The results of the study published in the scientific journal Planetary Science Journal suggest that the objects arrived millions of years ago from the nearest planetary system.
The appearance of the Interstellar Objects (ISO) ‘Oumuamua and Comet Borisov in 2017 and 2019, respectively, created a wave of interest. What were they? Where did they come from? Unfortunately, they did not stay to cooperate with our efforts to study them in detail. But they did show us something: objects from the Milky Way move through the galaxy.
We don’t know where either of the ISOs came from, but there must be more, many more. How many objects from our stellar neighbors could be visiting our Solar System?
Objects from our neighbor
The Alpha Centauri (AC) star system is our closest neighbor 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 moves towards us and presents an excellent opportunity to study how material could move between solar systems.
New research to be published in the Planetary Science Journal examines how much AC material could reach our Solar System and how much could already be here. It is entitled “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, but its origins and the 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 is likely to harbor planets and is moving toward us at a speed of 22 km/s2, or about 79,000 km per hour. In about 28,000 years it will reach its closest point and will be about 200,000 astronomical units (AU) from the Sun. According to Greg and Wiegert, the material expelled from AC can and will reach us, and some is already here.
AC is considered a mature star system of about five billion years old that harbors planets. Mature systems are expected to eject less material, but since AC has three stars and multiple planets, it is likely to eject a considerable amount of material. “Although mature star systems are less likely to eject material than those in their planet-forming years, the presence of multiple stars and planets increases the likelihood of gravitational scattering of the members of any remaining planetesimal reservoir, just 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 arrived in our Solar System, and we also know that interstellar dust has arrived in our system. The Cassini spacecraft detected something, and researchers reported on it in 2003. Existing models for the ejection of material from star systems are based in part 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 amounts of AC material. The authors write that “the current number of Alpha Centauri particles larger than 100 meters in diameter within our Oort Cloud is 106,” or 1 million. However, these objects are extremely difficult to detect. Most of them are probably in the Oort Cloud, a great distance from the Sun. The pair of researchers explain that “the observable fraction of such objects is still low” and that there is only a one in a million chance that one is within 10 AU of the Sun.
This animation brings some of the research results to life. “The orbit of Alpha Centauri around the Galactic Center seen in the xy and yz planes (top row), as well as the orbits of the ejection of Alpha Centauri seen in a movable frame (bottom row). Our Sun (Sol) is marked by a black hexagon, and its orbital path is indicated by a solid gray line (top row only). The location and path of Alpha Centauri are shown by a yellow star and a solid blue line (top row only). In the bottom row, the movable frame follows Alpha Centauri around its orbit while maintaining its orientation with the y-axis pointing towards the Galactic Center (blue arrow) and the speed of Alpha Centauri pointing in the -x direction (black arrow). This still image is taken at t? 3000 years (i.e. +3000 years from the present time) after ~ 100 Myr of integration. The colors of the ejection represent the third dimension of position, except that any particle that is at any time within 100,000 au of the Sun is represented in red. This shows the temporal evolution from t? -100 Myr to t? 10 Myr,” write the authors.
The researchers ran simulations to determine how much material may reach us from AC. The simulations ran for 110 million years from t= -100 myr to t= 10 myr. During that time, AC ejected 1,090,000 particles. They were ejected in random directions at different speeds, and only a small number came close to the Sun. “Only a small fraction of the CA ejection is within the CA distance (close approximation) from the Sun. In total, 350 particles had a CA with the Solar System, ~0.03% of the total ejection,” the authors explain.
The research shows that there are plausible ways for CA particles to reach our Solar System. How big can they be?
According to the authors, it is unlikely that the small particles that would appear as meteorites in the Earth’s atmosphere would reach us. They are subject to too many forces on their way, including magnetic fields, drag from the interstellar medium and destruction by sputtering or collisions. “Small particles traveling through the interstellar medium (ISM) are subject to a number of effects that are not modeled here,” they explain.
They calculated the minimum particle size that could make the journey. “We extracted the relevant parameters for each of the 350 ACs in our simulation and calculated the minimum size required for a grain traveling along that trajectory to survive the three effects,” the authors write. They found that a particle with a median of 3.30 micrometers can survive the journey.
“With 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, so our typical particles are effectively magnetically limited,” the researchers explain. ”In fact, all our particles are limited by magnetic forces.” The authors also point out that these small grain sizes are undetectable by meteor radar instruments such as the Zephyr Meteor Radar Network.
These results are hampered by our poor understanding of the rate of material ejection from our Solar System, on which the research is partly based. “Unfortunately, the rate of material ejection from Alpha Cen is poorly constrained,” write Greg and Wiegert.
This figure in the study focuses on the 360 particles that make close approaches. “The heliocentric equatorial radiant for the 350 close approaches at the time of their closest solar approach (”Time of Arrival”), with the current heliocentric equatorial coordinates of Alpha Cen plotted as a black star and the ‘effective radiant’ corresponding to the apparent velocity of Alpha Cen plotted as a red star. The purple shaded region is the combined projection of the effective cross-section of the Solar System (solid angle size as seen from Alpha Cen) from the beginning of the simulation to the present moment.
However, with that in mind, research shows that some material may reach us and is already here. Most of it traveled 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 meteorites in the Earth’s atmosphere today, and that numberincreases by a factor of ten over the next 28,000 years.
This research presents a concrete example of how our Solar System is anything but isolated. If material from stellar systems can move freely between them, it opens up another window into the process of planet formation. If AC harbors exoplanets, some of the material reaching us could be from the same reservoir of material from which those planets formed. It might be possible to learn something about these planets directly without having to overcome the vast distance between us and Alpha Centauri.
“A deep 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 up new avenues for exploring the interconnectedness of stellar systems and the potential for material exchange across the Galaxy,” the authors conclude.
