Stellar Flybys Leaves Permanent Mark on Newly Formed Planetary System
What do the UX Tauri, RW Aurigae, AS 205, Z CMajoris, and FU Orionis have in common? They are young star systems with disks where planets can form. It appears that the discs were disturbed by interstellar flybys or other close encounters in the past. Astronomers want to know: did the event interfere with the formation of planets in the disk? What are they doing? Does this happen on other systems? And, did our solar system experience strange encounters in its youth?
Some of the answers lie in research by astronomer Nicolás Cuello of the University of the Grenoble Alpes who heads a team studying the role of interstellar flight. In a recent paper, they discussed the processes this system goes through. They check the probability of a given disk having a flyby/encounter and classify the encounter type. The team is also studying a set of disks to understand what happens during each type of encounter and see the implications of flybys for planet formation in other systems. Finally, they looked at possible clues to the flybys that our own Solar System might experience.
Intruder Alert! Disk Attacked!
It all started when star birth took place in a cloud of gas and dust. This process creates a hot pool of young stars clustered together. Over time, some of these clusters disappeared. As stars leave their nests, they may pass close to other systems, causing disturbances in the planet-forming disks. Cuello and his team came to the conclusion that close encounters would excite or even disrupt these disks at some point in their evolution.
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“Stellar flybys and encounters are happening more frequently than previously thought,” Cuello said in an email discussion. “This probably happened when the star was very young (less than a million years old) and had a planet-forming disk around it. These disks are strongly affected by gravitational perturbations of nearby stars, which alter the initial conditions at the start of planet formation. This is why it should be taken into account in our model. ”
Flybys aren’t that rare, according to Cuello. “I would say that at least half of the stars and their disks were affected/formed by flybys,” he said. “One important aspect to highlight is that the probability of such a disturbance diminishes over time but never becomes zero. Thus, even more evolved stars (with surrounding planetary systems) can experience flybys during their lifetime. In this case, some planets may end up in orbits that are out of alignment with other planetary systems or even be captured by a perturbed star.”
How Much Damage Can a Stellar Flyby Do?
In typical star-forming regions, distance matters. The majority of stars with protoplanetary disks undergo close flybys—thousands of astronomical units apart. That equates to about half the distance from the Sun to the Oort Cloud in our Solar System. Some of those encounters can really interfere with the disk. For example, if the intruder star moves in a prograde direction, in a parabolic orbit that penetrates the disk, it can cause enough damage to deform the disk. Sometimes damage by intruders leads to the formation of a second disc of material.
This is exactly what happened with the FU star Orionis. Thanks to a close stellar flyby hitting its disk, FU Orionis appears to shine by a factor of a thousand in about a year. And, such disorders are also seen in other young systems.
During several encounters, the disc goes through what is called a “tidal cut”. It can erase up to 80 percent of disk mass. This had a catastrophic effect on planet formation because the encounter reduced the amount of material needed to form a protoplanet. Such flybys might also create dust traps. Theoretically, it could be a place where planetesimals could grow, if given enough time.
In some cases, close flybys can scatter planets within the system, or even eject planets. Those left behind could be moved into orbits reminiscent of Pluto—eccentric and out of line with the plane of the system. (To be clear, Pluto’s odd orbit isn’t due to flying past. It’s likely that the gravitational influence of Neptune and other giant planets has shaped its odd orbit.)
Flying Stars and Our Solar System
Did our solar system experience stellar flybys during its formation? It is possible that Cuello and his colleagues explored in their paper. Such encounters in or very close to our birth cloud could form a solar nebula. Ultimately it will have an influence on the size of the disk and its mass. It’s hard to know how many times this might happen, but amazingly, the protosolar nebula where the Sun was born is 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 arrangement of the solar system’s orbits could affect the distribution of transNeptune objects (the region just beyond Neptune, where Pluto orbits). It is also possible that one or more stars pass through and interfere with the Oort Cloud. Astronomers have found several candidates they are studying to see if this hypothesis works.
Of course, 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 located about 22 light years from us. The trajectory does not appear to affect the orbits of any planets, but may have a very small effect on the number of Oort Cloud objects ejected into long-term orbits around the Sun. However, it remains a useful example of the effect a passing star has on a planetary system or protoplanetary disk.
For more information
Close encounter: How flying through stars forms a planet-forming disk
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