that contains more carbon than oxygen. Carbon is the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. Marc Kuchner and Sara Seager coined the term "carbon ...
.


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.


See also

*
51 Pegasi b 51 Pegasi b, officially named Dimidium , and formerly unofficially dubbed Bellerophon , is an extrasolar planet approximately away in the constellation of Pegasus. It was the first exoplanet to be discovered orbiting a main-sequence star, the S ...


References


Further reading

* . * . * . * .


External links

*
Hubble Finds Three Surprisingly Dry Exoplanets
* * * * * *
AAVSO Variable Star Of The Season. Fall 2004: The Transiting Exoplanets HD 209458 and TrES-1
{{Atmospheres Exoplanets discovered in 1999 Giant planets Hot Jupiters Pegasus (constellation) Transiting exoplanets Exoplanets detected by radial velocity Exoplanets with proper names