HD 189733 b

HD 189733 b

Exoplanet		List of exoplanets
Size comparison of HD 189733 b with Jupiter.
Parent star
Star		HD 189733 A
Constellation		Vulpecula
Right ascension	(α)	20h 00m 43.71s[1]
Declination	(δ)	+22° 42′ 39.1″[1]
Apparent magnitude	(mV)	7.66
Distance		63.4 ± 0.9[1] ly (19.5 ± 0.3[1] pc)
Spectral type		K1–2V
Mass	(m)	6999846000000000000♠0.846+0.068 −0.049[2] M☉
Radius	(r)	6999805000000000000♠0.805±0.016[3] R☉
Temperature	(T)	7003487500000000000♠4875±43[3] K
Metallicity	[Fe/H]	−0.03 ± 0.04
Age		7000430000000000000♠4.3±2.8[3] Gyr
Orbital elements
Semi-major 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)	7000116199999999999♠1.162+0.058 −0.039[2] 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
Discovery information
Discovery date		5 October 2005
Discoverer(s)		Bouchy et al.
Discovery method		Doppler spectroscopy Transit
Other detection methods		Polarimetry Reflection/emission modulations
Discovery site		Haute-Provence Observatory
Discovery status		Confirmed
Database references
Extrasolar Planets Encyclopaedia		data
SIMBAD		data
Exoplanet Archive		data
Open Exoplanet Catalogue		data

HD 189733 b is an extrasolar planet approximately 63 light-years away from the Solar System in the constellation of Vulpecula. 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.^[4] 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 extraterrestrial life. Being the closest transiting hot Jupiter to Earth, HD 189733 b is a subject for extensive atmospheric examination. The atmosphere of HD 189733b has been extensively studied through high- and low-resolution instruments, both from ground and space.^[5] HD 189733 b was the first extrasolar planet for which a thermal map was constructed,^[6] to be detected through polarimetry, to have its overall color determined (deep blue),^[6][7] to have a transit detected in X-ray spectrum and to have carbon dioxide detected in its atmosphere.

In July, 2014, NASA announced finding very dry atmospheres on three exoplanets (HD 189733b, HD 209458b, WASP-12b) orbiting Sun-like stars.^[8]

Detection and discovery

Transit and Doppler spectroscopy

The infrared spectrum of HD 189733 b.

A global temperature map of HD 189733 b.

The planet's blue color was revealed using polarimetry.^[6]

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.

Infrared spectrum

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.^[9] 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.

Visible colour

In 2008, a team of astrophysicists managed to detect and monitor the planet's visible light using polarimetry, the first such success.^[10] This result was further improved by the same team in 2011.^[6] 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.^[6] The blue colour of the planet was subsequently confirmed in 2013,^[11] making HD 189733 the first planet to have its overall color 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 and 1540 K.^[12] The Rayleigh effect is confirmed in other models,^[13] and by the apparent lack of a cooler, shaded stratosphere below its outer atmosphere. In the visible region of the spectrum, thank to their high absorption cross sections, atomic sodium and potassium can be investigated. For example, using high-resolution UVES spectrograph on VLT, sodium has been detected on this atmosphere and further physical characteristics of the atmosphere such as temperature has been investigated.^[5]

X-ray spectrum

In July 2013, NASA reported the first observations of planet transit studied in X-ray spectrum. It was found that the planet's atmosphere blocks three times more X-rays than visible light.^[14]

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 exosphere of atomic hydrogen. HD 189733 b is the second planet after HD 209458 b for which atmospheric evaporation has been detected.^[15]

Physical characteristics

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.^[9] 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, and has neither satellites with greater than 0.8 the radius of Earth nor a ring system like that of Saturn.^[16]

The international team under the direction of Svetlana Berdyugina of Zurich University of Technology, using the Swedish 60-centimeter telescope KVA, which is located in Spain, was able to directly see the polarized light reflected from the planet. The polarization indicates that the scattering atmosphere is considerably larger (> 30%) than the opaque body of the planet seen during transits.^[17]

The atmosphere was at first predicted "pL class", lacking a temperature-inversion stratosphere; like L dwarfs which lack titanium and vanadium oxides.^[18] Follow-up measurements, tested against a stratospheric model, yielded inconclusive results.^[19] Atmospheric condensates form a haze 1000 km above the surface as viewed in the infrared. A sunset viewed from that surface would be red.^[20] 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.^[21] HD 189733 is also the first extrasolar planet confirmed to have carbon dioxide in its atmosphere.^[22]

Map of the planet

An artist's conception of HD 189733 b following the 2013 confirmation of the planet's blue color by the Hubble Space Telescope. The appearance of HD 189733 b beyond the blue color is unknown.

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.^[23]

An artist's impression of HD 189733 b showing rapid evaporation of the atmosphere

Scientists at the University of Warwick determined that it has winds of up to 8,700 km/h (5,400 mph) blowing from the day side to the night side.^[24] 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.^[25]

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.^[26] Follow-up observations made using the Hubble Space Telescope confirm the presence of water vapor, neutral oxygen and also the organic compound methane.^[13][27][28] Later, Very Large Telescope observations also detected the presence of carbon monoxide on the day side of the planet.^[2] 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.^[27][29]

Evolution

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.^[30]

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.^[31] 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

References

External links

• Media related to HD 189733 b at Wikimedia Commons
• Berdyugina; Berdyugin; Fluri; Piirola (2008-02-02). "First Detection of Polarized Scattered Light from an Exoplanetary Atmosphere". The Astrophysical Journal. 673: L83–L86. Bibcode:2008ApJ...673L..83B. arXiv:0712.0193 . doi:10.1086/527320. 
• Naeye, Robert (2005-10-07). "The Best Transiting Exoplanet Yet". Sky & Telescope. Retrieved 2008-06-21. 
• Deming; Harrington, Joseph; Seager, Sara; Jeremy Richardson (2006-02-20). "Strong Infrared Emission from the Extrasolar Planet HD189733b". The Astrophysical Journal. 644: 560–564. Bibcode:2006ApJ...644..560D. arXiv:astro-ph/0602443 . 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″