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Register a new userThe Impact of Stellar Clustering on the Observed Multiplicity of Super-Earth systems: Outside-in Cascade of Orbital Misalignments Initiated by Stellar Flybys
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A recent study suggests that the observed multiplicity of super-Earth (SE) systems is correlated with stellar clustering: stars in high phase-space density environments have an excess of single-planet systems compared to stars in low density fields. This correlation is puzzling as stellar clustering is expected to influence mostly the outer part of planetary systems. Here we examine the possibility that stellar flybys indirectly excite the mutual inclinations of initially coplanar SEs, breaking their co-transiting geometry. We propose that flybys excite the inclinations of exterior substellar companions, which then propagate the perturbation to the inner SEs. Using analytical calculations of the secular coupling between SEs and companions, together with numerical simulations of stellar encounters, we estimate the expected number of effective flybys per planetary system that lead to the destruction of the SE co-transiting geometry. Our analytical results can be rescaled easily for various SE and companion properties (masses and semi-major axes) and stellar cluster parameters (density, velocity dispersion and lifetime). We show that for a given SE system, there exists an optimal companion architecture that leads to the maximum number of effective flybys; this results from the trade-off between the flyby cross section and the companions impact on the inner system. Subject to uncertainties in the cluster parameters, we conclude that this mechanism is inefficient if the SE system has a single exterior companion, but may play an important role in SE + two companions systems that were born in dense stellar clusters.
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It has recently been shown that stellar clustering plays an important role in shaping the properties of planetary systems. We investigate how the multiplicity distributions and orbital periods of planetary systems depend on the 6D phase space density of stars surrounding planet host systems. We find that stars in high stellar phase space density environments (overdensities) have a factor 1.6 - 2.0 excess in the number of single planet systems compared to stars in low stellar phase space density environments (the field). The multiplicity distribution of planets around field stars is much flatter (i.e. there is a greater fraction of multi-planet systems) than in overdensities. This result is primarily driven by the combined facts that: (i) `hot Jupiters (HJs) are almost exclusively found in overdensities; (ii) HJs are predominantly observed to be single-planet systems. Nevertheless, we find that the difference in multiplicity is even more pronounced when only considering planets in the Kepler sample, which contains few HJs. This suggests that the Kepler dichotomy -- an apparent excess of systems with a single transiting planet -- plausibly arises from environmental perturbations. In overdensities, the orbital periods of single-planet systems are smaller than orbital periods of multiple-planet systems. As this difference is more pronounced in overdensities, the mechanism responsible for this effect may be enhanced by stellar clustering. Taken together, the pronounced dependence of planetary multiplicity and orbital period distributions on stellar clustering provides a potentially powerful tool to diagnose the impact of environment on the formation and evolution of planetary systems.
Revealing the mechanisms shaping the architecture of planetary systems is crucial for our understanding of their formation and evolution. In this context, it has been recently proposed that stellar clustering might be the key in shaping the orbital architecture of exoplanets. The main goal of this work is to explore the factors that shape the orbits of planets. We used a homogeneous sample of relatively young FGK dwarf stars with RV detected planets and tested the hypothesis that their association to phase space (position-velocity) over-densities (cluster stars) and under-densities (field stars) impacts the orbital periods of planets. When controlling for the host star properties, on a sample of 52 planets orbiting around cluster stars and 15 planets orbiting around field star, we found no significant difference in the period distribution of planets orbiting these two populations of stars. By considering an extended sample of 73 planets orbiting around cluster stars and 25 planets orbiting field stars, a significant different in the planetary period distributions emerged. However, the hosts associated to stellar under-densities appeared to be significantly older than their cluster counterparts. This did not allow us to conclude whether the planetary architecture is related to age, environment, or both. We further studied a sample of planets orbiting cluster stars to study the mechanism responsible for the shaping of orbits of planets in similar environments. We could not identify a parameter that can unambiguously be responsible for the orbital architecture of massive planets, perhaps, indicating the complexity of the issue. Conclusions. Increased number of planets in clusters and in over-density environments will help to build large and unbiased samples which will then allow to better understand the dominant processes shaping the orbits of planets.
Planet formation is generally described in terms of a system containing the host star and a protoplanetary disc, of which the internal properties (e.g. mass and metallicity) determine the properties of the resulting planetary system. However, (proto)planetary systems are predicted and observed to be affected by the spatially-clustered stellar formation environment, either through dynamical star-star interactions or external photoevaporation by nearby massive stars. It is challenging to quantify how the architecture of planetary systems is affected by these environmental processes, because stellar groups spatially disperse within <1 billion years, well below the ages of most known exoplanets. Here we identify old, co-moving stellar groups around exoplanet host stars in the astrometric data from the Gaia satellite and demonstrate that the architecture of planetary systems exhibits a strong dependence on local stellar clustering in position-velocity phase space, implying a dependence on their formation or evolution environment. After controlling for host stellar age, mass, metallicity, and distance from the Sun, we obtain highly significant differences (with $p$-values of $10^{-5}{-}10^{-2}$) in planetary (system) properties between phase space overdensities and the field. The median semi-major axis and orbital period of planets in overdensities are 0.087 au and 9.6 days, respectively, compared to 0.81 au and 154 days for planets around field stars. Hot Jupiters (massive, close-in planets) predominantly exist in stellar phase space overdensities, strongly suggesting that their extreme orbits originate from environmental perturbations rather than internal migration or planet-planet scattering. Our findings reveal that stellar clustering is a key factor setting the architectures of planetary systems.
The Pioneer 10 and 11 and Voyager 1 and 2 spacecraft, launched in the 1970s, are heading out of the solar system. Using the astrometric and radial velocity data from the second Gaia data release, we integrate the trajectories of 7.4 million stars, and the spacecraft, through a Galactic potential in order to identify those stars the spacecraft will pass closest to. The closest encounters for all spacecraft take place at separations between 0.2 and 0.5 pc within the next million years. The closest encounter will be by Pioneer 10 with the K8 dwarf HIP 117795, at 0.23 pc in 90 kyr at a high relative velocity of 291 km/s.
A recent observational study suggests that the occurrence of hot Jupiters (HJs) around solar-type stars is correlated with stellar clustering. We study a new scenario for HJ formation, called Flyby Induced High-e Migration, that may help explain this correlation. In this scenario, stellar flybys excite the eccentricity and inclination of an outer companion (giant planet, brown dwarf, or low-mass star) at large distance (10-300 au), which then triggers high-e migration of an inner cold Jupiter (at a few astronomical units) through the combined effects of von Zeipel-Lidov-Kozai (ZLK) eccentricity oscillation and tidal dissipation. Using semianalytical calculations of the effective ZLK inclination window, together with numerical simulations of stellar flybys, we obtain the analytic estimate for the HJ occurrence rate in this formation scenario. We find that this flyby induced high-e migration could account for a significant fraction of the observed HJ population, although the result depends on several uncertain parameters, including the density and lifetime of birth stellar clusters, and the occurrence rate of the cold Jupiter + outer companion systems.
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