Tidal Steams of Interstellar Objects May Flow Through the Milky Way Like Braided Rivers

We know that interstellar objects occasionally visit our solar system. So far, we have only discovered two interstellar objects (ISOs), but that’s mainly because we can only distinguish them from solar system bodies by their orbital motion, and that takes a series of observations over time. The two we have discovered, ?Oumuamua and Borisov, were only noticed because they had highly unusual orbits that moved through the inner solar system. But when sky survey telescopes such as the Vera Rubin Observatory come online, we will likely find new interstellar objects all the time. It’s estimated that several ISOs enter the solar system every year, and there could be hundreds of them passing by at any given time. But that raises an interesting question about how these objects arrive. Do they enter our solar system randomly from all directions, or do they appear in clusters a few at a time?

That is the question addressed in a new paper on the *arXiv*. It looks at how streams of interstellar bodies might escape a solar system and how they would then move through the Milky Way. While it is always possible for a random asteroid or comet to have a random flyby near a large planet that tosses it out of a system, most interstellar bodies would occur during the tumultuous early period of a star system when it is cleared of debris. In our solar system this process created the Oort cloud surrounding the Sun, which likely occurred in other systems. So star systems likely create bursts of ISOs, which then stream through the Milky Way until they encounter another star system.

To study all this, the team started with simulated bursts and then modeled how the resulting streams would evolve. Based on the distribution and typical ages of stars in our region of the Milky Way, they could then simulate streams that might intersect the Sun’s path through the galaxy. They found that the Sun likely intersects with streams regularly, which would give us plenty of interstellar visitors. They also found that just as Earth experiences meteor showers as it passes through the remnant debris streams of comets, the Sun would experience bursts of interstellar objects as it passes through a stream. With the abilities of Rubin and other observatories, we should be able to identify ISOs that are “siblings,” having originated from the same star system. Over time, this could give us a better idea of the composition and diversity of planetary systems. We may even be able to pinpoint the origin of sibling ISOs to a particular star.

One final interesting aspect of these interstellar streams is their overall appearance. Several of the authors of this work are from Aotearoa New Zealand, and they noticed that the simulated patterns have a braided appearance similar to the braided rivers such as Rakaia on the South Island. So they named these interstellar streams he awa whiria, which means the braiding rivers in te reo M?ori, as an homage to that region and its people. It’s a nice reminder of our humanity and the connection between the world around us and the sky above.

Reference: Forbes, John C., et al. “He awa whiria: the tidal streams of interstellar objects.” arXiv preprint arXiv:2411.14577 (2024).