Stellar Flybys leave a lasting mark on newly formed planetary systems

What do UX Tauri, RW Aurigae, AS 205, Z CMajoris and FU Orionis have in common? They are small star systems with disks in which planets can form. These disks appear to have been disrupted by stellar flybys or other close encounters in the recent past. Astronomers want to know: Did those events disrupt the formation of planets in the disks? What do they do? Does this happen in other systems? Did our solar system experience a strange encounter in its youth?

Some of the answers lie in a study by astronomer Nicolas Coelho of the University of Grenoble-Alpes, who leads a team studying the role of stellar flight. In a recent paper, they discuss the processes these systems undergo. They examined the chances of any given disk facing a flight/fight and categorized the types of encounters. The team also studied a set of disks to understand what happens during each type of encounter, and looked at the effects of flybys on planet formation in other systems. Finally, they looked at potential clues to flight that our solar system might have experienced.

Intruder alert! The disk is under attack!

It all begins when star births occur in clouds of gas and dust. The process creates clusters of hot young stars clustered together. Over time, some of these groups dissipate. When stars leave the nest, they may pass close to other systems, causing disturbances in the constituent disks of the planets. Coelho and his team came to the conclusion that close encounters would trigger or even disrupt these discs at some point in their development.

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FU Orionis and its associated nebula. It is possible that the nebula was disrupted by flight, and the brightness is one of the effects of the event. Image Editing: ESO

“Flights and stellar encounters happen more frequently than previously expected,” Coelho said in an email discussion. This likely occurs when stars are very young (less than a million years old) and have planet-forming disks around them. These disks are strongly affected by the gravitational perturbation of nearby stars, modifying the initial conditions at the beginning of planet formation. That’s why it has to be taken into account in our models.”

Flybys aren’t very rare, according to Coelho. “I would say at least half of the stars and their disks are affected/formed by flybys,” he said. “An important aspect to highlight is that the probability of such perturbations decreases with time but never reaches zero. Therefore, even the most advanced stars (with planetary systems around them) can experience flybys during their lifetime. In this case, they might Some planets end up in skewed orbits with respect to the rest of the planetary system or even get picked up by the perturbed star.”

How much damage can starbursts do?

In typical star-forming regions, distances are important. The majority of stars with protoplanetary disks experience close flybys—within a thousand astronomical units. This is about half the distance from the Sun to the Oort cloud in our Solar System. Some of these encounters can really upset the disc. For example, if an intrusive star is traveling in a forward direction, in a parabolic orbit that pierces the disk, it can do enough damage to change the shape of the disk. Sometimes the damage caused by the intruder causes the formation of a second disk of the substance.

This is, in fact, what is happening with the star FU Orionis. Thanks to a close stellar flyby smashing through its disk, FU Orionis appears to be shining by a factor of a thousand in about a year. Such disturbances manifest themselves in other young systems as well.

A group of flyby filters disrupted by stellar flybys is shown in scattered light. Images provided by François Maynard (ISO-Oph 2, DO Tau, RW Aur, and FU Ori Courtesy of Iain Hammond), Nicolas Coelho, Daniel J. Price.

During some encounters, the disc goes through what’s called a “tidal truncation.” It can remove up to 80 percent of the disk’s mass. This has a disastrous effect on planet formation because the encounter reduces the amount of material needed to form protoplanets. Such flybys may also create dust traps. In theory, these could be places where minor planets could grow, given enough time.

In some cases, close flyby can scatter planets within systems, or even eject a planet. Those left behind could teleport to Pluto-like orbits – eccentric and not aligned with the system plane. (To be clear, Pluto’s strange orbit is not due to a flyby of it. Rather, gravitational influences from Neptune and the other giant planets likely shaped its strange orbit.)

Stellar Flybys and Our Solar System

Did our solar system experience starbursts during its formation? Coelho and colleagues likely explore in their paper. Such an encounter in or near the natal cloud could have formed the solar nebula. Ultimately this had an effect on the size and mass of the disk. It’s hard to tell how often this could happen, but it is remarkable that the proto nebula in which the Sun was born is left in a somewhat circular shape and most of the planets move in fairly circular and regular orbits.

The solar system is in the making
Artist’s impression of the early solar system in the making. Stellar flights may have helped shape the cloud of planetary birth. Credit: NASA/JPL-Caltech

However, Coelho and his team concluded that the solar system’s orbital arrangement could have influenced the distribution of objects across Neptune (the region just behind Neptune, where Pluto orbits). It is also possible that one or more stars have passed through the Oort cloud and malfunctioned. Astronomers have found quite a few candidates to study to see if this hypothesis works.

Certainly, our solar system has seen other, more modern encounters during its long history. For example, Schulze’s star is believed to have passed through the Oort Cloud about 70,000 years ago. Currently, this binary star is located about 22 light-years away from us. The pass does not appear to affect the orbits of any of the planets, but it may have had very little effect on the numbers of Oort Cloud objects ejected into long-range orbits around the Sun. However, it remains a useful example of the effect that a passing star can have on a planetary system or protoplanetary disk.

for more information

Close Encounters: How Star Flies Shape Planetary Disks

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