The impact of internal versus external perturbations on close-in exoplanet architectures

TL;DR

The paper investigates how internal versus external perturbations shape close-in exoplanet architectures in young star-forming regions, using high-precision N-body simulations of four inner sub-Neptunes and a distant giant planet across multiple giant locations and masses. By resetting the inner system at 1 Myr, the authors isolate internal perturbations and compare them to external perturbations from stellar fly-bys, evolving all configurations to 500 Myr. They find that, for a 5 M_Jup outer giant, the end-state of the inner planet system is largely independent of whether the perturbation is internal or external, with similar eccentricity evolutions and inner-planet counts; the external fly-by generally dominates the outcome, and the giant’s mass has little effect on the inner system in this scenario. In the case of a lower-mass giant (1 M_Jup), external perturbations can be more disruptive than internal ones, but overall the study supports a robust tendency toward a common end-state across perturbation channels, particularly for closely spaced inner planets. These results have implications for interpreting the observed architectures of close-in exoplanets and the dynamical histories of planetary systems formed in dense stellar environments.

Abstract

Young planetary systems are subjected to different dynamical effects that can influence their orbital structure over time. In systems with more than one planet, other planets can internally influence each other, e.g. via planet-planet scattering. External perturbing effects also need to be taken into account, as stars do not form by themselves but together with other stars in young star-forming regions. This birth environment can externally affect young multi-planet systems, e.g. via fly-bys. Previous work has shown that the absence/presence and location of an outer giant planet around a close-in planet system do not change how these inner planets react to a single fly-by with another star. We further explore this by comparing the effects of these external perturbations on four close-in sub-Neptune planets to those caused by a situation where only the distant giant is perturbed by the same kind of encounter. Our results indicate that the close-in planet systems have a "preferred" end state after 500 Myr, which is reached regardless of how it was perturbed. In addition, the mass of the giant appears not to impact the reaction of the inner planet system in the scenario of an external perturbation in our tested set-ups, i.e. either a single 1 or 5 M_Jup giant placed at 2.5, 5, 10 or 20 au. However, the mass affects the subsequent evolution of the inner planets if only internal perturbations by the giant are considered. The reduction in mass leads to an absence of collisions during the 500 Myr.

Figures (15)

• Figure 1: Schematic showing the reset approach applied for the close-in four-planet system. The left graphic shows "exaggerated" orbits of the four planets after the encounter when they are on slightly eccentric, non-coplanar orbits with the giant planet outside the axis limits. Note: In our simulations, none of the planets show this amount of perturbation at the reset at 1 Myr. On the right graphic after the reset, the four-planet system is set back to its original circular, coplanar orbits, but the giant planet's orbit remains unchanged, being eccentric and/or inclined as shown in Fig. \ref{['fig:Inc_ecc_GP_only_10_14_lowmass']}. The inner planet system is reset to its initial inclination at the start of the simulations to isolate the effect of internal perturbations after 500 Myr.
• Figure 2: Comparison of the cumulative distributions at 1 Myr of the changes in orbital parameters of the higher-mass giant planet ('solid') and lower-mass giant planet ('dashed') in reaction to the fly-by (fly-bys occur within the first 20kyr). Left: Eccentricity for the two giant planet masses of all 10+14 R$_{\rm{MH}}$ (mutual Hill radii) simulations combined at 1 Myr. All giant locations show very similar cumulative distributions of their eccentricity. Right: Change of the inclinations for the two giant planet masses in all simulations at 1 Myr compared to the start. The cumulative distributions show similar shapes for all four giant planet locations, indicating a similar level of increase in inclination regardless of mass.
• Figure 3: Left: Comparison of the eccentricities of all remaining close-in planets at 500 Myr for the simulations with an initial separation of 10 R$_{MH}$ for the giant planet with 5 M$_{\rm{Jup}}$. Right: Comparison of the eccentricities of all remaining close-in planets at 500 Myr for the simulations with an initial separation of 14 R$_{MH}$ for the same outer giant mass. Both plots show that for the simulations where all planets were subjected to the fly-bys, the eccentricities of the remaining planets at the end of our simulations are very similar, indicating that the presence/absence and exact location of a distant giant planet make no apparent difference to this orbital characteristic. This is a similar result compared to other orbital characteristics, such as the average semi-major axis and inclination (compared to the start) of the close-in planets (see Appendix Figs. \ref{['fig:CDF_all_semax']} and \ref{['fig:CDF_all_incl']})
• Figure 4: Time evolution of the cumulative distributions of the eccentricity of all remaining inner planets in simulations with an initial separation of 10 R$_{MH}$ for different placements of the giant planet. We compare the eccentricity for the close-in planets subjected to a fly-by ('solid' lines) and those where only the distant giant was subjected to that fly-by ('dashed' lines).
• Figure 5: Time evolution of the cumulative distributions of the eccentricity of all remaining inner planets in simulations with initial separation of 14 R$_{MH}$ for different placements of the giant planet. We compare the eccentricity for the close-in planets subjected to a fly-by ('solid' lines) and those where only the distant giant was subjected to that fly-by ('dashed' lines).