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A Low-Mass Planet Candidate Orbiting Proxima Centauri at a Distance Of A low-mass planet candidate orbiting Proxima Centauri at a distance of 1.5 AU Mario Damasso, Fabio del Sordo, Guillem Anglada-Escudé, Paolo Giacobbe, Alessandro Sozzetti, Alessandro Morbidelli, Grzegorz Pojmanski, Domenico Barbato, R. Paul Butler, Hugh Jones, et al. To cite this version: Mario Damasso, Fabio del Sordo, Guillem Anglada-Escudé, Paolo Giacobbe, Alessandro Sozzetti, et al.. A low-mass planet candidate orbiting Proxima Centauri at a distance of 1.5 AU. Science Advances , American Association for the Advancement of Science (AAAS), 2020, 6 (3), pp.eaax7467. 10.1126/sciadv.aax7467. hal-02996631 HAL Id: hal-02996631 https://hal.archives-ouvertes.fr/hal-02996631 Submitted on 13 Nov 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. SCIENCE ADVANCES | RESEARCH ARTICLE PLANETARY SCIENCE Copyright © 2020 The Authors, some rights reserved; A low-mass planet candidate orbiting Proxima Centauri exclusive licensee American Association at a distance of 1.5 AU for the Advancement Mario Damasso1*, Fabio Del Sordo2,3*, Guillem Anglada-Escudé4, Paolo Giacobbe1, Alessandro Sozzetti1, of Science. No claim to 5 6 1,7 8 9 original U.S. Government Alessandro Morbidelli , Grzegorz Pojmanski , Domenico Barbato , R. Paul Butler , Hugh R. A. Jones , Works. Distributed 10 11 12 12 Franz-Josef Hambsch , James S. Jenkins , María José López-González , Nicolás Morales , under a Creative 11 12 12 12 Pablo A. Peña Rojas , Cristina Rodríguez-López , Eloy Rodríguez , Pedro J. Amado , Commons Attribution Guillem Anglada12, Fabo Feng8, Jose F. Gómez12 NonCommercial License 4.0 (CC BY-NC). Our nearest neighbor, Proxima Centauri, hosts a temperate terrestrial planet. We detected in radial velocities evidence +0.26 of a possible second planet with minimum mass mc sin ic = 5.8 ± 1.9M⊕ and orbital period P​​ c = ​5.21−0.22 years. The analysis of photometric data and spectro-scopic activity diagnostics does not explain the signal in terms of a stellar activity cycle, but follow-up is required in the coming years for confirming its planetary origin. We show that the existence of the planet can be ascertained, and its true mass can be determined with high accuracy, by combining Downloaded from Gaia astrometry and radial velocities. Proxima c could become a prime target for follow-up and characterization with next-generation direct imaging instrumentation due to the large maximum angular separation of ~1 arc second from the parent star. The candidate planet represents a challenge for the models of super-Earth formation and evolution. http://advances.sciencemag.org/ INTRODUCTION planets with minimum masses greater than 8.5 M⊕ and orbital pe- Over more than 15 years, our nearest stellar neighbor Proxima riods out to 100 days. It was because of the RV technique that the Centauri (GJ 551; hereafter Proxima) has been observed with different temperate, low-mass planet Proxima b, orbiting at a distance of techniques aimed at the detection of planetary companions. Proxima ∼0.05 AU, was discovered (9). M dwarfs have high occurrence rates is an M5.5V star 1.3008 ± 0.0006 pc away from the Sun (1); therefore, of small planets (1.0 to 2.8 R⊕), 3.5 times more than main-sequence it is an ideal target for astrometric (2) and direct imaging (3) searches. FGK stars (10); therefore, systems with multiple, small, low-mass To date, these methods excluded the existence of Jupiter mass planets planets are expected to be common around them. With their RV from 0.8 astronomical unit (AU) to farther than 5 AU (>2 AU for dataset, including measurements with the HARPS (High Accuracy masses m ≥ 4MJupiter) (2, 3, 4). Holman and Wiegert (5) predicted Radial Velocity Planet Searcher) and UVES (Ultraviolet and Visual a maximum stable orbital radius of 1700 AU for planets orbiting Echelle Spectrograph) spectrographs, Anglada-Escudé et al. (9) could Proxima, because the star orbits the double system aCen AB, as has not rule out the presence of an additional super-Earth in the system on September 14, 2020 been demonstrated with a high degree of confidence in (6, 7). More with orbital periods longer than that of Proxima b and with Doppler −1 recently, Kervella et al. (7) set a 1s upper limit of 0.3 MJupiter to semi-amplitude smaller than 3 m s . For the sake of an independent potential companions of Proxima up to 10 AU by analyzing its confirmation of the existence of Proxima b and to search for addi- proper motion taken from the Gaia Data Release 2 (DR2) and ex- tional low-mass companions, Damasso and Del Sordo (11) analyzed cluded the presence of planets between 10 and 50 AU in the mass the same RV measurements using a model that treats the imprint of range 0.3 to 8 MJupiter. Using the radial velocity (RV) technique, Endl the stellar activity in a different way than the method adopted in (9). and Kürster (8) excluded the presence of planets with minimum masses This analysis uncovered correlated variability in the RV data ascrib- greater than 1 MNeptune out to 1 AU, while they could have detected able to the stellar activity and modulated over the known stellar rotation period. By treating the stellar activity signal with a quasi- periodic model, they could not unambiguously detect a low-mass 1INAF-Osservatorio Astrofisico di Torino, Via Osservatorio 20, I-10025 Pino Torinese, companion with an orbital period longer than that of Proxima b. Italy. 2Department of Physics, University of Crete, Voutes, 70013 Heraklion, Greece. The search for additional planets in this system did not stop, and it 3Institute of Astrophysics, Foundation for Research and Technology-Hellas, PO Box 4 was at the origin of the Red Dots (RD) initiative (https://reddots. 1527, 71110 Heraklion, Crete, Greece. School of Physics and Astronomy, Queen Mary University of London, 327 Mile End Rd., E14NS London, UK. 5Laboratoire space/), which also focused on other nearby stars, and recently led to Lagrange, UMR7293, Université Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, the discovery of a candidate super-Earth orbiting Barnard’s star close 6 Boulevard de l’Observatoire, 06304, Nice Cedex 4, France. Warsaw University to the snowline (12). Because of the RD campaign, additional RVs Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland. 7Dipartimento di Fisica, Universitá degli Studi di Torino, Via Pietro Giuria 1, I-10125 Torino, Italy. 8Depart- of Proxima were collected with the HARPS spectrograph, extending ment of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad the time span by 549 days with respect to the dataset analyzed in (9). 9 Branch Road, NW, Washington, DC 20015-1305, USA. Centre for Astrophysics In this work, we present the results from the analysis of the Research School of Physics, Astronomy and Mathematics University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK. 10American Association of Variable Stars extended RV dataset carried out within the framework outlined in Observers, 49 Bay State Road, Cambridge, MA 02138, USA. 11Departamento de (11) and aimed at searching for additional low-mass planetary com- Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, 12 panions to Proxima. The conclusions of this study are supported by Santiago, Chile. Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la the analysis of spectroscopic activity diagnostics and of a photo- Astronomía s/n, 18008 Granada, Spain. *Corresponding author. Email: [email protected] (M.D.); fabiods@physics. metric light curve with a baseline longer than the time span of the uoc.gr (F.D.S.) RV dataset. Damasso et al., Sci. Adv. 2020; 6 : eaax7467 15 January 2020 1 of 13 SCIENCE ADVANCES | RESEARCH ARTICLE MATERIALS AND METHODS (to be eventually compared with any significant long-period signal RV extraction found in the RVs) is to deal with indexes extracted homogeneously The RVs from HARPS spectra were extracted using the TERRA both from the UVES and HARPS spectra, covering the whole time (Template-Enhanced Radial velocity Re-analysis Application) pipeline span of the observations. To do so, we used the UVES activity (13) and represent an updated dataset with respect to that published indexes already published in (9), and then we extracted the Ha in (9). As in previous works, all observations taken before (various index from HARPS spectra following the recipe used in (9) by programs) and after [including the Pale Red Dot 2016 (9) and Red Dots adapting the code ACTIN (18). The time series of the Ha index 2017 campaigns] the HARPS fiber upgrade in May 2015 were treated measured from HARPS spectra is listed in table S3. as coming from a separate instrument to account for the reported We excluded outliers potentially due to powerful flares through offset introduced by the fiber change. HARPS/ESO (European a 3s clipping of the data, resulting in two epochs removed from the Southern Observatory) reduced spectra have a known “residual” sys- UVES dataset and three epochs removed from the HARPS dataset, tematic effect with a ~1-year periodicity caused by a small pixel size therefore with a very limited impact on the analysis. Then, we difference every 512 pixels on the detector, often called the “stitching binned the Ha index extracted from the HARPS spectra on a nightly problem,” coupled to the barycentric motion of the Earth, which im- basis.
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