When ‘Oumuamua passed through our solar system in 2017 it was the first confirmed interstellar object (ISO) to do so. Then in 2019, comet 2l/Borisov did the same. These are the only two confirmed ISOs to have visited our solar system. Many more ISOs may have visited in our solar system’s long history, and many more will visit in the future. Obviously there are more of these objects, and many more are expected to be discovered by the upcoming Vera Rubin Observatory.
It’s possible that the Sun could capture an ISO or rogue planet in the same way that some planets have captured moons.
It all depends on the phase space.
What would happen if our mature, quiet solar system suddenly had another member? It would depend on the object’s mass and the final orbit in which it finds itself. It’s an interesting thought experiment; whereas Borisov and ‘Oumuamua were small bodies, a more massive rogue planet joining our solar system could generate orbital chaos. This could potentially change the course of life on Earth, although it is highly unlikely.
How likely is this scenario? A new research note in Celestial Mechanics and Dynamical Astronomy describes how our solar system could capture ISOs. It is titled “Permanent Capture in the Solar System,” and the authors are Edward Belbruno of the Department of Mathematical Sciences at Yeshiva University and James Green, formerly of NASA and now from Space Science Endeavor.
Phase space is a mathematical representation that describes the state of a dynamic system like our solar system. Phase space uses coordinates that represent both position and momentum. It is like a multidimensional space that contains all possible orbital configurations around the Sun.
Phase space captures the state of a dynamic system by tracking both position and momentum characteristics. The phase space of our solar system has capture points where an ISO could find itself gravitationally bound to the Sun.
Phase space is complex and based on Hamiltonian mechanics. Things like orbital eccentricity, semi-major axis, and orbital inclination are involved. Phase space is best understood as a multidimensional landscape.
The phase space of our solar system includes two types of capture points: weak and permanent.
Weak capture points are regions in space where an object can be temporarily pulled into a quasi-stable orbit. These points often occur where the outer edges of the objects’ gravitational boundaries meet. They are more like gravitational pushes than orbital lurches.
Permanent capture points are regions in space where an object can be permanently pulled into a stable orbit. The angular momentum and energy of an object is a precise configuration that allows it to maintain an orbit. In planetary systems, these permanent capture points are stable orbital configurations that persist for very long periods of time.
The phase space of our solar system is extremely complex and includes many moving objects and their changing coordinates. Subtle changes in the phase space coordinates can allow objects to transition between permanent capture states and weak capture states. Similarly, subtle differences in ISOs or rogue planets can move them to these points.
In their research note, the authors describe the permanent capture of ISOs as follows: “The permanent capture of a small body, P, orbiting about the Sun, S, from interstellar space occurs when P can never return to interstellar space and remains captured within the Solar System for all future times, continuing to move without colliding with the Sun.” Purists will note that nothing can be the same for all future times, but the point is there.
Other researchers have delved deeper into this scenario, but this work goes one step further. “In addition to being permanently captured, P is also weakly captured,” they write. This revolves around the notoriously difficult to solve three-body problem.
Unlike previous research that uses Jupiter as a third body, this work uses the tidal force of the galaxy as a third body, as well as P and S. “This tidal force has an appreciable effect on the structure of phase space for the velocity range and distance from the Sun we are considering,” they explain in their paper.
The paper focuses on phase space and the theoretical nature of ISO capture. It studies “the dynamical and topological properties of a special type of permanent capture, called permanent weak capture, which occurs for an infinite time.” An object in permanent weak capture will never escape, but will never reach a consistent stable orbit. It approaches the capture set without colliding with the star.
There is not much debate that rogue planets exist, possibly in large numbers. Stars form in clusters that eventually spread out over a wide area. Since stars host planets, some of these planets will disperse through gravitational interactions before the co-born stars achieve some separation from each other.