What do UX Tauri, RW Aurigae, AS 205, Z CMajoris and FU Orionis have in common? They are young star systems with disks where planets could form. It appears that those disks were disturbed by stellar flybys or other close encounters in the recent past. Astronomers want to know: Did those events disrupt planet formation in the disks? What do they do? Does this happen on other systems? And did our own solar system experience a strange encounter in its youth?
Some answers are found in a study by astronomer Nicolas Neck of the University of Grenoble Alpes, who heads a team studying the role of stellar flybys. In a recent article, they analyze the processes these systems go through. They examined the chances of any disc experiencing a flyby/encounter and classified the types of encounters. The team also studied a set of disks to understand what happens during each type of encounter and analyzed the implications of flybys for planet formation in other systems. Finally, they looked for possible clues about a flyby that our own Solar System might have experienced.
Intruder alert! Disc under attack!
It all begins when the birth of stars occurs in clouds of gas and dust. The process creates batches of hot, young stars clustered together. Over time, some of those groups dissipate. When stars leave the nest, they can pass close to other systems, causing disruptions in planet-forming disks. Neck and his team concluded that close encounters will awaken or even disrupt these disks at some point in their evolution.
“Stellar flybys and encounters occur more frequently than previously thought,” said Cuello in an email discussion. “This is likely to happen when stars are very young (less than a million years old) and have planet-forming disks around them. These disks are highly affected by gravitational perturbation from nearby stars, changing the initial conditions at the start of planet formation. That is why we have to take it into account in our models.”
Flyovers aren’t terribly rare, according to Cuello. “I would say that at least half of the stars and their disks are affected/formed by flybys,” he said. “An important aspect to note is that the probability of such disturbances decreases over time but never reaches zero. So even the most highly evolved stars (with planetary systems around them) can experience a flyby during their lifetime. In that case, some planets could end up in misaligned orbits relative to the rest of the planetary system or even be captured by the perturbing star.”
How much damage can a stellar flyby do?
In typical star-forming regions, distances matter. Most stars with protoplanetary disks experience close flybys, within a thousand astronomical units. That is equivalent to about half the distance between the Sun and the Oort Cloud in our Solar System. Some of those encounters can really disrupt a record. For example, if an intruder star travels in a progressive direction, in a parabolic orbit that penetrates the disk, it can cause enough damage to alter the shape of the disk. Sometimes damage from an intruder causes a second disk of material to form.
This is, in fact, what is happening with the star FU Orionis. Thanks to a close stellar flyby that crashed into its disk, FU Orionis appears to brighten by a factor of a thousand in about a year. And such interruptions are also evident in other young systems.
During some encounters, the puck goes through what is called “tidal truncation.” That can remove up to 80 percent of the disk’s mass. This has a catastrophic effect on planet formation because the encounter reduces the amount of material needed to form protoplanets. Such flybys could also create dust traps. Theoretically, those could be places where planetesimals could grow, given enough time.
In some cases, a close flyby can scatter planets within systems, or even eject a planet. Those left behind could move into orbits reminiscent of Pluto’s: eccentric and misaligned with the plane of the system. (To be clear, Pluto’s weird orbit isn’t due to a flyby. It’s more likely that gravitational influences from Neptune and other giant planets shaped its weird orbit.)
Stellar flybys and our solar system
Did our own solar system experience stellar flybys during its formation? It’s a possibility that Cuello and his colleagues explore in his article. Such an encounter in or very close to our birth cloud could have shaped the solar nebula. Ultimately, that would have influenced the size of the disk and its mass. It’s hard to know how many times this might have happened, but amazingly, the protosolar nebula where the Sun was born was left in a fairly circular shape and most of the planets move in fairly circular and regular orbits.
However, Cuello and his team concluded that the orbital arrangement of the solar system could have affected the distribution of trans-Neptunian Objects (the region just beyond Neptune, where Pluto orbits). It is also possible that one or more stars passed through and disrupted the Oort Cloud. Astronomers have found some candidates that they are studying to see if this hypothesis is true.
Certainly our solar system has experienced other more recent encounters during its long history. Scholz’s Star, for example, is thought to have passed through the Oort Cloud about 70,000 years ago. Currently, this binary star is about 22 light years from us. The passage didn’t seem to affect the orbits of any of the planets, but it probably had a very small effect on the number of Oort Cloud objects ejected into long-period orbits around the Sun. Still, it’s still a useful example. of the effect that a passing star can have on a planetary system or a protoplanetary disk.
For more information
Close Encounters: How Stellar Flybys Shape Planet-Forming Disks