A new study led by Assoc. Prof. LIU Huigen and Ph.D. candidate DAI Yuanzhe from Nanjing University showed evidence that the planet occurrence rate correlates with the stellar dynamical history, with the help of the latest Gaia astrometric data and LAMOST spectroscopic data.

The results were published in The Astronomical Journal. 

Current theories of star formation suggest that stars form in clustered environments and planets are born in protoplanetary disks around the stars.

Observation shows that stars form in environments of great diversity. Planets' formation and evolution in such different environments is an open issue. For example, in clusters, massive stars with strong radiation will photo-evaporate nearby gas. More interestingly, close stellar-flyby events occur frequently.

Studying how these dynamics and radiation environments of planet host stars influence planetary systems can help us understand how planets form and evolve on a large scale in the Milky Way. 

Screenshot of a simulation of the interaction between flyby stars and protoplanetary disks. (Image from https://www.physics.utah.edu) 

Limited by the observed samples of planets in clusters, most studies on how the environments influence the planetary systems are theoretical. However, with the help of Gaia DR2, previous works show that stars in clusters can maintain low relative velocities for billions of years. These studies indirectly suggest that stellar dynamics environments play an important role in shaping planetary systems. 

Based on the Gaia-Kepler Stellar Properties Catalog, DAI and his collaborators calculated the two-dimensional velocities of the single stars in main-sequence, and divided them into three groups according to their relative velocities, i.e., high-V, medium-V, and low-V stars.

Different stellar groups show different distributions of properties. Meanwhile, stellar properties are closely related to planet occurrence. For example, planet occurrence rate increases with increasing metallicity. To obtain robust results, such bias must be corrected. Fortunately, LAMOST provides robust metallicity measurement of stars which can be used to correct the influence of metallicity on planet occurrence rate.

The researchers finally found that super-Earth and sub-Neptune are easier to form or survive around low-V stars, while high-V stars tend to have more multiple-planets systems and a lower average eccentricity. 

The results can be explained by stellar dynamics. High-V stars escape from clusters in the early stage, experiencing less gravitational perturbation. While low-V stars retain in the clusters until the dispersal of the cluster, which suggests that they may experience secular gravitational perturbations. The perturbations can enhance the formation of close-in planets. Thus low-V stars, which stay in clusters for longer time, tend to have a higher occurrence rate of hot planets with 1-4 Earth radii.

For multi-planet system around the low-V star, mutual inclination becomes larger, coupled with the increasing eccentricity due to perturbations. Because it is blind for transit method to discover planets with high mutual inclination, the fraction of multi-planet systems around low-V stars will be lower than that of high-V stars. 

"This work correlated planetary formation and evolution to the dynamic evolution of stars by observing the velocity of stars, and opens a new window for studying the co-evolution of stellar dynamics and planetary systems," said LIU, the corresponding author of the paper.