HD 189733 b
Encyclopedia
HD 189733 b is an extrasolar planet
approximately 63 light-year
s away in the constellation
of Vulpecula
(the Fox
). 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 hot Jupiter
class Jovian planet, with a close orbit to its parent star. HD 189733 b was the first extrasolar planet to be mapped and the first to be discovered with carbon dioxide in its atmosphere.
. Real-time radial velocity
measurements detected the Rossiter-McLaughlin effect
caused by the planet passing in front of its star before photometric measurements confirmed that the planet was transiting. In 2006, a team led by Drake Deming announced a detection of strong infrared
thermal emission from the transiting extrasolar planet HD 189733 b, by measuring the flux decrement (decrease of total light) during its prominent secondary eclipse
(when the planet passes behind the star).
The mass of the planet is estimated to be 13% larger than Jupiter's
; with the planet completing an orbit around its host star every 2.2 days and an orbital speed
of 152.5 km/s. It is occasionally referred to as HD 189733 Ab to distinguish it from the red dwarf star HD 189733 B. The HD 189733 star system is 63 light years from Earth in the direction of the constellation Vulpecula
.
released news that the Spitzer Space Telescope
had measured detailed spectra from both HD 189733 b and HD 209458 b
. The release came simultaneously with the public release of a new issue of Nature
containing the first publication on the spectroscopic observation of the other exoplanet, HD 209458 b. A paper was submitted and published by the Astrophysical Journal Letters
. The spectroscopic observations of HD 189733 b were led by Carl Grillmair of NASA's Spitzer Science Center.
On October 22, a team of astrophysicists at the ETH Zürich
managed to "detect and monitor [its] visible light" using polarimetry
, the first such success. The authors claim a radius of 1.5+/-.2 Rj: over 30% larger than its transit disc. Its albedo
in blue light is greater than 0.14. The planet would appear deep blue to our eyes. This work will need to be confirmed, however, as the estimated radius is much larger than expected from measurements at other wavelengths.
The blueness of the planet may be the result of Rayleigh scattering
. In mid January 2008, spectral observation during the planet's transit using that model found that if molecular hydrogen
exists, it would have an atmospheric pressure
of 410 ± 30 mbar of 0.1564 solar radii. The Mie approximation model also found that there is a possible condensate in its atmosphere, magnesium silicate (MgSiO3) with a particle size
of approximately 10−2 to 10−1 μm. Using both models, the planet's temperature would be between 1340 to 1540 K. The Rayleigh effect is confirmed in other models, and by the apparent lack of a cooler, shaded stratosphere
below its outer atmosphere.
of atomic hydrogen. HD 189733 b is the second planet after HD 209458 b for which atmospheric evaporation has been detected.
This planet exhibits the largest photometric transit depth (amount of the parent star's light blocked) of any extrasolar planet
so far observed, of approximately 3%. The apparent longitude of ascending node of its orbit is 16 degrees +/- 8 away from north-south in our sky. It and HD 209458 b
were the first two planets to be directly spectroscopically
observed. The parent stars of these two planets are the brightest transiting-planet host stars, so these planets will continue to receive the most attention by astronomers. Like most hot Jupiters, this planet is thought to be tidally locked to its parent star, meaning it has a permanent day and night.
The planet is not oblate.
The atmosphere was at first predicted "pL class", lacking a temperature-inversion stratosphere
; like L dwarfs which lack titanium and vanadium oxides. Followup measurements, tested against a stratospheric model, yielded inconclusive results.
The condensates form a haze 1000 km above the surface as viewed in the infrared. A sunset viewed from that surface would be red.
Sodium and potassium signals were predicted by Tinetti 2007. These signals were at first obscured by the condensate haze. Sodium was then found at three times the level of HD 209458 b
. This is the first extra solar planet discovered to have carbon dioxide in the atmosphere.
was used to map the planet's temperature emissions. The planet+star system was observed for 33 consecutive hours, starting when only the night side of the planet was in view. Over the course of one-half of the planet's orbit, more and more of the day side came into view. A temperature range of 973 ± 33 K to 1,212 ± 11 K was discovered, indicating that the absorbed energy from the parent star is distributed fairly evenly through the planet's atmosphere. Interestingly, the region of peak temperature was offset 30 degrees east of the substellar point, as predicted by theoretical models of Hot Jupiters taking into account a parameterized day to night redistribution mechanism.
Assuming the planet is tidally locked with its star, this suggests that powerful easterly winds moving at more than 9,600 kilometers per hour are responsible for redistributing the heat. NASA released a brightness map of the surface temperature of HD 189733 b; it is the first map ever published of an extra-solar planet.
confirm the presence of water vapor and also the organic compound methane
. It is currently unknown how the methane originated as the planet's high temperature (700°C, 1292°F) should cause the water and methane to react, replacing the atmosphere with carbon monoxide
.
, the shifting in photospheric
spectral lines caused by the planet occulting a part of the rotating stellar surface. Due to its high mass and close orbit the parent star has a very large semi-amplitude (K)
, the "wobble" in the star's radial velocity
, of 205 m/s.
The Rossiter-McLaughlin effect
allows the measurement of the angle between the planet's orbital plane and the equatorial plane of the star. These are well aligned. By analogy with HD 149026 b
, the formation of the planet was peaceful and probably involved interactions with the protoplanetary disc. A much larger angle would have suggested a violent interplay with other protoplanets.
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...
approximately 63 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 away in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
of Vulpecula
Vulpecula
Vulpecula is a faint constellation in the northern sky. Its name is Latin for "little fox", although it is commonly known simply as the fox. It was identified in the seventeenth century, and is located in the middle of the Summer Triangle .-Stars:There are no stars brighter than 4th magnitude in...
(the Fox
Fox
Fox is a common name for many species of omnivorous mammals belonging to the Canidae family. Foxes are small to medium-sized canids , characterized by possessing a long narrow snout, and a bushy tail .Members of about 37 species are referred to as foxes, of which only 12 species actually belong to...
). The planet was discovered orbiting the star HD 189733
HD 189733
HD 189733, also catalogued as V452 Vulpeculae, is a binary star system approximately 63 light-years away in the constellation of Vulpecula . The primary star is suspected to be an orange dwarf star, while the secondary star is a red dwarf star...
on October 5, 2005, when astronomers in France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
observed the planet 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...
across the face of the star. The planet is classified as a hot Jupiter
Hot Jupiter
Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...
class Jovian planet, with a close orbit to its parent star. HD 189733 b was the first extrasolar planet to be mapped and the first to be discovered with carbon dioxide in its atmosphere.
Transit and Doppler spectroscopy
On October 6, 2005, a team of astronomers announced the discovery of transiting planet HD 189733 b. The planet was then detected using Doppler spectroscopyDoppler spectroscopy
Doppler spectroscopy, also known as radial velocity measurement, is a spectroscopic method for finding extrasolar planets. It involves the observation of Doppler shifts in the spectrum of the star around which the planet orbits....
. Real-time radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
measurements detected the Rossiter-McLaughlin effect
Rossiter-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...
caused by the planet passing in front of its star before photometric measurements confirmed that the planet was transiting. In 2006, a team led by Drake Deming announced a detection of strong 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...
thermal emission from the transiting extrasolar planet HD 189733 b, by measuring the flux decrement (decrease of total light) during its prominent secondary eclipse
Eclipse
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer...
(when the planet passes behind the star).
The mass of the planet is estimated to be 13% larger than Jupiter's
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,...
; with the planet completing an orbit around its host star every 2.2 days and an orbital speed
Orbital speed
The orbital speed of a body, generally a planet, a natural satellite, an artificial satellite, or a multiple star, is the speed at which it orbits around the barycenter of a system, usually around a more massive body...
of 152.5 km/s. It is occasionally referred to as HD 189733 Ab to distinguish it from the red dwarf star HD 189733 B. The HD 189733 star system is 63 light years from Earth in the direction of the constellation Vulpecula
Vulpecula
Vulpecula is a faint constellation in the northern sky. Its name is Latin for "little fox", although it is commonly known simply as the fox. It was identified in the seventeenth century, and is located in the middle of the Summer Triangle .-Stars:There are no stars brighter than 4th magnitude in...
.
Infrared spectrum
On February 21, 2007, 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 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...
had measured detailed spectra from both HD 189733 b and HD 209458 b
HD 209458 b
HD 209458 b is an extrasolar planet that orbits the Solar analog star HD 209458 in the constellation Pegasus, some 150 light-years from Earth's solar system, with evidence of water vapor....
. The release came simultaneously with the public release of a new issue of Nature
Nature (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...
containing the first publication on the spectroscopic observation of the other exoplanet, HD 209458 b. A paper was submitted and published by the Astrophysical Journal Letters
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....
. The spectroscopic observations of HD 189733 b were led by Carl Grillmair of NASA's Spitzer Science Center.
ETH Zurich
The Swiss Federal Institute of Technology Zurich or ETH Zürich is an engineering, science, technology, mathematics and management university in the City of Zurich, Switzerland....
managed to "detect and monitor [its] visible light" using polarimetry
Polarimetry
Polarimetry is the measurement and interpretation of the polarization of transverse waves, most notably electromagnetic waves, such as radio or light waves...
, the first such success. The authors claim a radius of 1.5+/-.2 Rj: over 30% larger than its transit disc. Its 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...
in blue light is greater than 0.14. The planet would appear deep blue to our eyes. This work will need to be confirmed, however, as the estimated radius is much larger than expected from measurements at other wavelengths.
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...
. In mid January 2008, spectral observation during the planet's transit using that model found that if molecular 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...
exists, it would have an atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
of 410 ± 30 mbar of 0.1564 solar radii. The Mie approximation model also found that there is a possible condensate in its atmosphere, magnesium silicate (MgSiO3) with a particle size
Particle size (grain size)
Particle size, also called grain size, refers to the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which is the size of a single crystal inside the...
of approximately 10−2 to 10−1 μm. Using both models, the planet's temperature would be between 1340 to 1540 K. The Rayleigh effect is confirmed in other models, and by the apparent lack of a cooler, shaded 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...
below its outer atmosphere.
Evaporation
In March 2010, transit observations using HI Lyman-alpha found that this planet is evaporating at a rate of 1-100 gigagrams per second. This indication was found by detecting the extended exosphereExosphere
The exosphere is the uppermost layer of Earth's atmosphere. In the exosphere, an upward travelling molecule moving fast enough to attain escape velocity can escape to space with a low chance of collisions; if it is moving below escape velocity it will be prevented from escaping from the celestial...
of atomic hydrogen. HD 189733 b is the second planet after HD 209458 b for which atmospheric evaporation has been detected.
Satellites
The transits reveal no moons of 0.8 Earth radius or larger, and no ring system comparable to Saturn's.Physical characteristics
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...
so far observed, of approximately 3%. The apparent longitude of ascending node of its orbit is 16 degrees +/- 8 away from north-south in our sky. It and HD 209458 b
HD 209458 b
HD 209458 b is an extrasolar planet that orbits the Solar analog star HD 209458 in the constellation Pegasus, some 150 light-years from Earth's solar system, with evidence of water vapor....
were the first two planets to be directly 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...
observed. The parent stars of these two planets are the brightest transiting-planet host stars, so these planets will continue to receive the most attention by astronomers. Like most hot Jupiters, this planet is thought to be tidally locked to its parent star, meaning it has a permanent day and night.
The planet is not oblate.
The atmosphere was at first predicted "pL class", lacking a temperature-inversion 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...
; like L dwarfs which lack titanium and vanadium oxides. Followup measurements, tested against a stratospheric model, yielded inconclusive results.
The condensates form a haze 1000 km above the surface as viewed in the infrared. A sunset viewed from that surface would be red.
Sodium and potassium signals were predicted by Tinetti 2007. These signals were at first obscured by the condensate haze. Sodium was then found at three times the level of HD 209458 b
HD 209458 b
HD 209458 b is an extrasolar planet that orbits the Solar analog star HD 209458 in the constellation Pegasus, some 150 light-years from Earth's solar system, with evidence of water vapor....
. This is the first extra solar planet discovered to have carbon dioxide in the atmosphere.
Map of the planet
In 2007, the Spitzer space telescopeSpitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...
was used to map the planet's temperature emissions. The planet+star system was observed for 33 consecutive hours, starting when only the night side of the planet was in view. Over the course of one-half of the planet's orbit, more and more of the day side came into view. A temperature range of 973 ± 33 K to 1,212 ± 11 K was discovered, indicating that the absorbed energy from the parent star is distributed fairly evenly through the planet's atmosphere. Interestingly, the region of peak temperature was offset 30 degrees east of the substellar point, as predicted by theoretical models of Hot Jupiters taking into account a parameterized day to night redistribution mechanism.
Water vapor and organic compounds
On July 11, 2007, a team led by Giovanna Tinetti published the results of their observations using the Spitzer Space Telescope concluding there is solid evidence for significant amounts of water vapor in the planet's atmosphere. Follow-up observations made using 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...
confirm the presence of water vapor and also the organic compound 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...
. It is currently unknown how the methane originated as the planet's high temperature (700°C, 1292°F) should cause the water and methane to react, replacing the atmosphere with carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
.
Evolution
While transiting the system also clearly exhibits the 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...
, the shifting in photospheric
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
spectral lines caused by the planet occulting a part of the rotating stellar surface. Due to its high mass and close orbit the parent star has a very large semi-amplitude (K)
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
, the "wobble" in the star's radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
, of 205 m/s.
The Rossiter-McLaughlin effect
Rossiter-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...
allows the measurement of the angle between the planet's orbital plane and the equatorial plane of the star. These are well aligned. By analogy with HD 149026 b
HD 149026 b
HD 149026 b is an extrasolar planet approximately 257 light-years away in the constellation of Hercules. The planet was discovered after it transited its parent star, HD 149026...
, the formation of the planet was peaceful and probably involved interactions with the protoplanetary disc. A much larger angle would have suggested a violent interplay with other protoplanets.