Stellar Flyby Disruptions

• Stellar flybys are close encounters that dramatically alter planetary systems and circumstellar disks through gravitational perturbations.
• They exhibit strong and weak regimes, leading from immediate planet ejections to long-term orbital instabilities.
• Simulation and observational studies reveal that these events shape disk morphology and influence planet formation in clustered environments.

Stellar flyby disruptions refer to the profound dynamical alterations of planetary systems and circumstellar disks caused by the close passage of another star. These events—frequent in clustered star-forming environments—produce a complex interplay of immediate gravitational perturbation, delayed planet–planet interactions, and long-term evolutionary consequences for planets, disks, and debris structures. The dynamical response depends intricately on the geometry and proximity of the encounter, the mass and speed of the intruder, the presence and state of gaseous disks, and the phase of system evolution at the time of the event. A combination of analytical formulations, large-scale N-body simulations, and direct observations (notably with ALMA and Gaia) has established stellar flybys as a central mechanism in shaping planetary architectures, disk morphology, and the statistics of extrasolar planet populations.

1. Dynamical Regimes and Mechanisms of Disruption

Disruption strength is governed by the closest approach distance ($r_\mathrm{min}$) and encounter kinematics. Two primary regimes characterize the dynamical pathways:

• Strong regime ($r_\mathrm{min} \lesssim 100$ au): In this regime, the intruding star passes within a distance comparable to the semi-major axes of giant planets or the extent of a protoplanetary disk. Numerical scattering experiments (over $6 \times 10^5$ encounters) show that $15$–$31\%$ of such flybys result in immediate ejection (“ionization”) of one or more planets or, less commonly, capture (“exchange”) of a planet by the intruder. The ejection probability increases with intruder mass, and the outcome distribution is highly sensitive to encounter geometry (Malmberg et al., 2010).
• Weak regime ($100$ au $< r_\mathrm{min} < 1000$ au): Distant passages impart non-negligible perturbations to planetary eccentricities and inclinations, parameterized by analytical expressions such as

$$\delta e = \frac{15}{8} \frac{m_I \left|m_H - m_p\right|}{M_\mathrm{ps}^2} \left[ \sqrt{M_\mathrm{ps} M_\mathrm{tot}} \left(\frac{a_p}{r_\mathrm{min}}\right)^{5/2} \frac{1}{e_I^3 (1+e_I)^{5/2}} \times \cdots \right]^{1/2} ,$$

where eccentricity excitation is a sensitive function of $r_{\min}$, relative orientation, and mass ratios (Malmberg et al., 2010). These “kicks” can place systems in dynamically unstable configurations (e.g., orbit crossing, high angular momentum deficit), leading to delayed planet–planet scattering and eventual ejection events over $10^6$–$10^8$ yr (Malmberg et al., 2010).

Disruption of circumstellar and protoplanetary disks is governed by tidal truncation models. The final disk radius after a parabolic, coplanar, prograde flyby is typically

$$r_\mathrm{disc} = 0.28\, r_\mathrm{peri} \, M_{21}^{-0.32}$$

where $M_{21} = M_2/M_1$ is the mass ratio of perturber to host (Pfalzner et al., 2021, Cuello et al., 2022). Deep encounters produce sharp, steep-edged outer boundaries and partially unbound large-scale tidal features.

2. Simulation Approaches and Analytical Modeling

State-of-the-art simulation frameworks span:

• Cluster N-body modeling: Using direct N-body codes (e.g., NBODY6) to simulate rich (N = 150–1000) young open clusters, flyby rates are statistically derived by registering close passages (typically within 1000 au). In reference clusters ($N=700$, $r_{h, i} = 0.38$ pc), $73.5\%$ of solar-type stars experience at least one flyby in their embedded lifetime (Malmberg et al., 2010).
• Planetary system integration: Post-encounter evolution is traced using high-precision symplectic or Bulirsch–Stoer integrators (e.g., MERCURY6) over $\sim$100 Myr. Scattering events, Hill-radius crossings, and final system architecture statistics are cataloged (Malmberg et al., 2010).
• Hydrodynamical SPH/mesh-based simulations: 3D SPH codes such as VINE and PHANTOM model the dynamical and thermodynamical response of gas/dust disks to tidal perturbation, including the vertical structure, gas–dust decoupling, and thermally driven accretion bursts (Picogna et al., 2014, Cuello et al., 2018, Borchert et al., 2022). Radiative transfer codes (e.g., MCFOST) post-process outputs for direct comparison with ALMA and H-band scattered light data (Cuello et al., 2019).
• Impulse and perturbative analytical estimates: For weak/far flybys, the response of test particles or planets is computed using multipole expansions and secular torque theory (Moore et al., 2020). Changes in orbital elements (eccentricity, inclination) are estimated by integrating the tidal or impulsive force over the encounter duration.

Key formulae include:

• Classical gravitational focusing cross-section:

$$\sigma = \pi r_\mathrm{min}^2 \left[1 + \frac{2G(m_1 + m_2)}{r_\mathrm{min} v_\infty^2} \right]$$

where gravitational focusing dominates in low-dispersion environments (Malmberg et al., 2010).

• Angular momentum deficit (AMD), quantifying dynamical "heating":

$$\mathrm{AMD} = \sum_k \Lambda_k \left[1 - \sqrt{1-e_k^2} \cos i_k\right]$$

with $\Lambda_k = m_k \sqrt{\mu a_k}$ (Malmberg et al., 2010).

3. Environmental Dependence and Statistical Outcomes

The frequency and dynamical impact of flybys are dictated by local stellar density ($n$), velocity dispersion ($v$), and cluster mass–radius scaling ($M_c = C R_c^\gamma$). Unexpectedly, low-mass clusters in the Solar neighborhood have higher than expected central densities due to their compact radii, leading to frequent flybys and environmental influence on protoplanetary disks (Pfalzner et al., 2021).

• In typical open clusters, $5$–$15\%$ of Solar-system–like planetary systems suffer planet ejections within $10^8$ yr post-flyby (Malmberg et al., 2010). High-mass clusters and "strong regime" encounters raise this perturbation fraction.
• In low-mass clusters ($N\sim200$), $10$–$15\%$ of disks are predicted to be truncated below $30$ au, producing compact, steep-edged disks observationally distinguishable from primordially small disks (Pfalzner et al., 2021).
• Field star encounters (e.g., in the Solar neighborhood) seldom produce dynamically significant events inside $\sim100$ au, but can excite the outer Oort cloud, as demonstrated by Scholz’s star, which passed within $0.25^{+0.11}_{-0.07}$ pc (52$^{+23}_{-14}$ kAU) 70 kyr ago, but with negligible impact on comet influx due to low mass and high velocity (Mamajek et al., 2015).

Environmental comparisons show that planetary systems in globular clusters, the Galactic center, or central bulge face efficient disruption and Oort cloud stripping, whereas open clusters and local field environments are less hostile, preserving habitable-zone stability in most cases (Jiménez-Torres et al., 2013).

4. Observational Signatures and Case Studies

ALMA, JVLA, and scattered-light imaging have provided direct evidence of ongoing or fossil flyby disruptions:

• RW Aurigae displays multiple tidal arms, misaligned gas disks (CO radii 58 and 38 au), and extended streams, requiring repeated non-coplanar encounters for modeling. Disk misalignment (∼12° or 57°) and recurring deep optical dimming events are interpreted as signatures of sustained flyby-driven morphological evolution (Rodriguez et al., 2018).
• Z CMa hosts a 2000 au streamer and a third component at ∼4700 au interpreted as the intruder. High-resolution imaging reveals mass flows and spiral features consistent with SPH flyby simulations with inclination $\beta = 45^\circ$ and pericenter $\sim3000$ au. Dust mass estimates in the streamer approach 119 Earth masses, indicative of significant mass redistribution (Dong et al., 2022).
• Statistical follow-up in the Sco-Cen OB association (using Gaia DR2) identifies >100 stars with recent flyby events within their Hill radius. However, only a minority of disk-bearing stars show strong asymmetry or morphology changes, highlighting the diversity and possibly transient visibility of flyby-induced features (Ma et al., 2022).
• Transient accretion events: Disc–disc flybys can trigger sustained, high-rate accretion outbursts (FU Orionis-like), with the angular momentum cancellation from misaligned infalling alien gas being the governing trigger (Borchert et al., 2022). Retrograde disc–disc flybys favor decades-long outbursts on the primary, with a significant fraction of the accreted mass originating from the secondary's disc.

5. Long-Term Evolution: Planetary Architectures and Demographic Implications

Stellar flybys have pronounced effects on both short-term dynamics and the secular evolution of planetary systems:

• Delayed planet–planet scattering: Weak flybys do not directly unbind planets, but initiate dynamical heating (increased eccentricity and angular momentum deficit). This increases the probability of close encounters between planets, doubling the fraction of ejections after $10^8$ yr compared to immediate post-flyby statistics (Malmberg et al., 2010). Surviving planets are left on eccentric orbits, closely matching the observed extrasolar eccentricity distributions.
• Stability of resonance chains: Long-period mean-motion resonance chains (e.g., 3:2–3:2, 2:1–2:1) are fragile to perturbation. Compact 3:2–3:2 chains are susceptible to dissolution even from distant flybys with $q/a_\mathrm{out} \sim 25$, while 2:1–2:1 chains are more robust, but easily broken by deeper encounters (Charalambous et al., 13 Mar 2025). This provides an explanation for the observed scarcity of long-period resonant exoplanet systems.
• Inclination excitation in debris disks: Analytical and N-body models show that flybys generally leave the bulk of disk inclinations low, but prograde encounters produce a distinct high-inclination population (including retrograde orbits) among particles with pericenter $>50$ au. This is a plausible explanation for the high-$i$ population among trans-Neptunian objects (Moore et al., 2020). The maximum excited inclination for massive stellar perturbers approximately follows

$i_{t, \max} \approx 163.6^\circ \cdot \sin i_s$

where $i_s$ is the flyby inclination.

• Architectural evolution of compact multiplanet systems: Single flybys in young clustered environments can induce mutual inclination growth and planet–planet collisions/cascades among close-in Super-Earth/sub-Neptune systems, with later depletion of multiplicity and reduction in the number of observable, coplanar transits. This mechanism is a contributor to the so-called Kepler Dichotomy—excess of single-transiting systems despite substantial underlying multiplicity (Schoettler et al., 31 Jul 2024).
• Disk mass reservoir and outer planet formation: Repeated flybys or a single strong flyby in the embedded phase can truncate disks to much smaller radii (10–30 au), potentially precluding the formation of wide-orbit giant planets and generating observable steep outer disk edges (Pfalzner et al., 2021, Cuello et al., 2022).

6. Timescales, Damping Processes, and Limitations

• Timescales: Direct dynamical consequences (planet ejection, capture) occur on encounter timescales (years–$10^3$ yr). Delayed instabilities (planet–planet scattering, inclination growth, resonance breaking) unfold over $10^6$–$10^8$ yr post-flyby (Malmberg et al., 2010, Charalambous et al., 13 Mar 2025, Schoettler et al., 31 Jul 2024).
• Circumstellar disk damping: If the flyby occurs while the system is still embedded in a massive gas disk, planet–disk interactions efficiently damp out excited eccentricities and inclinations on timescales $\tau_e \sim 10^4$ yr; after this, the system is restored to a nearly circular, coplanar state (Marzari et al., 2012, Picogna et al., 2014). Differences between 2D and 3D disk models arise due to vertical structure and mass exchange, but 3D results support even more rapid damping and random-walk misalignment of only a few degrees.
• Observational visibility windows: Morphological features such as spiral arms, bridges, and warps in disks persist for $10^3$-$10^4$ yr post-encounter before they wind up or dissipate (Cuello et al., 2018, Cuello et al., 2019). Recurring accretion outbursts are supported by simulations of misaligned or colliding disks (Borchert et al., 2022).
• Habitable zone and Oort cloud survival: In open clusters and the Solar neighborhood, habitable-zone orbits ($<$10 au) are robust to flybys unless the cluster is extremely long-lived or dense; Oort cloud objects are more vulnerable to stripping in dense cluster cores or during unusually close field star passages (e.g., Gliese 710 approaching within $\sim10$ kAU in $\sim$1.3 Myr) (Jiménez-Torres et al., 2013, Marcos et al., 2018).

7. Broader Implications and Theoretical Synthesis

Stellar flyby disruptions reshape the landscape of planetary and disk architectures in several foundational respects:

• Origin of extrasolar eccentricity/inclination distributions: The prevalence of eccentric and misaligned exoplanets can be explained by cumulative or singular flyby events followed by delayed planet–planet dynamical instabilities (Malmberg et al., 2010).
• Diversity of planetary system architectures: Environmental stochasticity—arising from the timing, geometry, and cluster properties of flybys—leads to wide variance in the endpoint multiplicity, semimajor axis, and coplanarity distributions in exoplanetary systems (Schoettler et al., 31 Jul 2024, Charalambous et al., 13 Mar 2025).
• Role in disk mass partitioning and planetesimal formation: Disk truncation and the induction of local density enhancements (spiral arms, warps) alter both the mass reservoir for outer planet formation and the dust/gas fraction critical for planetesimal coagulation (Cuello et al., 2018, Lu et al., 2022).
• Mixing and accretion of alien material: In multi-disc encounters, the transfer and accretion of non-native (“alien”) gas and solids can drive accretion events and produce chemical signatures difficult to explain by isolated evolution (Borchert et al., 2022).
• Interpretation of observed system topologies: Case studies (RW Aur, Z CMa, FU Orionis, UX Tauri) offer empirical templates for recognizing flyby signatures in resolved structures, photometric variability, and disk–star misalignment (Rodriguez et al., 2018, Dong et al., 2022, Cuello et al., 2022).

Stellar flybys are a fundamental route to planetary system excitation and restructuring, operating through a spectrum of pathways from immediate disruption to long-term instability, and from disk truncation to compositional mixing. Their prevalence and effects are key determinants of the emerging diversity of planetary systems across the Galaxy.