Aims. The goal of this research is to study how the fragmentation of planetary embryos can affect the physical and dynamical properties of terrestrial planets around solar-type stars. Our study focuses on the formation and evolution of planets and water delivery in the habitable zone (HZ). We distinguish class A and class B HZ planets, which have an accretion seed initially located inside and beyond the snow line, respectively. Methods. We develop an N-body integrator that incorporates fragmentation and hit-and-run collisions, which is called D3 N-body code. From this, we perform 46 numerical simulations of planetary accretion in systems that host two gaseous giants like Jupiter and Saturn. We compare two sets of 23 N-body simulations, one of which includes a realistic collisional treatment and the other one models all impacts as perfect mergers.Results. The final masses of the HZ planets formed in runs with fragmentation are about 15 % -20 % smaller than those obtained without fragmentation. As for the class A HZ planets, those formed in simulations without fragmentation experience very significant increases in mass respect to their initial values, while the growth of those produced in runs with fragmentation is less relevant. We remark that the fragments play a secondary role in the masses of the class A HZ planets, providing less than 30 % of their final values. In runs without fragmentation, the final fraction of water of the class A HZ planets keeps the initial value since they do not accrete water-rich embryos. In runs with fragmentation, the final fraction of water of such planets strongly depends on the model used to distribute the water after each collision. The class B HZ planets do not show significant differences concerning their final water contents in runs with and without fragmentation. From this, we find that the collisional fragmentation is not a barrier to the survival of water worlds in the HZ.

Context. Planets orbiting low-mass stars such as M dwarfs are now considered a cornerstone in the search for planets with the potential to harbour life. GJ 273 is a planetary system orbiting an M dwarf only 3.75 pc away, which is 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 its interesting character, the GJ 273 planetary system has been poorly studied thus far. Aims. We aim to 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 a 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 we searched for regions in GJ 273 which may harbour minor bodies in stable orbits, that is, the main asteroid belt and Kuiper belt analogues. Results. From our set of dynamical studies, we find 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 that is likely to be composed of an Earth-mass planet, a super-Earth and two mini-Neptunes. Based on planetary formation models, we determine that GJ 273b is likely an efficient water captor while GJ 273c is probably a dry planet. We find that the system may have several stable regions where minor bodies might reside. Collectively, these results are used to offer a comprehensive discussion about the habitability of GJ 273b.

Context. The small (≤ 135 km mean radius) satellites of Saturn are closely related to its rings and together they constitute a complex dynamical system where formation and destruction mechanisms compete against each other. The Cassini-Huygens mission provided high-resolution images of the surfaces of these satellites and therefore allowed for the calculation of observational crater counts. Aims. We model the cratering process by Centaur objects on the small Saturnian satellites, and compare our results with the observational crater counts obtained from the Voyager and Cassini missions. Methods. Using a theoretical model previously developed we calculate the crater production on these satellites considering two slopes of the size-frequency distribution (SFD) for the smaller objects of the Centaur population and compare our results with the available observations. In addition, we consider the case of catastrophic collisions between these satellites and Centaur objects and calculate the age of formation of those satellites that suffer one or more disruptions.Results. In general we find that the observed crater distributions are best modeled by the crater size distribution corresponding to the s 2 = 3.5 index of the SFD of impactors with diameters smaller than 60 km. However, for crater diameters D 3 − 8 km (which correspond to impactor diameters d ∼ 0.04 − 0.15 km), the observed distributions become flatter and deviate from our results, which may evidence processes of erosion and/or crater saturation at small crater sizes or a possible break in the SFD of impactors at d ∼ 0.04 − 0.15 km to a much shallower differential slope of ∼ −1.5. Our results suggest that Pan, Daphnis, Atlas, Aegaeon, Methone, Anthe, Pallene, Calypso, and Polydeuces suffered one or more catastrophic collisions over the age of the solar system, the younger being associated to arcs with ages of ∼ 10 8 years. We have also calculated surface ages for the satellites, which indicate ongoing resurfacing processes.