Interstellar Comets vs. Dark Matter express
Recently, astronomers spotted three interstellar comets coming from deep space. One of them, 3I/ATLAS, turned out to be unexpectedly large: its nucleus is several kilometers across. This got scientists thinking: there might be countless such icy wanderers throughout the Galaxy, with a combined mass comparable to all visible stars combined, around 50 billion solar masses.
This much ordinary matter changes the picture of mass distribution. For a long time, the fast rotation of galaxies was explained by the presence of invisible dark matter, a hypothesis proposed by Fritz Zwicky and confirmed by observations by Vera Rubin. But now there's an alternative: we simply overlook a huge amount of ordinary objects, like weighing a bag without realizing it's stuffed with thick books. The weight seems to appear from nowhere, but it's actually hidden in plain sight.
New calculations suggest that the density of dark matter near the Sun could be almost half of previous estimates. To be fair, explaining all rotation with just comets would require an absurdly large number. But the discovery itself is a reminder: we still have a poor idea of how much ordinary matter lurks in the interstellar darkness. And these comets themselves are time travelers: unchanged for billions of years, they even witnessed the birth of our Sun.
Original abstract
as written by the authors arXiv:2605.04801
The recent discovery of three interstellar comets in the solar system indicates the presence of so-far unaccounted baryonic matter in the Galaxy as a population of inter-stellar objects (ISO). The contribution of ISOs to the overall mass budget of the Galaxy affects the estimates on mass of the non-baryonic dark matter halo. We are attempting to estimate the mass density of non-baryonic Dark Matter after including a Galactic ISO contribution to the Galactic rotation curve. The object 3I/ATLAS is a surprisingly massive object with estimates of the nuclear radius reaching up to few kilo-metres. The observed incidence rate of interstellar objects (ISO) passing through the inner solar system in combination with estimates on the mass density and size provides an estimate of the local mass density if ISOs in the interstellar medium. The resulting estimate carries large uncertainties which are the consequence of the difficulties to constrain or measure the nuclear radius. The large kinematic age of 3I/ATLAS motivates a model where ISO objects are distributed in a thick (0.8~kpc) disk with a large radial scale length of $$\approx 7$$~kpc estimated from a fit to rotational velocity measurements from GAIA DR3 data. We find that the ISO contribution to the baryonic mass budget could reach a total mass of $$5\times 10^{10}~M_\odot$$ which leads to a reduction of the local Dark Matter halo density to $$0.24$$~GeV/cm$$^3$$. Even though this scenario requires an overly optimistic fraction of matter to be released in the form of ISO objects, it is plausible that the local Dark Matter halo density is biased towards large values given our ignorance of non-detectable baryonic matter in the Galaxy.
Some interstellar comets, like 3I/ATLAS, have been wandering the Galaxy almost since its birth: their age rivals that of the Milky Way itself.
This concept is reminiscent of Arthur C. Clarke's 'Rendezvous with Rama,' only with giant comets wandering for millions of years instead of an alien spaceship.