A&A 628, A39 (2019) Astronomy https://doi.org/10.1051/0004-6361/201935801 & © ESO 2019 Astrophysics Planetary system around the nearby M dwarf GJ 357 including a transiting, hot, Earth-sized planet optimal for atmospheric characterization? R. Luque1,2, E. Pallé1,2, D. Kossakowski3, S. Dreizler4, J. Kemmer5, N. Espinoza3, J. Burt6,??, G. Anglada-Escudé7,8, V. J. S. Béjar1,2, J. A. Caballero9, K. A. Collins10, K. I. Collins11, M. Cortés-Contreras9, E. Díez-Alonso12,13, F. Feng14, A. Hatzes15, C. Hellier16, T. Henning3, S. V. Jeffers4, L. Kaltenegger17, M. Kürster3, J. Madden17, K. Molaverdikhani3, D. Montes12, N. Narita1,18,19,20, G. Nowak1,2, A. Ofir21, M. Oshagh4, H. Parviainen1,2, A. Quirrenbach5, S. Reffert5, A. Reiners4, C. Rodríguez-López8, M. Schlecker3, S. Stock5, T. Trifonov3, J. N. Winn22, M. R. Zapatero Osorio23, M. Zechmeister4, P. J. Amado8, D. R. Anderson16, N. E. Batalha24, F. F. Bauer8, P. Bluhm5, C. J. Burke6, R. P. Butler14, D. A. Caldwell25,26, G. Chen27, J. D. Crane28, D. Dragomir6,???, C. D. Dressing29, S. Dynes6, J. M. Jenkins26, A. Kaminski5, H. Klahr3, T. Kotani18,20, M. Lafarga30,31, D. W. Latham10, P. Lewin32, S. McDermott33, P. Montañés-Rodríguez1,2, J. C. Morales30,31, F. Murgas1,2, E. Nagel34, S. Pedraz35, I. Ribas30,31, G. R. Ricker6, P. Rowden36, S. Seager6,37,38, S. A. Shectman28, M. Tamura18,20,39, J. Teske28,???, J. D. Twicken25,26, R. Vanderspeck6, S. X. Wang14, and B. Wohler25,26 (Affiliations can be found after the references) Received 29 April 2019 / Accepted 27 June 2019 ABSTRACT We report the detection of a transiting Earth-size planet around GJ 357, a nearby M2.5 V star, using data from the Transiting Exoplanet Survey Satellite (TESS). GJ 357 b (TOI-562.01) is a transiting, hot, Earth-sized planet (Teq = 525 ± 11 K) with a radius of Rb = 1:217 ± 0:084 R⊕ and an orbital period of Pb = 3:93 d. Precise stellar radial velocities from CARMENES and PFS, as well as archival data from HIRES, UVES, and HARPS also display a 3.93-day periodicity, confirming the planetary nature and leading to a planetary mass of Mb = 1:84 ± 0:31 M⊕. In addition to the radial velocity signal for GJ 357 b, more periodicities are present in the data indicating the presence of two further planets in the system: GJ 357 c, with a minimum mass of Mc = 3:40 ± 0:46 M⊕ in a 9.12 d orbit, and GJ 357 d, with a minimum mass of Md = 6:1 ± 1:0 M⊕ in a 55.7 d orbit inside the habitable zone. The host is relatively inactive and exhibits a photometric rotation period of Prot = 78 ± 2 d. GJ 357 b is to date the second closest transiting planet to the Sun, making it a prime target for further investigations such as transmission spectroscopy. Therefore, GJ 357 b represents one of the best terrestrial planets suitable for atmospheric characterization with the upcoming JWST and ground-based ELTs. Key words. planetary systems – techniques: photometric – techniques: radial velocities – stars: individual: Gl 357 – stars: late-type 1. Introduction Only six of the abovementioned eleven systems con- tain planets with masses below 10 M⊕: LHS 1140 b and c To date nearly 200 exoplanets have been discovered orbiting (GJ 3053, Dittmann et al. 2017; Ment et al. 2019), K2-3 b approximately 100 M dwarfs in the solar neighborhood (e.g., and c (PM J11293–0127, Almenara et al. 2015; Sinukoff et al. Bonfils et al. 2013; Rowe et al. 2014; Trifonov et al. 2018; Ribas 2016), K2-18 b (PM J11302+0735, Cloutier et al. 2017; Sarkis et al. 2018). Some of these orbit near to or in the habitable et al. 2018), GJ 1214 b (LHS 3275 b, Harpsøe et al. 2013), zone (e.g., Udry et al. 2007; Anglada-Escudé et al. 2013, 2016; GJ 1132 (Berta-Thompson et al. 2015; Bonfils et al. 2018), Tuomi & Anglada-Escudé 2013; Dittmann et al. 2017; Reiners and b–g planets of TRAPPIST-1 (Gillon et al. 2016, 2017). et al. 2018). However, only 11 M dwarf planet systems have been However, only three planets with masses similar to Earth orbit detected with both the transit as well as the radial velocity (RV) M dwarfs of moderate brightness (J = 9.2–9.8 mag): GJ 1132 b method, which allows us to derive their density from their mea- (1:66 ± 0:23 M⊕), LHS 1140 c (1:81 ± 0:39 M⊕), and K2-18 b sured radius and mass, informing us about its bulk properties. +2:1 (2.1−1:3 M⊕). Systems hosting small terrestrial exoplanets orbit- When transit timing variation (TTV) mass measurements are ing bright stars are ideal not only from the perspective of precise included, TRAPPIST-1 (2MUCD 12171, Gillon et al. 2017) rep- mass measurements with ground-based instruments, but also for resents the 12th M dwarf planet system with mass and radius further orbital (e.g., obliquity determination) and atmospheric measurements. characterization using current and future observatories (see, e.g., ? RV data are only available at the CDS via anonymous ftp to Batalha et al. 2018). cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc. The Transiting Exoplanet Survey Satellite (TESS, Ricker u-strasbg.fr/viz-bin/qcat?J/A+A/628/A39 et al. 2015) mission is an observatory that was launched to find ?? Torres Fellow. small planets transiting small, bright stars. Indeed, since the ??? NASA Hubble Fellow. start of scientific operations in July 2018, TESS has already Article published by EDP Sciences A39, page 1 of 18 A&A 628, A39 (2019) uncovered over 600 new planet candidates, and is quickly Li et al. 2019), for example, even-odd transits comparison, eclips- increasing the sample of known Earths and super-Earths around ing binary discrimination tests, ghost diagnostic tests to help rule small M-type stars (Vanderspek et al. 2019; Günther et al. out scattered light, or background eclipsing binaries, among oth- 2019; Kostov et al. 2019). In this paper, we present the dis- ers. The report indicates that the dimming events are associated covery of three small planets around a bright M dwarf, one with significant image motion, which is usually indicative of a of which, GJ 357 b, is an Earth-sized transiting exoplanet dis- background eclipsing binary. However, in this case, the reported covered using photometry from the TESS mission. To date, information is meaningless because the star is saturated. On the GJ 357 b is the second nearest (d = 9:44 pc) transiting planet to other hand, the transit source is coincident with the core of the the Sun after HD 219134 b (Motalebi et al. 2015, d = 6:53 pc), stellar point spread function (PSF), so the transit events happen and the closest around an M dwarf. Besides, it is amenable to on the target and not, for example, on a nearby bright star. future detailed atmospheric characterization, opening the door to We also performed an independent analysis of the TESS light new studies for atmospheric characterization of Earth-like planet curve in order to confirm the DVR analysis and search for addi- atmospheres (Pallé et al. 2009). tional transit signals. An iterative approach was employed: in The paper is structured as follows. Section2 presents the each iteration the same raw data were detrended and outliers- TESS photometry used for the discovery of GJ 357 b. Section3 rejected, a signal was identified and then modeled, and that presents ground-based observations of the star including seeing- model was temporarily divided-out during the detrending of the limited photometric monitoring, high-resolution imaging, and next iteration to produce a succession of improving models, until precise RVs. Section4 presents a detailed analysis of the stellar the χ2 converges. The raw photometry was detrended by fitting properties of GJ 357. Section5 presents an analysis of the avail- a truncated Fourier series, starting from the natural period of able data in order to constrain the planetary properties of the sys- twice the data span, and all of its harmonics, down to some tem, including precise mass constraints on GJ 357 b along with “protected” time span to make sure the filter does not modify the a detection and characterization of two additional planets in the shape of the transit itself. We used a protected time span of 0.5 d, system, GJ 357 c and GJ 357 d. Section6 presents a discussion and this series was iteratively fitted with 4σ rejection. Finally, of our results and, finally, Sect.7 presents our conclusions. OptimalBLS (Ofir 2014) is used to identify the transit signal, which is then modeled using the Mandel & Agol(2002) model and the differential evolution Markov chain Monte Carlo algo- 2. TESS photometry 2 rithm (ter Braak & Vrugt 2008). The final model has χν = 1:017 Planet GJ 357 (TIC 413248763) was observed by TESS in 2-min and the resultant transit parameters are consistent with the TESS short-cadence integrations in Sector 8 (Camera #2, CCD #3) DVR. We also checked for odd-even differences between the from February 2, 2019 until February 27, 2019 (see Fig.1), transits, additional transit signals, and parabolic TTVs (Ofir et al. and will not be observed again during the primary mission. At 2018) – all with null results. BJD = 2 458 531:74, an interruption in communications between the instrument and spacecraft occurred, resulting in an instru- 2.2. Limits on photometric contamination ment turn-off until BJD = 2 458 535:00. Together with the satel- 00 lite repointing for data downlink between BJD = 2 458 529:06 Given the large TESS pixel size of 21 , it is essential to ver- and BJD = 2 458 530:44, a gap of approximately 6 d is present ify that no visually close-by targets are present that could affect in the photometry.