The object 3I/ATLAS has generated considerable excitement over the past year, prompting astronomers to re-evaluate their understanding of planetary systems beyond our own, as well as our own solar system. However, this third interstellar visitor might unexpectedly influence our grasp of dark matter. In a recently published paper, available as a preprint on arXiv and authored by researchers from the University of Hamburg, an attempt is made to quantify the impact that a large population of interstellar objects (ISOs) could have on our calculations of dark matter within our galaxy.
A portion of dark matter calculations relies on what is known as “missing mass.” This figure is derived from a phenomenon called the Galaxy’s rotation curve, which essentially describes the speed at which stars orbit the center of the Milky Way. The observed rotation curve indicates a much higher speed than would be predicted based solely on the visible stars, suggesting that some unseen mass must be contributing to this increased rotational velocity.
It has long been theorized that dark matter is the source of this missing mass. However, because it interacts minimally, if at all, with anything other than gravity, it remains undetectable by conventional means, making its study exceedingly difficult. Current estimates, informed by data from the Gaia mission, place the concentration of dark matter in our galaxy at approximately 0.44 gigaelectronvolts per cubic centimeter. But what if there’s an alternative explanation for at least a portion of this missing mass?
Interstellar objects themselves possess mass and can be detected through various means. To date, only three have been observed: 1I/’Oumuamua, 2I/Borisov, and 3I/ATLAS, with the largest measuring between 0.16 and 2.8 kilometers in radius. Given that mass scales cubically with radius, even this range of sizes has a significant impact when estimating the mass of this largest known interstellar visitor. Crucially, we also infer that billions, if not trillions, of other ISOs are likely present in the galaxy at any given time.
The central question the researchers sought to answer was straightforward: what proportion of the galaxy’s “missing mass” could be accounted for by stray ISOs that simply elude our current observational techniques? To determine this, they employed a statistical approach known as Poisson distribution to estimate the local density of rogue bodies comparable in size to 3I/ATLAS. Their findings suggest that a substantial number of such free-floating objects may inhabit our region of the galaxy.
By extending this analysis, they calculated the percentage of the galaxy’s “missing mass” that could be attributed to these ISOs. Their results indicated that these objects could conceivably account for approximately 13% to 45% of the galactic mass currently ascribed to dark matter.
It is important to acknowledge certain limitations inherent in this extrapolation process. The most significant is that the calculation is based on extrapolating from a sample size of just one object (3I/ATLAS) to the entire galactic population of ISOs. The authors themselves concede that the upper limit of their estimate, which posits ISOs constituting up to half the missing mass, requires an “overly optimistic” amount of ejected material into interstellar space.
Nonetheless, the underlying mathematical framework is sound and has implications for both existing and future research. Direct dark matter detection experiments, such as LZ and XENONnT, rely on estimates of local dark matter density to predict the flux of weakly interacting massive particles (WIMPs) passing through their xenon-filled detectors. If this density proves to be even 18% lower than initially assumed by the experimental designers, the sensitivity of these instruments may need recalibration.
Fortunately, definitive evidence to support or refute this theory is likely not far off. Data from upcoming next-generation sky surveys are becoming available, and these are expected to identify tens, and possibly hundreds, of new interstellar objects. Once we have a clearer understanding of the prevalence, size distribution, and forms of ISOs, we will be better equipped to assess their potential contribution to our galaxy’s missing mass and, consequently, to refine our understanding of dark matter itself.
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