What Happens to Interstellar Objects Captured by the Solar System?

Scientists are eager to learn more about interstellar objects (ISOs), now that they have visited our Solar System. What is the best way to capture them? What happens to them if they are captured? What number of them could be in the Solar System?
One group of researchers is working to find the answers.

Two ISOs are known for certain: Oumuamua, and comet 2I/Borisov. There could have been many more. We have only recently been able to view them. We will likely find many more thanks to new facilities such as the Vera C. Rubin Observatory.

A trio of researchers investigated the issue of ISOs in the Solar System, and submitted a paper to The Planetary Science Journal. The paper's title is "On the Fate of Interstellar Objects Captured By Our Solar System". Kevin Napier, from the Dept. University of Michigan.

There is no way to reliably identify individual objects captured. It would be amazing if astronomers could capture an ISO as it is being captured. The Solar System is complex and makes it difficult to identify ISOs. The authors explain that it is difficult to identify whether an object is interstellar in origin due to the complex dynamical architecture found in the outer Solar System.

Oumuamua and comet 2I/Borisov are the only two ISOs that we know of. Images Credits: Left: Original: ESO/M. KornmesserDerivative: nagualdesign Derivative of http://www.eso.org/public/images/eso1737a/, shortened (65%) and reddened and darkened, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=64730303. Right: By NASA, ESA, and D. Jewitt (UCLA) https://imgsrc.hubblesite.org/hvi/uploads/image_file/image_attachment/31897/STSCI-H-p1953a-f-11061106.png, Public Domain, https://commons.wikimedia.org/w/index.php?curid=83146132

Borisov and Oumuamua were not much of a possibility. Their hyperbolic excess velocity was what identified them as ISOs. This means that an object is able to travel at a speed and trajectory that allows it to escape gravity. The Sun is the central object in this example.

Could ISOs then be captured? Quite likely. This question can be answered by rigorously researching it. The authors describe the first step to calculating a capture cross section for interstellar object as a function hyperbolic excess velocity.

According to the authors, this is only the beginning. Although the cross-section is the first step in determining the mass of alien rock in our solar system, the researchers also needed to determine the life expectancy of captured objects.

Three trends were identified by the researchers:

Captured objects must be able to lift their pericenters above Jupiter in order to survive for more than a few millions of years. (In this instance, survival is about staying connected to the Solar System.

Objects in highly inclined orbits are more likely to survive than objects on planar orbits.

Permanent trans-Neptunian status was not achieved by any object (ie q=30 AAU).

If an ISO cannot lift its pericenter above Jupiter, it will likely be pulled into the gas giant, and eventually destroyed. The second scenario is where objects orbiting in highly inclined orbits are less likely than those on planes that are out of the Solar Systems plane to encounter a planet. Planar orbits are more likely than those on inclined orbits to encounter a planet, and then be disturbed and sent back into interstellar. It would be very difficult for an ISO in the third scenario to attain permanent trans-Neptunian status. This is because it would require a very unlikely sequence of events.

The study's figure shows some simulation results. Each blue line represents an ISO. The top shows the osculating distance from the pericenter in AUs. The bottom represents the inclination in degrees. Individual objects are not distinguishable in their simulations until approximately 100 million years. If a blue line crosses, the ISO has left our Solar System. Image Credit: Napier et al 2021.

These simulations are not perfect, as the authors point out. They only account for the Solar System's four largest planets, and the Sun. They don't account for smaller bodies, which are either too small to have any effect or dwarfed by their Sun counterparts. They ignore out-gassing and radiation pressure from Sun. They explain that each of these approximations are very small, so they would not make a significant difference in our conclusions.

The simulation indicates that most of the captured bodies will be ejected from Solar System over time. However, it takes some time. It is because most ISOs will pass through the system. Those that are captured into unstable orbits of some kind would then go through many orbits (30 in this case) before being ejected. Captured objects are typically held at 1000 AU and have orbital periods of around 30,000 years. It takes approximately one million years for any captured ISOs to be ejected.

The study's figure shows the percentage of ISOs that survived over time. The simulation data is shown in black. The blue line represents the best fit. It takes approximately 1,000,000 years for enough orbits to occur for an ISO to be released. Image Credit: Napier et al 2021.

Researchers also estimated the current populations of ISOs captured in our Solar System. The researchers point out that objects that are interesting can be captured in two distinct times periods. In the early days of our Solar System, when the Sun is still within its birth cluster of stars and objects could be captured from that cluster, this is the first. The second is when there is a Sun in the field.

The scientists used 276,691 synthetically captured interstellar objects in their simulations. Only 13 of those survived for 500,000,000 years and only three survived for a billion years. These results have some caveats, which are best explained in this paper.

Their simulations may be helpful in understanding panspermia, the authors suggest. The ISOs are likely to play a part in ensuring that the chemicals required for life or life can travel between solar systems. Perhaps the most important role.

They also mentioned the possibility of a Planet Nine scenario. Konstantin Batygin was one of the authors. He also wrote with Michael E. Brown that he believed there would be a "Planet Nine". According to the Planet Nine hypothesis, another planet with a mass approximately 5-10 times that of Earth orbits in a wide orbit and a semi-major direction of 400-800 AUs. If Planet Nine exists, it would take 10,000 to 20,000 years for one orbit around Sun.

This paper shows that Planet Nine produced rich dynamics when it was included in simulations. It did not appear in simulations that only four of the known giant planets.

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