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First direct detection of an exoplanet by optical interferometry. Astrometry and K-band spectroscopy of HR 8799 e

Lacour, S. and Wang, J. (2019) First direct detection of an exoplanet by optical interferometry. Astrometry and K-band spectroscopy of HR 8799 e. Astronomy and Astrophysics, 623 . Art. No. L11. ISSN 0004-6361.

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Aims. To date, infrared interferometry at best achieved contrast ratios of a few times 10^(−4) on bright targets. GRAVITY, with its dual-field mode, is now capable of high contrast observations, enabling the direct observation of exoplanets. We demonstrate the technique on HR 8799, a young planetary system composed of four known giant exoplanets. Methods. We used the GRAVITY fringe tracker to lock the fringes on the central star, and integrated off-axis on the HR 8799 e planet situated at 390 mas from the star. Data reduction included post-processing to remove the flux leaking from the central star and to extract the coherent flux of the planet. The inferred K band spectrum of the planet has a spectral resolution of 500. We also derive the astrometric position of the planet relative to the star with a precision on the order of 100 μas. Results. The GRAVITY astrometric measurement disfavors perfectly coplanar stable orbital solutions. A small adjustment of a few degrees to the orbital inclination of HR 8799 e can resolve the tension, implying that the orbits are close to, but not strictly coplanar. The spectrum, with a signal-to-noise ratio of ≈5 per spectral channel, is compatible with a late-type L brown dwarf. Using Exo-REM synthetic spectra, we derive a temperature of 1150 ± 50 K and a surface gravity of 10^(4.3 ± 0.3) cm s^2. This corresponds to a radius of 1.17_(−0.11)^(+0.13) R_(Jup) and a mass of 10_(−4)^(+7) M_(Jup), which is an independent confirmation of mass estimates from evolutionary models. Our results demonstrate the power of interferometry for the direct detection and spectroscopic study of exoplanets at close angular separations from their stars.

Item Type:Article
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Lacour, S.0000-0002-6948-0263
Wang, J.0000-0003-0774-6502
Additional Information:© 2019 S. Lacour et al. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Received: 12 February 2019; Accepted: 28 February 2019; Published online 27 March 2019. The reduced spectrum is available at the CDS via anonymous ftp to ( or via Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, ID 60.A-9102(G). GRAVITY was developed via a collaboration of the Max Planck Institute for Extraterrestrial Physics, LESIA of Paris Observatory and IPAG of Université Grenoble Alpes/CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the Centro Multidisciplinar de Astrofisica Lisbon and Porto, and the European Southern Observatory. Part of this work was supported by the European Union under ERC grant 639248 LITHIUM. J.W. is supported by the Heising-Simons Foundation 51 Pegasi b postdoctoral fellowship. A.A., P.G., and N.A. acknowledge funding from Fundação para a Ciência e Tecnologia through grants PTDC/CTE-AST/116561/2010, SFRH/BD/52066/2012, COMPETE FCOMP-01-0124-FEDER-019965, UID/FIS/00099/2013, and SFRH/BSAB/142940/2018. R.G.L. acknowledges funding by H2020 Marie Curie fellowship 706320.
Funding AgencyGrant Number
European Research Council (ERC)639248 LITHIUM
Heising-Simons FoundationUNSPECIFIED
Fundação para a Ciência e Tecnologia (FCT)PTDC/CTE-AST/116561/2010
Fundação para a Ciência e Tecnologia (FCT)SFRH/BD/52066/2012
Fundação para a Ciência e Tecnologia (FCT)COMPETE FCOMP-01-0124-FEDER-019965
Fundação para a Ciência e Tecnologia (FCT)UID/FIS/00099/2013
Fundação para a Ciência e Tecnologia (FCT)SFRH/BSAB/142940/2018
Marie Curie Fellowship706320
Subject Keywords:stars: individual: HR 8799 – planets and satellites: atmospheres – planet-star interactions – techniques: interferometric
Record Number:CaltechAUTHORS:20190327-142743110
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Official Citation:First direct detection of an exoplanet by optical interferometry - Astrometry and K-band spectroscopy of HR 8799 e. GRAVITY Collaboration, S. Lacour, et. al., A&A, 623 (2019) L11. DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:94222
Deposited By: Tony Diaz
Deposited On:27 Mar 2019 21:40
Last Modified:28 Oct 2019 21:54

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