The appearance of the Interstellar Objects (ISOs) Oumuamua and Comet Borisov in 2017 and 2019, respectively, created a surge of interest. What were they? Where did they come from? Unfortunately, they didn’t stick around and wouldn’t cooperate with our efforts to study them in detail. Regardless, they showed us something: Milky Way objects are moving around the galaxy.
We don’t know where either ISO came from, but there must be more—far more. How many other objects from our stellar neighbours could be visiting our Solar System?
The Alpha Centauri (AC) star system is our nearest stellar neighbour and consists of three stars: Alpha Centauri A and Alpha Centauri B, which are in a binary relationship, and Proxima Centauri, a dim red dwarf. The entire AC system is moving toward us, and it presents an excellent opportunity to study how material might move between Solar Systems.
New research to be published in the Planetary Science Journal examines how much material from AC could reach our Solar System and how much might already be here. It’s titled “A Case Study of Interstellar Material Delivery: Alpha Centauri.” The authors are Cole Greg and Paul Wiegert from the Department of Physics and Astronomy and the Institute for Earth and Space Exploration at the University of Western Ontario, Canada.
“Interstellar material has been discovered in our Solar System, yet its origins and details of its transport are unknown,” the authors write. “Here we present Alpha Centauri as a case study of the delivery of interstellar material to our Solar System.” AC likely hosts planets and is moving toward us at a speed of 22 km s?1, or about 79,000 km per hour. In about 28,000 years it will reach its closest point and be about 200,000 astronomical units (AU) of the Sun. According to Greg and Wiegert, material ejected from AC can and will reach us, and some is already here.
AC is considered a mature star system about five billion years old that hosts planets. Mature systems are expected to eject less material, but since AC has three stars and multiple planets, it likely ejects a considerable amount of material. “Though mature star systems likely eject less material than those in their
planet-forming years, the presence of multiple stars and planets increases the likelihood of gravitational scattering of members from any remnant planetesimal reservoirs, much as asteroids or comets are currently being ejected from our Solar System,” the authors write.
We know that macro objects like Borisov and Oumuamua have reached our Solar System, and we also know that interstellar dust has reached our system. The Cassini probe detected some, and researchers reported on it in 2003. Existing models for material ejection from star systems are partly based on what we know about our Solar System and how it ejects material, and Greg and Wiegert based their work on those models.
The research shows that there are potentially large quantities of material from AC. The authors write that “the current number of Alpha Centauri particles larger than 100 m in diameter within our Oort Cloud to be 106,” or 1 million. However, these objects are extremely difficult to detect. Most of them are likely in the Oort Cloud, a long distance from the Sun. The pair of researchers explain that “the observable fraction of such objects remains low” and that there is only a one-in-a-million chance that one is within 10 AU of the Sun.
The researchers ran simulations to determine how much material can reach us from AC. The simulations ran for 110 million years from t= -100 myr to t= 10 myr. During that span, AC ejected 1,090,000 particles. They were ejected in random directions at different speeds, and only a tiny amount came anywhere near the Sun. “Only a small fraction of the AC ejecta come within the CA (close approach) distance of the Sun. In total, 350 particles had a CA with the Solar System, ~0.03% of the total ejecta,” the authors explain.
The research shows that there are plausible pathways for particles from AC to reach our Solar System. How large can they be?
According to the authors, small particles that would appear as meteors in Earth’s atmosphere are not likely to reach us. They’re subjected to too many forces on their way, including magnetic fields, drag from the interstellar medium, and destruction through sputtering or collisions. “Small particles travelling through the interstellar medium (ISM) are subject to a number of effects not modelled here,” they explain.
They computed the minimum size of particles that could make the journey. “We extracted the relevant parameters for each of the 350 CAs from our simulation and computed the minimum size needed for a grain travelling along that trajectory to survive all three effects,” the authors write. They found that a particle with a median of 3.30 micrometres can survive the journey.
“At this size and speed, the particle can travel 125 pc in the ISM before grain destruction becomes relevant, 4200 pc for ISM drag, and only 1.5 pc for magnetic forces, and thus our typical particles are effectively magnetically limited,” the researchers explain. “In fact, all of our particles are limited by magnetic forces.” The authors also point out that these tiny grain sizes are undetectable by meteor radar instruments like the Zephyr Meteor Radar Network.
These results are hampered by our poor understanding of our Solar System’s material ejection rate, on which the research is partly based. “Unfortunately, the rate of ejection of material from Alpha Cen is poorly constrained,” write Greg and Wiegert.
However, with that in mind, the research shows that some material can reach us and is already here. Most of it travelled for less than 10 Myr to reach us, but it has to be larger than about 10 microns to survive the journey. It also estimates that about 10 particles from Alpha Centauri become detectable meteors in Earth’s atmosphere currently, with that number increasing by a factor of ten in the next 28,000 years.
This research presents a concrete example of how our Solar System is anything but isolated. If material from star systems can move freely to and from one another, it opens up another window into the planet formation process. If AC does host exoplanets, some of the material reaching us could be from the same reservoir of material that those planets formed from. It could be possible to learn something about those planets directly without having to overcome the vast distance between us and Alpha Centauri.
“A thorough understanding of the mechanisms by which material could be transferred from Alpha Centauri to the Solar System not only deepens our knowledge of interstellar transport but also opens new pathways for exploring the interconnectedness of stellar systems and the potential for material exchange across the Galaxy,” the authors conclude.
Research: A Case Study of Interstellar Material Delivery: Alpha Centauri