https://doi.org/10.1140/epjp/s13360-022-03602-w
Review
Close encounters: How stellar flybys shape planet-forming discs
1
CNRS, IPAG / UMR 5274, Univ. Grenoble Alpes, 38000, Grenoble, France
2
School of Physics and Astronomy, Monash University, 3800, Clayton, VIC, Australia
a
nicolas.cuello@univ-grenoble-alpes.fr
Received:
19
July
2022
Accepted:
15
December
2022
Published online:
9
January
2023
We review the role of stellar flybys and encounters in shaping planet-forming discs around young stars, based on the published literature on this topic in the last 30 years. Since most stars 
Myr old harbour protoplanetary discs, tidal perturbations affect planet formation. First, we examine the probability of experiencing flybys or encounters: More than 50% of stars with planet-forming discs in a typical star-forming environment should experience a close stellar encounter or flyby within 1000 au. Second, we detail the dynamical effects of flybys on planet-forming discs. Prograde, parabolic, disc-penetrating flybys are the most destructive. Grazing and penetrating flybys in particular lead to the capture of disc material by the secondary to form a highly misaligned circumsecondary disc with respect to the disc around the primary. One or both discs may undergo extreme accretion and outburst events, similar to the ones observed in FU Orionis-type stars. Warps and broken discs are distinct signatures of retrograde flybys. Third, we review some recently observed stellar systems with discs where a stellar flyby or an encounter is suspected—including UX Tau, RW Aur, AS 205, Z CMa, and FU Ori. Finally, we discuss the implications of stellar flybys for planet formation and exoplanet demographics, including possible imprints of a flyby in the Solar System in the orbits of trans-Neptunian objects and the Sun’s obliquity.
Focus Point on Environmental and Multiplicity Effects on Planet Formation. Guest editors: G. Lodato, C.F. Manara.
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© The Author(s), under exclusive licence to Società Italiana di Fisica and Springer-Verlag GmbH Germany, part of Springer Nature 2023. Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.
