Fly-bye, Baby: a review of the impact of stellar flybys on protoplanetary disks

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

Authors: Nicolás Cuello, François Ménard, Daniel J. Price

First Author’s Institution: Univ. Grenoble Alpes, CNRS, IPAG / UMR 5274, F-38000 Grenoble

Status: Invited review article submitted to EPJ+ [closed access]

All planetary systems, including our own Solar System, began their lives as pancakes of gas and dust that encircled newly formed stars. These “protoplanetary disks” don’t exist in isolation, however. When massive clouds of gas and dust collapse in on themselves, many thousands of stars are created in close proximity to one another, and take millions of years to disperse from their star-forming region out into the galaxy. While they disperse, nearby stars pass by each other, gravitationally influencing the trajectories of their neighbors. When two stars pass uncomfortably close, enough to dramatically change their motion, astronomers refer to the passing event as a “close stellar encounter” or “flyby.” In theory, stellar flybys can disrupt the protoplanetary disks around young stars, leading to changes in how planets form within the affected disks. Today’s bite covers a recent article by Ménard & Price that reviews how often young stars experience flybys, theory and simulations that show how flybys can impact protoplanetary disks, and recent observations of disks expected to have experienced close encounters.

“Never Tell Me The Odds”

The second section of this review (after its introduction) tackles the occurrence rate of stellar flybys for young stars hosting disks. They combine estimates of the occurrence rate of protoplanetary disks around young stars from the literature with an analytical estimate for the number of stars that experience a stellar flyby. They find that 50% or more of all stars with planet forming disks are likely to have been affected by flybys. Wow! Half of all infant planetary systems experience these events, which must mean that stellar flybys have a large effect on the population of exoplanets in our galaxy. 

Spiraling Out of Control

The next section tackles the physics, the theories and modeling that predict how stellar flybys of different orientations affect extended disk structures. They divide their descriptions into two types of flybys, a “prograde” flyby and a “retrograde” flyby. During a prograde flyby, the orbital motion of the passing star is aligned with the rotational motion of the impacted star (see the Figure below). Prograde flybys trigger spiral arms within the outer reaches of the affected protoplanetary disk, as the disk material forms a “bridge” between the two passing bodies.

Figure 1: Three different types of prograde flybys, where the rotation of the primary star is aligned with the passing orbit of the secondary. These prograde orbits generate short lived spiral arms that reach from each disk to form a “bridge” between the two forming solar systems (Figure 3 in the text).

For a retrograde flyby, where the rotation of the impacted star is misaligned with the orbital motion of the passing star, the transport of angular momentum between the two stellar systems is much less efficient, and so spiral arms do not form. These kinds of flybys can lead to warps and increased eccentricities (a more oval, less circular shape) for the disks involved. 

I Spy With My Little Eye: A Stellar Flyby

The fourth section of this review paper is by far my favorite, because the authors collate suspected observations of stellar flybys from the literature. They record 9 different systems whose morphology is best explained by a recent or ongoing stellar close encounter. Each system is unique in its own right, exhibiting a different set of spirals, bridges, or truncated and warped disks. Figure 2 shows a gallery of images of these dramatic systems. 

Figure 2: Observational evidence of stellar flybys in protoplanetary systems observed in scattered light using the SPHERE instrument (Figure 6 in the text). 

What’s the Impact?

The authors then discuss what impact these close encounters have on the formation of planets within the affected disks, and on formed and forming planetary systems.

If planets already formed by the time a flyby occurs, they could be perturbed to higher eccentricity orbits during the period of closest approach. In the most extreme cases, planets could be ripped from their host and ejected, or stolen into the passing system. Whether the planets return to circular, stable orbits depends a lot on how much gas remains within the disk, because gas can exhibit a drag force on the eccentric planet, returning it to a low eccentricity configuration.

It appears unlikely that flybys destroy a high number of disks, but they do lead to mass loss, resulting in less overall material to form planets from. They also effectively destroy large planetesimals by increasing the relative velocities and eccentricities of planetesimals within the disk, leading to breaking collisions. On the flip side, these events could help trigger planet formation by generating warps, rings, and dust traps that allow dust to clump together rapidly, enabling the formation of rocky planets.

So flybys and close encounters have a big impact on planet formation! In the future, discoveries of more flyby affected systems with better high contrast images or more sensitive interferometers will allow astronomers to model these flyby events in more detail. Better population level statistics (of star forming regions, disk evolution, and exoplanets) can help inform our understanding of the effect of flybys on planets in our galaxy. For now though, we can all just marvel at the fantastical images and simulations of these dynamic stellar dances.

Astrobite edited by Macy Huston

Featured image credit: Figure 6 in Close encounters: How stellar flybys shape planet-forming discs

About William Balmer

William Balmer (they/them) is a PhD student at JHU/STScI studying the formation, evolution, and composition of giant planets, brown dwarfs, and very low mass stars. They enjoy reading, tabletop games, cycling, and astrophotography.

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