HD 189733 b

Extrasolar planet		List of extrasolar planets
Size comparison of HD 189733 b with Jupiter.
Parent star
Star		HD 189733 A
Constellation		Vulpecula
Right ascension	(α)	20h 00m 43.7133s
Declination	(δ)	+22° 42′ 39.070″
Apparent magnitude	(mV)	7.66
Distance		62.9 ly (19.3 pc)
Spectral type		K1-K2V
Mass	(m)	0.8 ± 0.4 M☉
Radius	(r)	0.781 ± 0.051 R☉
Temperature	(T)	4939 ± 158 K
Metallicity	[Fe/H]	-0.03 ± 0.04
Age		>0.6 Gyr
Orbital elements
Semimajor axis	(a)	0.03099 ± 0.0006 AU (4.636 ± 0.09 Gm)
Periastron	(q)	0.03096 AU (4.632 Gm)
Apastron	(Q)	0.03102 AU (4.641 Gm)
Eccentricity	(e)	0.0010 ± 0.0002
Orbital period	(P)	2.2185733 ± 0.00002 d
		(53.245759 h)
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
Physical characteristics
Mass	(m)	1.13 ± 0.03 MJ
Radius	(r)	1.138 ± 0.027 RJ
Surface gravity	(g)	21.2 m/s²
Temperature	(T)	1117 ± 42 K
Discovery information
Discovery date		5 October 2005
Discoverer(s)		Bouchy et al.
Detection method		Doppler spectroscopy Transit
Discovery site		Haute-Provence Observatory
Discovery status		Confirmed
Database references
Extrasolar Planets Encyclopaedia		data
SIMBAD		data

HD 189733 b is an extrasolar planet approximately 63 light-years away in the constellation of Vulpecula (the Fox). The planet was discovered orbiting the star HD 189733 on October 5, 2005 (more precisely HD 189733 A since a star companion has been confirmed by then), when astronomers in France observed the planet transiting across the face of the star.^[1] 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.

Detection and discovery[link]

Transit and Doppler spectroscopy[link]

File:HD 189733 b rv.pdf

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 HD 189733 star system is 63 light years from Earth in the direction of the constellation Vulpecula.

Infrared spectrum[link]

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.^[2] 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.

The infrared spectrum of HD 189733 b.

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.^[3][4] This work will need to be confirmed, however, as the estimated radius is much larger than expected from measurements at other wavelengths.

Measured global temperature map of HD 189733 b.

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.^[5] The Rayleigh effect is confirmed in other models,^[6] and by the apparent lack of a cooler, shaded stratosphere below its outer atmosphere.

Evaporation[link]

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.^[7]

Satellites[link]

The transits reveal no moons of 0.8 Earth radius or larger, and no ring system comparable to Saturn's.^[8]

Physical characteristics[link]

An artist's impression of the planet that is in agreement with the global temperature map.^{[dubious – discuss]}

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.^[2] 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.^[9] Followup measurements, tested against a stratospheric model, yielded inconclusive results.^[10]

The condensates form a haze 1000 km above the surface as viewed in the infrared. A sunset viewed from that surface would be red.^[11]

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.^[12] This is the first extra solar planet discovered to have carbon dioxide in the atmosphere.^[13]

Map of the planet[link]

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.^[14]

An artist's impression of HD 189733 b

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.^[15] 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.^[16]

Water vapor and organic compounds[link]

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.^[17] Follow-up observations made using the Hubble Space Telescope confirm the presence of water vapor and also the organic compound methane.^[6][18] 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.^[18][19]

Evolution[link]

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.^[20]

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.^[21] 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.

See also[link]

• HD 209458 b
• 51 Pegasi b

References[link]

External links[link]

Media related to HD 189733 b at Wikimedia Commons

• Berdyugina et al.; Berdyugin; Fluri; Piirola (2008-02-02). "First Detection of Polarized Scattered Light from an Exoplanetary Atmosphere". The Astrophysical Journal 673: L83–L86. arXiv:0712.0193. Bibcode 2008ApJ...673L..83B. DOI:10.1086/527320. 
• Naeye, Robert (2005-10-07). "The Best Transiting Exoplanet Yet". Sky & Telescope. http://skyandtelescope.com/news/article_1606_1.asp. Retrieved 2008-06-21. 
• Deming et al.; Harrington, Joseph; Seager, Sara; Jeremy Richardson (2006-02-20). "Strong Infrared Emission from the Extrasolar Planet HD189733b". The Astrophysical Journal 644: 560–564. arXiv:astro-ph/0602443. Bibcode 2006ApJ...644..560D. DOI:10.1086/503358. 

• HD 189733 b on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images

Coordinates: 20^h 00^m 43.7133^s, +22° 42′ 39.07″