An extremely low-density and temperate giant exoplanet

Creators: Santerne, A.; Dai, F.; Fulton, B. J.; Howard, A. W.; Sinukoff, E.

Abstract

Transiting extrasolar planets are key objects in the study of the formation, migration, and evolution of planetary systems. In particular, the exploration of the atmospheres of giant planets, through transmission spectroscopy or direct imaging, has revealed a large diversity in their chemical composition and physical properties. Studying these giant planets allows one to test the global climate models that are used for the Earth and other solar system planets. However, these studies are mostly limited either to highly-irradiated transiting giant planets or directly-imaged giant planets at large separations. Here we report the physical characterisation of the planets in a bright multi-planetary system (HIP41378) in which the outer planet, HIP41378 f is a Saturn-sized planet (9.2 ± 0.1 R⊕) with an anomalously low density of 0.09 ± 0.02 g cm⁻³ that is not yet understood. Its equilibrium temperature is about 300 K. Therefore, it represents a planet with a mild temperature, in between the hot Jupiters and the colder giant planets of the Solar System. It opens a new window for atmospheric characterisation of giant exoplanets with a moderate irradiation, with the next-generation space telescopes such as JWST and ARIEL as well as the extremely-large ground-based telescopes. HIP41378 f is thus an important laboratory to understand the effect of the irradiation on the physical properties and chemical composition of the atmosphere of planets.

Additional Information

We are grateful to Mikkel Lund for sharing the manuscript on the asteroseismic constraints of HIP41378 before its publication. This has been extremely useful for the analyses presented here and to reach these results. The HARPS team is grateful to the HARPS observers who conducted part of the visitor-mode observations at La Silla Observatory: David Anderson, Nicola Astudillo, Xavier Bonfils, Romina Ibañez Bustos, David Ehrenreich, Melissa Hobson, Kristine Lam, Baptiste Lavie, Ester Linder, Felipe Murgas, Lorenzo Pino, Julia Seidel, & Aurélien Wyttenbach. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programmes 198.C-0169 and 0102.C-0171. Based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Instituto Nazionale di Astrofísica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile. Based in part on observations made at Observatoire de Haute Provence (CNRS), France. This paper includes data collected by the K2 mission. Funding for the K2 mission is provided by the NASA Science Mission directorate. Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii. This publication makes use of The Data & Analysis Center for Exoplanets (DACE), which is a facility based at the University of Geneva (CH) dedicated to extrasolar planets data visualisation, exchange and analysis. DACE is a platform of the Swiss National Centre of Competence in Research (NCCR) PlanetS, federating the Swiss expertise in Exoplanet research. The DACE platform is available at https://dace.unige.ch. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France. The original description of the VizieR service was published in A&AS 143, 23. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/ California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. The French group acknowledges financial support from the French Programme National de Planétologie (PNP, INSU). The Porto group is supported by the Fundação para a Ciência e a Tecnologia (FCT) / MCTES through national funds (PIDDAC) and by FEDER - Fundo Europeu de Desenvolvimento Regional through COMPETE2020 - Programa Operacional Competitividade e Internacionalização by these grants: UID/FIS/04434/2019; PTDC/FIS-AST/32113/2017 & POCI-01-0145-FEDER-032113; PTDC/FIS-AST/28953/2017 & POCI-01-0145-FEDER-028953. VAd, SGS, EDM, SCCB acknowledge the support from through Investigador FCT contract nr. IF/00650/2015/CP1273/CT0001, IF/00028/2014/CP1215/CT0002, IF/00849/2015/CP1273/CT0003, IF/01312/2014/CP1215/CT0004 (respectively). SHo acknowledges support by the fellowships PD/BD/128119/2016 funded by FCT (Portugal). ODSD is supported in the form of work contract (DL 57/2016/CP1364/CT0004) funded by national funds through FCT. The Geneva group thanks the Swiss National Science Foundation (SNSF) and the Geneva University for their continuous support to the exoplanet researches. This work has been in particular carried out in the frame of the National Centre for Competence in Research PlanetS supported by SNSF. XDu is grateful to the Branco-Weiss Fellowship–Society in Science for its continuous support. This research has been funded by the Spanish State Research Agency (AEI) Projects No.ESP2017-87676-C5-1-R and No. MDM-2017-0737 Unidad de Excelencia "María de Maeztu" - Centro de Astrobiología (INTACSIC). DJA acknowledges support from the STFC via an Ernest Rutherford Fellowship (ST/R00384X/1). DB acknowledges support from an NSERC PGS-D scholarship. DJAB acknowledges support from the UK Space Agency (UKSA). DLP acknowledges support through a Royal Society Wolfson Merit Award. IJMC acknowledges support from the NSF through grant AST-1824644, and from NASA through Caltech/JPL grant RSA-1610091. MRK acknowledges support from the NSF Graduate Research Fellowship, grant No. DGE 1339067. MDa acknowledges financial support from Progetto Premiale 2015 FRONTIERA funding scheme of the Italian Ministry of Education, University, and Research. CDD acknowledges support from the K2 Guest Observer program (80NSSC18K0304 & 80NSSC19K0099) and the TESS Guest Investigator Program (80NSSC18K1583). Parts of this work have been supported by the National Aeronautics and Space Administration under grant No. NNX17AB59G issued through the Exoplanets Research Program. LMa acknowledges support from PLATO ASI-INAF agreement n.2015-019-R.1-2018. AMo acknowledges support from Senior Kavli Institute Fellowships at the University of Cambridge. HPO acknowledges support from Centre National d'Etudes Spatiales (CNES) grant 131425-PLATO. JKT acknowledges that support for this work was provided by NASA through Hubble Fellowship grant HST-HF2-51399.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. AVa's work was performed under contract with the California Institute of Technology / Jet Propulsion Laboratory funded by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute. CAW would like to acknowledge support from UK Science Technology and Facility Council grant ST/P000312/1. PJW has been supported by STFC through consolidated grants ST/L000733/1 and ST/P000495/1. ACC acknowledges support from the Science and Technology Facilities Council (STFC) consolidated grant number ST/R000824/1 and UKSA grant ST/R003203/1. LMW acknowledges support from the Beatrice Watson Parrent Fellowship. JFD acknowledges funding from the European Research Council (ERC) under the H2020 research & innovation programme (grant agreement #740651 NewWorlds). The authors declare that they have no competing financial interests. Authors Contributions: ASa led the Analysis Task Force to which LMa, MRK, XDu, AVa, NCH, AMo, TAL are members and wrote the manuscript. CDD coordinated the observations from the radial-velocity instruments. HARPS programs CoIs and associated collaborators: VAd, ADJA, DBar, SCCB, DBay, IBo, ASB, FBo, DJAB, MRD, EDM, ODe, RFD, GH´e, PFi, SHo, JLB, TAL, CLo, OMo, LDN, HPO, DLP, ASa (PI), NCS, SGS, SUd, AVi, PJW. HARPS-N CoIs and associated collaborators: ASB, LAB, ACC, RCo, MDa, CDD, XDu, PFi, AGh, AHa, DWL, MLM (chair), CLo, LMa, AFMF, EMo, AMo, CNa, DFP, EPo, KRi, DS´e, ASo, SUd (chair), AVa, CAW. HIRES CoIs, major observers, and data reduction: CDD, MRK, DBe, IJMC (PI), BJF, LAH, AWH, HIs, EAP, ESi, LMW. PFS CoIs, builders, and data reduction: FDa, RPB, JDC, SAS, JKT, SXW, JNW. ESPaDOnS CoIs and data reduction: IBo, DBar, JFD, MRD, JLB, TAL, CMo, ASa (PI), NCS. Data Availability Statement: The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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