HR 8799 b

Size comparison of HR 8799 b (gray) with Jupiter.
Discovery
Discovered by	Maroiset al.
Discovery site	Keck andGemini observatories inHawaii
Discovery date	November 13, 2008
Detection method	Direct imaging
Orbital characteristics
Semi-major axis	~ 68AU
Orbital period (sidereal)	~ 460[1][note 1][note 2]y
Star	HR 8799
Physical characteristics
Mean radius	1.2 ± 0.1[1]RJ
Mass	5+2 −1[2]MJ
Meandensity	5.041 g/cm3[3]
Temperature	870+30 −70[1]

HR 8799 b is anextrasolar planet located approximately 129light-years away in theconstellation ofPegasus, orbiting the 6thmagnitudeLambda Boötis starHR 8799. It has a mass between 4 and 7^[2]Jupiter masses and a radius from 10 to 30% larger thanJupiter's. It orbits at 68AU from HR 8799 (or 7 AU inside the inner edge of thedust disk orbiting the star) with an unknown eccentricity and a period of 460 years, and is the outermost known planet in the HR 8799 system. Along with two other planets orbiting HR 8799, the planet was discovered on November 13, 2008 by Maroiset al., using theKeck andGemini observatories inHawaii. These planets were discovered using thedirect imaging technique.^{[1][4][5][6][7]}

In 2009 it was discovered that theHubble Space Telescope had in fact directly imaged HR 8799 b eleven years earlier, in 1998, suggesting that more exoplanets might be revealed through analysis of HST photographic archives.^[8] Additionalprecovery images were also obtained by reanalyzing data taken in 2002 at theSubaru Telescope and in 2005 and 2007 at the W. M. Keck Observatory.^[9][10][11]

Broadband photometry of HR 8799 b shows that it has thicker clouds in its atmosphere than do older, higher surface gravity substellar objects of the sameeffective temperature.^[12]

Near infrared H band and K band spectroscopy of HR 8799 b published in May 2011 indicate a hydrogen rich, dusty atmosphere with disequilibrium CO / CH₄ chemistry.^[13]

Near infrared spectroscopy made with the Palomar Observatory show evidence of ammonia and/or acetylene as well as carbon dioxide, and some methane.^[14] In 2015, the signals of water, carbon monoxide and methane were reported in infrared spectra taken at theKeck telescope.^[15] However, a reanalysis of the same data in 2018 shows that water and carbon monoxide are present, but that the methane detection may have been spurious. Observations by theJames Webb Space Telescope will be capable of confirming or disproving the presence of methane in HR 8799 b's atmosphere.^[16]

• Fomalhaut b

Media related toHR 8799 b at Wikimedia Commons

• "HR 8799 b".Exoplanets. Archived fromthe original on 2012-03-04. Retrieved2008-12-02.

• HR 8799
• Exoplanets discovered in 2008
• Giant planets
• Pegasus (constellation)
• Exoplanets detected by direct imaging

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