A close stellar flyby could fundamentally alter the growth path of a planet.

A joint team of researchers from Nanjing University and the Shanghai Astronomical Observatory, Chinese Academy of Sciences, has proposed a new pathway for the formation of giant planets: Flyby-Induced Second Accretion (FISA). The study shows that a planet initially only about the mass of Earth could gain a “second chance” to grow after its low-mass host star experiences a close encounter with another star. By reaccreting substantial amounts of solids and gas, the planet could eventually evolve into a gas giant. The study has been published in The Astrophysical Journal Letters.

Where do giant planets around M dwarfs get their material?

M dwarfs are the most common type of star in the Milky Way, accounting for roughly three quarters of all stars. Compared with the Sun, they are much less massive and are generally surrounded by smaller protoplanetary disks. In the classical picture of planet formation, a gas giant first needs to build a sufficiently massive solid core and then rapidly accrete a large amount of gas. For M dwarfs, however, the available material in their disks is often thought to be insufficient for this process.

Yet observations have revealed several striking exceptions. Astronomers have discovered giant planets orbiting stars much less massive than the Sun, including GJ-3512 b, LHS-3154 b, and TOI-6894 b. Among them, TOI-6894 b has a mass of about 0.17 times that of Jupiter, yet orbits only about 0.026 astronomical units from a star with roughly one-fifth of the Sun’s mass.

Where did these giant planets get the material needed to grow? 

The research team turned to the stellar “neighborhood” in which stars are born.

Most stars do not form in isolation. Instead, they are born together with many stellar companions in young clusters, where close stellar encounters can occur. Previous studies suggest that more than 10% of stars may experience encounters within about 100 astronomical units during their early evolution. Observations with facilities such as ALMA have also revealed protoplanetary disks that may have been disturbed by stellar flybys. Some studies have even proposed that the young Solar System may have experienced a stellar encounter at a distance of roughly 110 astronomical units.

A “second chance” for planetary growth

The team proposes the Flyby-Induced Second Accretion (FISA) mechanism. In a young stellar cluster, an M dwarf hosting an Earth-like planet may pass close to the protoplanetary disk of another young star. The gravitational interaction can strip and capture part of the disk’s gas and dust. The captured material can subsequently settle around the M dwarf and form a second-generation disk, effectively providing a delayed supply of material to its planetary system.

Figure 1. Schematic illustration of the Flyby-Induced Second Accretion (FISA) mechanism. From left to right: capture of disk material during the stellar flyby, formation of a new disk, and secondary accretion by the planet.

Using hydrodynamic simulations, the researchers found that, under favorable flyby conditions, an M dwarf with a mass of about 0.1–0.2 solar masses can capture enough material from another star’s protoplanetary disk to form a new, relatively compact disk within a few thousand years. The pre-existing planet can then resume pebble and gas accretion within this second-generation disk while migrating inward toward its host star. The simulations indicate that the process remains viable for a range of encounter inclinations and velocities.

Formationof the second-generation disk based on hydrodynamic simulations. Connecting stellar evolution, planet formation, and celestial dynamics

An important feature of this study is its interdisciplinary approach. The mechanism connects three areas of research: stellar evolution and protoplanetary disks, planet formation and evolution, and celestial dynamics.

Hydrodynamic modeling addresses how much material can be captured during a stellar encounter and how the second-generation disk forms. Planet formation models then follow how the pre-existing planet can resume pebble and gas accretion and migrate toward its host star. Finally, dynamical considerations allow the researchers to estimate how frequently such stellar encounters may occur in realistic stellar environments.

Taking into account the frequency of close stellar encounters, encounter velocities and inclinations, the properties of the donor protoplanetary disk, and the efficiency of subsequent giant-planet formation, the team estimates that the FISA mechanism could produce a close-in gas giant around roughly 3X10^-5 M-dwarf systems. This order-of-magnitude estimate is broadly consistent with the very small observed population of close-in giant planets around low-mass M dwarfs.

The implications of FISA may extend beyond M dwarfs. Interactions between stars, the replenishment of protoplanetary disks, and the subsequent migration of planets could all influence the final architecture of planetary systems. The birth of a planetary system may therefore not be the story of a single star and its disk alone, but a story shaped by the wider stellar environment in which the system was born.

Paper:Zhao, S., Shi, X., Lei, H., Deng, H., Lu, X., Hui, M.T., Wang, G. and Shi, J., 2026. Flyby-induced Second Accretion as a Pathway to Giant Planets Around M Dwarfs. The Astrophysical Journal Letters, 1008(1), p.L27
https://iopscience.iop.org/article/10.3847/2041-8213/ae984f

Scientific contacts:
Shunjing Zhao: 602024260017@smail.nju.edu.cn
Xian Shi: shi@shao.ac.cn
Hanlun Lei: leihl@nju.edu.cn