Study of the dynamics of exocomets and the formation of cometary reservoirs in the Kepler-90 extrasolar system

Study of the dynamics of exocomets and the formation of cometary reservoirs in the Kepler-90 extrasolar system

Nilce da Silva dos Santos¹, Rafael Ribeiro de Sousa¹ and Silvia Maria Giuliatti Winter¹

¹Orbital Dynamics and Planetology Group (GDOP), São Paulo State University-UNESP

 

   Comets are bodies composed mainly of water and dust that exist in our Solar System, with the main cometary reservoirs being the Kuiper belt, located in the trans-Neptunian region, that is, beyond the orbit of Neptune, between approximately 30 and 50 AU (astro- nomical units) and in the Oort cloud, whose inner edge is located at approximately 2000 AU.

   Models of planetary system formation indicate that cometary reservoirs, like comets, are bodies left over from the formation process of these systems. During the formation process, the gravitational interaction between small objects that do not accrete to form planets (called planetesimals) and large bodies, such as gas giant planets during the plane- tary migration phase, enables the formation of reservoirs. Thus, the existence of cometary reservoirs appears to be a by-product of the formation of systems.

   When colliding with planets, comets can transport water and matter of astrobiological importance, such as argon and xenon (O’Brien et al., 2015; Dvorak et al., 2020). Given the possibilities of collisions, in this work we study the dynamics of hypothetical comets in the extrasolar system Kepler-90 (K90).

   It is important to emphasize that we consider a reservoir of hypothetical comets (exo- comets) since these bodies, to date, have not been identified in the system in question. As previously planned, the existence of comets appears to be natural to the formation of plane- tary systems. Although exocomets have not been identified, there is observational evidence of their existence (Dvorak et al., 2020). In addition, it is worth mentioning the passage through the Solar System of the object Oumuamua in 2017, the first interstellar object to pass through our system and be classified as a comet.

   In this work, we focus on the study of the extrasolar system Kepler-90 (K90), discovered by the Kepler spacecraft. The interest in this system must be due to its similarities with our Solar System: K90 is formed by a host star with a mass and radius respectively equal to 1.2 solar masses and 1.2 solar radii. In addition, the system has eight planets in a hierarchical arrangement: the innermost are rocky planets and the outermost are gas giant planets. The main difference between the two systems is that K90 is considered compact, with the outermost planet, K90-h, located at an orbital radius of approximately 1 AU.

   For the dynamic analysis of comets in K90, we consider a reservoir with highly eccentric bodies, as observed in the Solar System. These bodies (test particles) are under the gravitational influence of the planets of K90 and have a semi-major axis ranging from 5 to 7 AU; and inclinations ranging from 0 to 180 degrees, that is, they reach the inner region of the system from any direction. We consider the argument of the pericentre, longitude of the ascending node and mean anomaly varying randomly from 0 to 360 degrees.

   For the analysis, we consider three sets of simulations for different values of eccentricities of the planets: 0 and 0.001. Gaslac et al., 2024, showed that, for these values, the K90 system is stable. Thus, for the eccentricities, we considered the particles tested with the same initial conditions, totaling approximately 8,300 particles in the system. The numerical simulations were performed using the Mercury program (Chambers, 1999) and the Bulirsch- Stoer integrator.

   For the cases of eccentricities of the planets investigated, we observed similar results: although the majority of the particles are ejected from the system, approximately 2% of the total considered collide with the planets of the system, with the majority of collisions occurring with the gas giant planets. Even so, the majority of the particles that collide have initial inclinations of 0 or 180 degrees, that is, they are the particles that reach the system with a direction close to the ecliptic plane. Thus, we can conclude that the transport of water to the planets of the system is possible given the initial conditions considered.

   The initial conditions of the set of test particles were assumed based on the reinforced values for the test particles in previous work (Dvorak et al. 2020). In the current stage, we study the formation of cometary reservoirs in the Kepler-90 system by scattering of planetesimals during the stages of the system formation process. We consider the migration of giant planets that can cause the propagation of planetesimals. The consideration of this phase of the work aims to make it more realistic. In addition to the gravitational influence of the planets, we will consider the galactic tide and the passage of stars on the movement of exocomets in the formed reservoir.

Acknowledgments: SMG thanks FAPESP (Proc. 2016/24561-0), CNPq (Proc. 316991/2023- 6) and Capes.

References:

DVORAK, R.; LOIBNEGGER, B.; CUNTZ, M. On the dynamics of comets in extrasolar planetary systems. In: The Trans-Neptunian Solar System. Elsevier, 2020. p. 331- 350.

GASLAC GALLARDO, D. M. et al. Analysing the dynamics of the Kepler-90 planetary system. Monthly Notices of the Royal Astronomical Society, v. 535, n. 4, p. 3198-3210, 2024.