Astronomy & Astrophysics manuscript no. dinev_gj273 c ESO 2020 June 18, 2020 GJ 273: On the formation, dynamical evolution and habitability of a planetary system hosted by an M dwarf at 3.75 parsec Francisco J. Pozuelos1; 2;y, Juan C. Suárez3; 4, Gonzalo C. de Elía5; 6, Zaira M. Berdiñas7, Andrea Bonfanti1; 10, Agustín Dugaro5; 6, Michaël Gillon2, Emmanuël Jehin1, Maximilian N. Günther8; 9, Valérie Van Grootel1, Lionel J. Garcia2, Antoine Thuillier1; 11, Laetitia Delrez1; 2, and Jose R. Rodón4 1 Space sciences, Technologies & Astrophysics Research (STAR) Institute, Université de Liège, 4000 Liège, Belgium 2 EXOTIC Lab, UR Astrobiology, AGO Department, University of Liège, 4000 Liège, Belgium 3 Dpt. Física Teórica y del Cosmos, Universidad de Granada, Campus de Fuentenueva s/n, 18071, Granada, Spain 4 Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008, Granada, Spain 5 Instituto de Astrofísica de La Plata, La Plata , Argentina 6 Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata. LaPlata, Argentina 7 Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile, Casilla 36-D 8 Department of Physics, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cam- bridge, MA 02139, USA 9 Juan Carlos Torres Fellow 10 Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, A-8042 Graz, Austria 11 Observatoire de Paris, 61 Avenue de l’Observatoire 75014 Paris, France y e-mail: [email protected] Received ; accepted ABSTRACT Context. Planets orbiting low-mass stars such as M dwarfs are now considered a cornerstone in the search for life-harbouring planets. GJ 273 is a planetary system orbiting an M dwarf only 3.75 pc away, composed of two confirmed planets, GJ 273b and GJ 273c, and two promising candidates, GJ 273d and GJ 273e. Planet GJ 273b resides in the habitable zone. Currently, due to a lack of observed planetary transits, only the minimum masses of the planets are known: Mb sin ib=2.89 M⊕,Mc sin ic=1.18 M⊕,Md sin id=10.80 M⊕, and Me sin ie=9.30 M⊕. Despite being an interesting system, the GJ 273 planetary system is still poorly studied. Aims. We aim at precisely determine the physical parameters of the individual planets, in particular to break the mass–inclination degeneracy to accurately determine the mass of the planets. Moreover, we present thorough characterisation of planet GJ 273b in terms of its potential habitability. Methods. First, we explored the planetary formation and hydration phases of GJ 273 during the first 100 Myr. Secondly, we analysed the stability of the system by considering both the two- and four-planet configurations. We then performed a comparative analysis between GJ 273 and the Solar System, and searched for regions in GJ 273 which may harbour minor bodies in stable orbits, i.e. main asteroid belt and Kuiper belt analogues. Results. From our set of dynamical studies, we obtain that the four-planet configuration of the system allows us to break the mass– inclination degeneracy. From our modelling results, the masses of the planets are unveiled as: 2:89 ≤ Mb ≤ 3:03 M⊕, 1:18 ≤ Mc ≤ 1:24 M⊕, 10:80 ≤ Md ≤ 11:35 M⊕ and 9:30 ≤ Me ≤ 9:70 M⊕. These results point to a system likely composed of an Earth-mass planet, a super-Earth and two mini-Neptunes. From planetary formation models, we determine that GJ 273b was likely an efficient water captor while GJ 273c is probably a dry planet. We found that the system may have several stable regions where minor bodies might reside. Collectively, these results are used to comprehensively discuss the habitability of GJ 273b. Key words. GJ 273 – M dwarfs – Planetary systems – Planetary dynamics – Tides – Minor-body reservoir analogues – Habitability 1. Introduction a rate of at least one planet per star (e. g., Bonfils et al. 2013; Dressing & Charbonneau 2013, 2015; Tuomi et al. 2014). This arXiv:2006.09403v1 [astro-ph.EP] 16 Jun 2020 In the last decade, interest in planetary systems around M-type fact is also highlighted by the number of dedicated surveys tar- stars has significantly grown. The scientific interest is twofold. geting M dwarfs, notably in the search of rocky planets in their On one side, M-type stars are the most abundant star type in habitable zones (HZs). Some examples are the SPECULOOS the Solar neighbourhood, and indeed the Universe (see e. g., (Gillon 2018; Burdanov et al. 2018; Delrez et al. 2018; Je- Henry et al. 1994; Chabrier & Baraffe 2000; Winters et al. 2015). hin et al. 2018), CARMENES (Quirrenbach et al. 2010; Rein- However, their physical properties are poorly known since they ers et al. 2018), and MEarth (Nutzman & Charbonneau 2008) are generally quite faint, as compared with other stellar types, projects. and have only recently become the cornerstone in the detec- More interestingly, the huge number of M-type stars has tion of low-mass planets. Indeed, recent studies based on radial– significantly increased the chances of finding Earth-mass rocky velocity (RV) and transit photometry surveys have hinted at the planets in temperate orbits, i.e. within the host star’s HZ. Never- high occurrence of low-mass planets orbiting M dwarfs, with theless, planets in the HZs of M dwarfs are close to the star and Article number, page 1 of 20 A&A proofs: manuscript no. dinev_gj273 thus are generally exposed to intense radiation and magnetic ful- etary parameters and the general system architecture, while in gurations. Whether a planet can remain habitable in the presence Section 5 we explore the similarities of GJ 273 with the Solar of UV and X-ray flares is still a matter of debate: while a single System while searching for minor-body reservoir analogues. In flare probably does not compromise a planet’s habitability, plan- Section 6, we discuss on the habitability of planet GJ 273b, and ets orbiting M dwarfs may suffer frequent and strong flares at finally, in Section 7 we present our conclusions. younger ages (Armstrong et al. 2016). Among all the planets discovered so far orbiting M dwarfs, some of the most interesting are also the closest to Earth. One 2. Formation and evolution of the planetary system such example is the temperate Earth-mass planet orbiting Prox- GJ 273: N-body experiments ima Centauri-b, which is located only 1.2 pc away (Anglada- In the present section, we analyse the processes of formation and Escudé et al. 2016), while the seven Earth-size planets orbit- evolution of the planetary system GJ 273 using N-body simula- ing the TRAPPIST-1 system at a distance of 12.1 pc, where at tions. To do this, we first describe the properties of the proto- least three of them are in the HZ (Gillon et al. 2016, 2017) are planetary disk and then determine the physical and orbital initial equally tantalizing. In this context, GJ 273 is a highly interesting conditions to be used in our model. Then, we define the differ- M dwarf (also known as Luyten’s star) hosting a multi-planet ent scenarios to be modelled and finally, we discuss the results system, which is one of the closest stars to the Sun located at obtained from our N-body experiments. only 3.75 pc (Gatewood 2008). Compared with other M-type stars, GJ 273 is a bright star (9.87 Vmag, Koen et al. 2010), with a main-sequence spectral 2.1. Protoplanetary disk: properties type of M3.5V (Hawley et al. 1996), which shows a small pro- −1 An important parameter that determines the distribution of the jected rotational velocity (v sin i < 2:5 kms , Browning et al. material in a protoplanetary disk is the surface density. The gas- 2010). In the work by Astudillo-Defru et al. (2017b), the au- surface density profile Σg(R) adopted in the present research is thors made use of a data set of 280 RVs measured with HARPS given by (mounted on ESO’s 3.5-m La Silla telescope in Chile) which !−γ covered 12 years of intense monitoring of GJ 273. Astudillo- R 2−γ −(R=Rc) Defru et al. (2017b) reported the presence of two short-period Σg(R) = Σ0g e ; (1) R planets, GJ 273b and GJ 273c, with minimum masses of 2.89 c and 1.18 M⊕ and orbital periods of 18.6 and 4.7 d, respectively. where R is the radial coordinate in the disk mid-plane, Σ0g a nor- Planet GJ 273b was shown to be located in the HZ of GJ 273, malization constant, Rc the characteristic radius, and γ an expo- which along with its close proximity to Earth, sparked interest nent that determines the density gradient. At the same way, the in this particular system. In addition, in the recent study pre- solid-surface density profile Σs(R) is written by sented by Tuomi et al. (2019) the authors made use of a data !−γ R 2−γ set that combined HARPS, HIRES (Keck), PFS (Subaru) and −(R=Rc) Σs(R) = Σ0sηice e ; (2) APF (Lick), for a total of 466 RVs measurements, and fully Rc confirmed the existence of the two planets previously detected by Astudillo-Defru et al. (2017b) and suggested the presence of where Σ0s is a normalization constant and parameter ηice repre- two extra planets at larger heliocentric distances: GJ 273d with a sents the change in the amount of solid material due to the con- minimum mass of 10.8 M⊕ and an orbital period of 413.9 d, and densation of water beyond the snow line, which is assumed to be GJ 273e with a minimum mass of 9.3 M⊕ and an orbital period initially located at Rice.