Researchers from Nanjing University and the Shanghai Astronomical Observatory of the Chinese Academy of Sciences have proposed a new pathway for giant planet formation, known as Flyby-Induced Second Accretion (FISA).

The mechanism suggests that a planet initially 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 was recently 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 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.

To investigate where these giant planets could obtain the material needed to grow, the researchers turned to the stellar "neighborhoods" in which stars are born.

Most stars do not form in isolation. Instead, they are born alongside other stars 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.

AI-generated 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. (Image by SHI et al.)

A "second chance" for planetary growth

Under the proposed FISA mechanism, an M dwarf hosting an Earth-like planet in a young stellar cluster 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, providing a delayed supply of material for its planetary system.

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 across a range of encounter inclinations and velocities.

The study combines three areas of research: stellar evolution and protoplanetary disks, planet formation and evolution, and celestial dynamics. Hydrodynamic modeling was used to determine how much material can be captured during a stellar encounter and how the second-generation disk forms. Planet formation models then tracked how the pre-existing planet can resume pebble and gas accretion and migrate toward its host star. Finally, dynamical considerations were used 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 in roughly 3 × 10^-5 of 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.

According to the researchers, 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. Planetary system formation may therefore be shaped not only by a host star and its disk, but also by the wider stellar environment in which the system forms.