|Extrasolar planet||List of extrasolar planets|
|Hubble Space Telescope.|
|Star||HD 189733 A|
|Right ascension||(α)||20h 00m 43.71s|
|Declination||(δ)||+22° 42′ 39.1″|
|Distance||63.4 ± 0.9 ly|
(19.5 ± 0.3 pc)
|Mass||(m)||0.846+0.068/-0.049  M☉|
|Radius||(r)||0.781 ± 0.051 R☉|
|Temperature||(T)||4939 ± 158 K|
|Metallicity||[Fe/H]||-0.03 ± 0.04|
|Semimajor axis||(a)||0.03099 ± 0.0006 AU|
(4.636 ± 0.09 Gm)
|Eccentricity||(e)||0.0010 ± 0.0002|
|Orbital period||(P)||2.2185733 ± 0.00002 d|
|Orbital speed||(υ)||152.5 km/s|
|Inclination||(i)||85.76 ± 0.29°|
|Time of transit||(Tt)||2,453,988.80336 ± 0.00024 JD|
|Semi-amplitude||(K)||205 ± 6 m/s|
|Mass||(m)||1.162+0.058/-0.039  MJ|
|Radius||(r)||1.138 ± 0.027 RJ|
|Stellar flux||(F⊙)||275 ⊕|
|Geometric Albedo||(Ag)||0.40 ± 0.12 (290-450 nm)|
< 0.12 (450-570 nm)
|Surface gravity||(g)||21.2 m/s²|
|Temperature||(T)||1117 ± 42 K|
|Discovery date||5 October 2005|
|Discoverer(s)||Bouchy et al.|
|Discovery method||Doppler spectroscopy|
|Other detection methods||Polarimetry|
Orbital light variations
|Discovery site||Haute-Provence Observatory|
HD 189733 b, more formally HD 189733 Ab, is an extrasolar planet approximately 63 light-years away from the Solar System in the constellation of Vulpecula, the Fox. The planet was discovered orbiting the star HD 189733 A on October 5, 2005, when astronomers in France observed the planet transiting across the face of the star. HD 189733 b was the first extrasolar planet for which a thermal map was constructed, to be detected through polarimetry, to have its overall colour determined (deep blue), to have a transit detected in x-ray spectrum and to have carbon dioxide in its atmosphere. With a mass 13% higher than that of Jupiter, HD 189733 b orbits its host star once every 2.2 days at an orbital speed of 152.5 kilometres per second (341,000 mph), making it a hot Jupiter with poor prospects for life as we know it.
Detection and discovery
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 spectroscopy. 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 secondary star in the same system.
On February 21, 2007, NASA 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.
In 2008, a team of astrophysicists managed to detect and monitor the planet's visible light using polarimetry, the first such success. This result was further improved by the same team in 2011. They found that the planet albedo is significantly larger in blue light than in the red, most probably due to Rayleigh scattering and molecular absorption in the red. The blue colour of the planet was subsequently confirmed in 2013, making HD 189733 the first planet to have its overall colour determined by two different techniques.
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.
In July 2013, NASA reported the first observations of planet transit studied in X-ray spectrum. It was found that the planet blocks three times as much x-rays than visible light.
File:Hubble, Swift Detect First-ever Changes in an Exoplanet Atmosphere.ogv
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 exosphere of atomic hydrogen. HD 189733 b is the second planet after HD 209458 b for which atmospheric evaporation has been detected.
This planet exhibits one of the largest photometric transit depth (amount of the parent star's light blocked) of extrasolar planets so far observed, 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 atmosphere was at first predicted "pL class", lacking a temperature-inversion stratosphere; like L dwarfs which lack titanium and vanadium oxides. Follow-up measurements, tested against a stratospheric model, yielded inconclusive results. Atmospheric 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. First obscured by the haze of condensates, sodium was eventually observed at three times the concentration of HD 209458 b's sodium layer. HD 189733 is also the first extrasolar planet confirmed to have carbon dioxide in its atmosphere.
Map of the planet
In 2007, the Spitzer space telescope 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 2,700 metres per second (9,700 km/h) 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.
Water vapor, oxygen 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 Telescope confirm the presence of water vapor, neutral oxygen and also the organic compound methane. Later, Very Large Telescope observations also detected the presence of carbon monoxide on the day side of the planet. It is currently unknown how the methane originated as the planet's high 700 °C temperature should cause the water and methane to react, replacing the atmosphere with carbon monoxide.
While transiting the system also clearly exhibits the Rossiter–McLaughlin effect, 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.
- HD 189733 b on Articles and images
es:HD 189733#HD 189733b