HD 209458 b
Encyclopedia
HD 209458 b is an extrasolar planet
(unofficially referred to as Osiris) that orbits the Solar analog star HD 209458
in the constellation Pegasus
, some 150 light-year
s from Earth's solar system, with evidence of water vapor.
The radius of the planet's orbit is 7 million kilometres, about 0.047 astronomical unit
s, 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 2.5 times greater than that of Jupiter. The high mass and great volume of HD 209458 b indicate that it is 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
, one of the first two extrasolar planets to be directly observed spectroscopically
and the first extrasolar gas giants to have its superstorm measured, and the first planet to have its orbital speed measured, determining its mass directly. 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.
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 partial transit, seeing only the ingress. 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
. 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.
. 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.
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.
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
. 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 micrometre
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, 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.
On 23 June 2010, astronomers announced they have measured a superstorm (with windspeeds of up to 7000 km/h) for the first time in the atmosphere of HD 209458 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.
and so spin-orbit angle was that of Winn in 2005. This is -4.4 +/- 1.4 degrees.
s 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.
The atmosphere is at a pressure 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 2200+/-260 K.
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%.) 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. 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
of cooler gas. 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 tholin
s cannot be ruled out as of yet. 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.
detected sodium
, the first planetary atmosphere outside our solar system to be measured. This detection was predicted by Sara Seager
in late 2001. 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
.
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 kilometres 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. It may be possible that the planet's magnetic field may prevent this loss, as the exosphere would become ionized by the star, and the magnetic field would contain the ions from loss.
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.
. Indeed, the Einsteinian correction to the third Kepler
law
would be, in principle, measurable.
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
(unofficially referred to as Osiris) that orbits the Solar analog star HD 209458
HD 209458
HD 209458 is an 8th magnitude star in the constellation Pegasus. It is very similar to our Sun, and it is classified as a yellow dwarf . Because it is located at a distance of about 150 light years, it is not visible to the unaided eye...
in the constellation Pegasus
Pegasus (constellation)
Pegasus is a constellation in the northern sky, named after the winged horse Pegasus in Greek mythology. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations.-Stars:...
, some 150 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s from Earth's solar system, with evidence of water vapor.
The radius of the planet's orbit is 7 million kilometres, about 0.047 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
s, or one eighth the radius of Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
'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
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
masses) and its volume is some 2.5 times greater than that of Jupiter. The high mass and great volume of HD 209458 b indicate that it is a gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
.
HD 209458 b represents a number of milestones in extraplanetary research. It was the first of many categories: a transiting
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...
extrasolar planet discovered, an extrasolar planet known to have an atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
, an extrasolar planet observed to have an evaporating hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
atmosphere, an extrasolar planet found to have an atmosphere containing oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, one of the first two extrasolar planets to be directly observed spectroscopically
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
and the first extrasolar gas giants to have its superstorm measured, and the first planet to have its orbital speed measured, determining its mass directly. 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
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
in its atmosphere.
Transits
SpectroscopicAstronomical spectroscopy
Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...
studies first revealed the presence of a planet around HD 209458 on November 5, 1999. Astronomers had made careful photometric
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
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
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...
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 Charbonneau
David Charbonneau is a Professor of Astronomy at Harvard University. His research focuses on the development of novel techniques for the detection and characterization of planets orbiting nearby, Sun-like stars...
including Timothy Brown and others, and the other by Gregory W. Henry
Gregory W. Henry
Gregory W. Henry is an astronomer and research scientist for Tennessee State University. In 1999, Henry led one of two teams that discovered the first transiting extrasolar planet, HD 209458 b. The other team was led by David Charbonneau....
, were able to detect a transit
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...
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. 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
Astrophysical Journal
The Astrophysical Journal is a peer-reviewed scientific journal covering astronomy and astrophysics. It was founded in 1895 by the American astronomers George Ellery Hale and James Edward Keeler. It publishes three 500-page issues per month....
. 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
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...
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 JupiterJupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
. The occurrence of transits allowed astronomers to calculate the planet's radius, which had not been possible for any previously known exoplanet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
, and it turned out to have a radius some 35% larger than Jupiter's.
Direct detection
On March 22, 2005, NASANASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
released news that infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
light from the planet had been measured by the Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...
, 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.
Spectral observation
On February 21, 2007, NASA and NatureNature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...
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
HD 189733 b is an extrasolar planet approximately 63 light-years away in the constellation of Vulpecula . The planet was discovered orbiting the star HD 189733 on October 5, 2005, when astronomers in France observed the planet transiting across the face of the star. The planet is classified as a...
. 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
Goddard Space Flight Center
The Goddard Space Flight Center is a major NASA space research laboratory established on May 1, 1959 as NASA's first space flight center. GSFC employs approximately 10,000 civil servants and contractors, and is located approximately northeast of Washington, D.C. in Greenbelt, Maryland, USA. GSFC,...
spectrally measured HD 209458 b's atmosphere in the range of 7.5 to 13.2 micrometre
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
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, which the investigators did not have an explanation for. A separate team led by Mark Swain of the Jet Propulsion Laboratory
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...
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 7000 km/h) for the first time in the atmosphere of HD 209458 b. The very high-precision observations done by ESO
ESO
ESO, as a three-letter abbreviation, may stand for:* European Southern Observatory* Ensemble Studios Online* English Symphony Orchestra* Edmonton Symphony Orchestra* Executive Stock Options...
’s Very Large Telescope
Very Large Telescope
The Very Large Telescope is a telescope operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The VLT consists of four individual telescopes, each with a primary mirror 8.2m across, which are generally used separately but can be used together to...
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.
Rotation
As of August 2008, the most recent calculation of HD 209458 b's Rossiter-McLaughlin effectRossiter-McLaughlin effect
The Rossiter–McLaughlin effect is a spectroscopic phenomenon observed when either an eclipsing binary's secondary star or an extrasolar planet is seen to transit across the face of the primary or parent star. As the main star rotates on its axis, one quadrant of its photosphere will be seen to be...
and so spin-orbit angle was that of Winn in 2005. This is -4.4 +/- 1.4 degrees.
Physical characteristics
It had been previously hypothesized that hot JupiterHot Jupiter
Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...
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
Tidal heating occurs through the tidal friction processes: orbital and rotational energy are dissipated as heat in the crust of the moons and planets involved. Io, a moon of Jupiter, is the most volcanically active body in the solar system, with no impact craters surviving on its surface...
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.
Stratosphere and upper clouds
Where the pressure is 33+/-5 millibars, the atmosphere is clear (probably hydrogen) and its Rayleigh effect
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
is detectable. At that pressure the temperature is 2200+/-260 K.
Observations by the orbiting Microvariability and Oscillations of STars telescope
Microvariability and Oscillations of STars telescope
The Microvariability and Oscillations of STars telescope, better known simply as MOST, is Canada's first and only space telescope. It is also the smallest space telescope in the world...
initially limited the planet's albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
(or reflectivity) below 30%, making it a surprisingly dark object. (The geometric albedo has since been measured to 3.8 ± 4.5%.) 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. 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
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
of cooler gas. 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 tholin
Tholin
Tholin [after the ancient Greek word meaning "not clear"] is a heteropolymer molecule formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane. Tholins do not form naturally on modern-day Earth, but are found in great abundance on the surface of icy bodies in...
s cannot be ruled out as of yet. 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
Surrounding that level, on November 27, 2001 the Hubble Space TelescopeHubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
detected sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
, the first planetary atmosphere outside our solar system to be measured. This detection was predicted by Sara Seager
Sara Seager
Sara Seager is a Canadian-American astronomer who is currently a professor at the Massachusetts Institute of Technology and known for her work on extrasolar planets. She was born in Toronto, Canada. In 1994, she earned the degree of Bachelor of Science in Mathematics and Physics from the...
in late 2001. 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
HD 189733 b is an extrasolar planet approximately 63 light-years away in the constellation of Vulpecula . The planet was discovered orbiting the star HD 189733 on October 5, 2005, when astronomers in France observed the planet transiting across the face of the star. The planet is classified as a...
.
In 2003-4, astronomers used the Hubble Space Telescope Imaging Spectrograph to discover an enormous ellipsoidal envelope of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
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
Escape velocity
In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...
, 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
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
" created by its evaporating hydrogen atmosphere. The hydrogen tail streaming from the planet is approximately 200,000 kilometres 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. It may be possible that the planet's magnetic field may prevent this loss, as the exosphere would become ionized by the star, and the magnetic field would contain the ions from loss.
Presumed atmospheric water vapor
Travis Barman
Travis Barman, is an astronomer at the Lowell Observatory in Arizona. While studying a combination of the previously published Hubble Space Telescope measurements and new theoretical models, Barman found strong evidence for water absorption in the atmosphere of HD 209458 bA planet about 150 light...
of the Lowell Observatory
Lowell Observatory
Lowell Observatory is an astronomical observatory in Flagstaff, Arizona. Lowell Observatory was established in 1894, placing it among the oldest observatories in the United States, and was designated a National Historic Landmark in 1965....
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
Harvard University
Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...
, from the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
, 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
David Charbonneau
David Charbonneau is a Professor of Astronomy at Harvard University. His research focuses on the development of novel techniques for the detection and characterization of planets orbiting nearby, Sun-like stars...
, 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
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
, carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, and methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
in the atmosphere.
Possible tests of fundamental physics
Given the exquisite accuracy with which its orbital period was measured, it was proposed to use HD 209458b to test general relativityGeneral relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
. Indeed, the Einsteinian correction to the third Kepler
Johannes Kepler
Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...
law
Kepler's laws of planetary motion
In astronomy, Kepler's laws give a description of the motion of planets around the Sun.Kepler's laws are:#The orbit of every planet is an ellipse with the Sun at one of the two foci....
would be, in principle, measurable.