# Close encounters: How stellar flybys shape planet-forming discs

**Authors:** Nicolás Cuello, François Ménard, Daniel J. Price  
**arXiv:** [2207.09752](https://arxiv.org/abs/2207.09752)  
**Date:** 2022-07-20  
**Summary page:** https://www.summarizepaper.com/en/paper/2207.09752v1/

## Key points

- Stellar flybys and encounters can significantly affect planet formation in protoplanetary discs around young stars
- Over 50% of stars with planet-forming discs in a typical star-forming environment should experience a close encounter or flyby within 1000 au
- Prograde, parabolic, disc-penetrating flybys are the most destructive while grazing and penetrating flybys can lead to the capture of disc material by the secondary star
- Retrograde flybys leave distinct signatures such as warps and broken discs
- Recently observed stellar systems where a stellar flyby or encounter is suspected include UX Tau, RW Aur, AS 205, Z CMa, and FU Ori
- Possible imprints of a past flyby in our own Solar System are discussed in relation to trans-Neptunian objects' orbits and the Sun's obliquity
- Detailed figures show face-on views of gas column density for different angles (β) at various times after periastron passage during prograde encounters. The size of the disc increases with β: prograde encounters truncate it more heavily compared to retrograde ones.
- The authors also present edge-on views of the gas column density for different angles and times as well as xy- and xz-views of dust column density in centimeter-sized grains during prograde versus retrograde encounters.
- Overall, this review highlights the significant impact that stellar flybys can have on planet formation while providing insights into observed systems where such events may have occurred.

## Summary

This review article explores the impact of stellar flybys and encounters on planet-forming discs around young stars. With most stars under 2 million years old hosting protoplanetary discs, tidal perturbations can significantly affect planet formation. The authors first examine the probability of experiencing flybys or encounters, finding that over 50% of stars with planet-forming discs in a typical star-forming environment should experience a close encounter or flyby within 1000 au. The paper then delves into the dynamical effects of different types of flybys on planet-forming discs, with prograde, parabolic, disc-penetrating flybys being the most destructive. Grazing and penetrating flybys can lead to the capture of disc material by the secondary star, forming a highly misaligned circumsecondary disc with respect to the primary's disc. One or both discs may undergo extreme accretion and outburst events similar to those observed in FU Orionis-type stars. Retrograde flybys leave distinct signatures such as warps and broken discs. The authors also review recently observed stellar systems where a stellar flyby or encounter is suspected, including UX Tau, RW Aur, AS 205, Z CMa, and FU Ori. Finally, they discuss the implications of stellar flybys for planet formation and exoplanet demographics. Possible imprints of a past flyby in our own Solar System are discussed in relation to trans-Neptunian objects' orbits and the Sun's obliquity. The paper includes detailed figures showing face-on views of gas column density for different angles (β) at various times after periastron passage during prograde encounters. The size of the disc increases with β: prograde encounters truncate it more heavily compared to retrograde ones. A bridge appears between both stars shortly after periastron for prograde orbits; remarkably for β=45° , a bridge of material connects both stars around 500 years after periastron passage. The authors also present edge-on views of the gas column density for different angles and times as well as xy- and xz-views of dust column density in centimeter-sized grains during prograde versus retrograde encounters. Overall, this review highlights the significant impact that stellar flybys can have on planet formation while providing insights into observed systems where such events may have occurred.