A&A 631, A7 (2019) https://doi.org/10.1051/0004-6361/201935922 Astronomy & © ESO 2019 Astrophysics Pebbles versus planetesimals: the case of Trappist-1 G. A. L. Coleman, A. Leleu?, Y. Alibert, and W. Benz Physikalisches Institut, Universität Bern, Gesellschaftsstr. 6, 3012 Bern, Switzerland e-mail: [email protected] Received 20 May 2019 / Accepted 11 August 2019 ABSTRACT We present a study into the formation of planetary systems around low mass stars similar to Trappist-1, through the accretion of either planetesimals or pebbles. The aim is to determine if the currently observed systems around low mass stars could favour one scenario over the other. To determine these differences, we ran numerous N-body simulations, coupled to a thermally evolving viscous 1D disc model, and including prescriptions for planet migration, photoevaporation, and pebble and planetesimal dynamics. We mainly examine the differences between the pebble and planetesimal accretion scenarios, but we also look at the influences of disc mass, size of planetesimals, and the percentage of solids locked up within pebbles. When comparing the resulting planetary systems to Trappist-1, we find that a wide range of initial conditions for both the pebble and planetesimal accretion scenarios can form planetary systems similar to Trappist-1, in terms of planet mass, periods, and resonant configurations. Typically these planets formed exterior to the water iceline and migrated in resonant convoys into the inner region close to the central star. When comparing the planetary systems formed through pebble accretion to those formed through planetesimal accretion, we find a large number of similarities, including average planet masses, eccentricities, inclinations, and period ratios. One major difference between the two scenarios was that of the water content of the planets. When including the effects of ablation and full recycling of the planets’ envelope with the disc, the planets formed through pebble accretion were extremely dry, whilst those formed through planetesimal accretion were extremely wet. If the water content is not fully recycled and instead falls to the planets’ core, or if ablation of the water is neglected, then the planets formed through pebble accretion are extremely wet, similar to those formed through planetesimal accretion. Should the water content of the Trappist-1 planets be determined accurately, this could point to a preferred formation pathway for planetary systems, or to specific physics that may be at play. Key words. planetary systems – planets and satellites: formation – planets and satellites: dynamical evolution and stability – planet-disk interactions 1. Introduction (Anglada-Escudé et al. 2016) and Ross 128 (Bonfils et al. 2018b) each contain a planet that is very similar in mass and period The recent discovery of seven Earth-sized planets orbiting the to Trappist-1 g. More recently, planets have also been observed low mass star Trappist-1 (Gillon et al. 2017; Luger et al. 2017) has around Barnard’s Star (Ribas et al. 2018), LHS 1140 (Dittmann led to many questions about the formation and evolution of such et al. 2017; Ment et al. 2019), and GJ 1214 (Luque et al. 2018), a complex system. Not only are all seven planets orbiting close significantly increasing the number of planets observed around to their parent star (with orbital periods ≤20 d), but they also low mass stars. all appear to form a resonant chain, such that their orbital peri- Following the discovery of Proxima b (Anglada-Escudé et al. ods are near integer ratios of each other. The formation of such 2016), Coleman et al.(2017a) presented numerous formation sce- resonant chains are a natural outcome of interactions between narios for such a planet orbiting such a low-mass star. These the planets and their nascent protoplanetary discs (Cresswell & scenarios ranged from in situ formation, to migration of a sin- Nelson 2008). These resonant chains have been observed in gle or multiple planetary embryos from outside the iceline after other compact planetary systems (Lissauer et al. 2011; Fabrycky accreting either planetesimals or pebbles. They showed that each et al. 2014; Mills et al. 2016), and have also been formed in scenario yielded subtly different observational signatures such complex planet formation simulations involving multiple bod- as multiplicity, planet composition, and orbital architectures. ies (Hellary & Nelson 2012; Coleman & Nelson 2014, 2016a,b). In situ formation produced numerous volatile-poor Earth-sized Whilst Trappist-1 may be the most high-profile planetary sys- planets with little evidence for resonant chains; migration of a tem around low mass stars, it is interesting to note that a number single embryo from outside the iceline formed a single volatile- of similar planetary systems have also been recently observed. rich Earth-sized planet on a circular orbit; whilst the migration For example, two planets with periods less than five days have of multiple embryos formed numerous Earth-sized planets rich been confirmed around YZ Ceti, with a third planet still await- in volatiles, often displaying mean-motion resonances (MMRs) ing confirmation (Astudillo-Defru et al. 2017a; Robertson 2018). between neighbouring planets. More recently Alibert & Benz GJ 1132 (Berta-Thompson et al. 2015; Bonfils et al. 2018a) (2017) studied the formation and composition of planets around and GJ 3323 (Astudillo-Defru et al. 2017b) each have two low mass stars, finding that close-in planets have similar masses super-Earths orbiting close to their central star, whilst Proxima and radii (peaking at ∼1 R⊕), and also that the properties of the ? CHEOPS Fellow. protoplanetary disc and their correlation with the stellar mass Article published by EDP Sciences A7, page 1 of 24 A&A 631, A7 (2019) are important in determining the characteristics of the planet 2. Physical model (e.g. water content). Since these papers were either only aiming to form a single planet, or only involved single-planet-in-a- The physical model we adopt for this study is based on the planet system simulations, they did not address the formation of such formation models of Coleman & Nelson(2014, 2016a). These a complex system as Trappist-1. models run N-body simulations using the Mercury-6 symplectic A scenario that has recently been proposed for the formation integrator (Chambers 1999), adapted to include the disc models of the Trappist-1 planetary system through pebble accretion is and physical processes described below. outlined in Ormel et al.(2017). In their scenario, they assume (i) We solve the standard diffusion equation for a 1D that the planetary embryos form at the water iceline in the disc, viscous α-disc model (Shakura & Sunyaev 1973; Lynden-Bell after millimeter- or centimeter-sized particles (i.e. pebbles) have & Pringle 1974). Disc temperatures are calculated by balancing accumulated there. Once a planetary embryo forms, it accretes black-body cooling against viscous heating and stellar irradi- × −3 the surrounding pebbles before migrating in towards the central ation. The viscous parameter αvisc = 1 10 throughout most star. As the embryo migrates inwards, it accretes dry pebbles, of the disc, but increases to αactive = 0:005 in regions where ≥ further increasing its mass, whilst a new embryo forms at the T 1000 K to mimic the fact that fully developed turbulence iceline and goes through the same process. The planets then can develop in regions where the temperature exceeds this value migrate to the inner edge of the disc, where their migration (Umebayashi & Nakano 1988; Desch & Turner 2015). ceases, allowing the planets to enter into first-order mean motion (ii) The final stages of disc removal occur through a pho- resonances. As the disc disperses, the first order resonances can toevaporative wind. We use a standard photoevaporation model for most of the disc evolution (Dullemond et al. 2007), corre- be broken allowing the planets to dynamically rearrange into new sponding to a photoevaporative wind being launched from the configurations (Coleman & Nelson 2016a; Izidoro et al. 2017). upper and lower disc surfaces. Direct photoevaporation of the Recently Schoonenberg et al.(2019) explored this scenario in disc is switched on during the final evolution phases when an a more quantitative setting. They found that this method was inner cavity forms in the disc, corresponding to the outer edge unable to form planetary systems similar to Trappist-1. How- of the disc cavity being exposed directly to the stellar radiation ever, instead of forming planetary embryos one after another, (Alexander & Armitage 2009). they were able to form planetary systems similar to Trappist-1 if (iii) The N-body simulations consist of a number of planetary multiple planetary embryos formed at the iceline on short time- embryos that can mutually interact gravitationally and collide. scales (e.g. 1000 yr). These embryos could then mutually interact In addition, some models also include planetesimals (bodies and accrete dry and wet pebbles whilst slowly migrating closer to with radii either 100 m ≤ ≤ 1 km). Planetesimals orbit- the central star. The planets in the simulated systems then com- Rp ing in the gaseous protoplanetary disc experience size depen- pared favourably to Trappist-1 in terms of planet masses and dent aerodynamic drag (Adachi et al. 1976; Weidenschilling water fractions. 1977). Collisions between protoplanets and other protoplanets In this paper we used up-to-date models of planet forma- or planetesimals always result in perfect merging. Planetesimal- tion utilising either planetesimal or pebble accretion, in studying planetesimal interactions and collisions are neglected for reasons the formation of planetary systems around low mass stars, of computational speed. with a specific goal in forming planetary systems similar to (iv) We use the torque formulae from Paardekooper et al. Trappist-1. We used the Mercury-6 symplectic integrator to (2010, 2011) to simulate type I migration due to Lindblad and compute the dynamical evolution and collisional accretion of corotation torques acting on the planetary embryos.