Here’s an interesting thought. We know that at least two objects from outside our Solar System have appeared in our skies, the comet 2I/Borisov and the still enigmatic object called ‘Oumuamua. Most attention on these visitors has focused on their composition and the prospects of one day visiting such an interloper, for it is assumed that with new technologies like the Vera Rubin Observatory and its Legacy Survey of Space and Time (LSST), we will be picking up more of the same.
But consider origins. Extrapolating backward to figure out where either object came from quickly exhausts the most patient researcher, for it only takes the slightest changes in trajectory to widen the search field so broadly as to be useless. That’s especially true since we don’t know the ages of the objects, which may span hundreds of millions of years.
Enter Cole Gregg (University of Western Ontario), who has embarked on a project to study the question from a different perspective. Gregg asks how likely it is that material from another star could be passing through Sol’s neighborhood. As presented at the American Astronomical Society’s Division for Planetary Sciences meeting in Boise, Idaho earlier this month, his calculations explore the gravitational interactions such objects would experience. And he studies whether ‘streams’ of such material could identify by their common characteristics the likely system of origin.
The question is intriguing given how difficult it is to get a probe to Alpha Centauri at present levels of technology. Material from that system, if identified as such, would have interesting implications, especially with regard to the dispersal of chemical elements and organic molecules. The idea of panspermia rides on the possibility that life itself might move between planets and even stars in such material, so getting our hands on something from another star would offer obvious benefits.
Gregg is working on this project with colleague Paul Wiegert, who is perhaps best known for pioneering work with Matt Holman back in the 1990s on planetary orbits possible in the Alpha Centauri system. That work established that rocky terrestrial worlds could remain on stable orbits around Centauri A or B within 3 AU of the host stars, and a bit further out if the planetary orbit was retrograde. As these orbits were stable for billions of years, this finding was an incentive for deeper investigation into Alpha Centauri as a possible home for life.
Gregg and Wiegert now turn their attention to what they call ‘interstellar meteor streams,’ analyzing the development of such streams as they form and “as the material evolves in time through a time-independent, asymmetric potential model for our galaxy.” Their method is to conduct simulations with existing galactic models that involve ejected material in the galactic bulge, the halo and the disk, with the direction of ejection oriented randomly and at speeds of up to 15 kilometers per second.
We’re early in the process here (remember that this work has just been presented), but quoting from a brief write-up made available for a 2023 conference:
As expected, since the physics of their motion is identical, we see the ejecta travel through the disk of the Galaxy in much the same way that stars do. However, through the progression of the simulation we see complicated and interesting effects on the ejecta swarm, hinting at the complicated evolution of interstellar meteoroid streams. This talk will also discuss the implications of the modeled motion on the identification of interstellar meteoroid streams.
For stars in the galactic disk (which is where we are), material ejected from their systems evolves in a way similar to meteoroid streams we see in the Solar System, undergoing ‘orbital shear’. These streams can be long-lived and may include a flux of material passing through our Solar System that, while small, would be of great interest. Gregg sees his ongoing work as refining the process of interstellar transfer and producing population estimates for interstellar visitors currently nearby.
Image: A hypothetical interstellar meteoroid stream after 185 Myr of evolution from a burst ejection originating in the Alpha Centauri system. Credit: Cole Gregg.
As to the Centauri system, Gregg told Sky & Telescope in an article published this month that his calculations using a simplified model of the Milky Way show that about 0.03 percent of material ejected from Alpha Centauri could reach the Solar System, and perhaps most important, be recognized as coming from that source. We’re talking about material that could be as small as dust grains and as large as a comet or asteroid, but moving within a stream that has similar orbital velocities within the galaxy and similar positions, the latter traits making their source identifiable.
Gregg’s work is intriguing and also preliminary, for he intends to produce a full follow-up study looking to refine the galactic model and include better estimates on the likely size of the material that would have made such a crossing. So at this point what we have is only a possibility that may be excluded if the further analysis rules it out. But it’s a worthwhile study given the implications of finding such a stream.
The Sky & Telescope article is from October 18, 2024, titled “Are Objects from Alpha Centauri Streaming by Earth?” (available here). Gregg’s writeup for a 2023 conference is “The Development of Interstellar Meteoroid Streams” (full text). A PowerPoint presentation can be found here. The Wiegert and Holman paper, a key paper in Alpha Centauri studies, is “The Stability of Planets in the Alpha Centauri System,” Astronomical Journal 113 (1997), 1445–1450 (abstract).
