The term “dark comets” was recently attributed in a series of papers to a new class of anomalous Solar system objects that pass near Earth and exhibit weak non-gravitational acceleration without showing evidence for an associated cometary tail. The concept “dark comets” is an oxymoron because a comet is identified visually by its coma. The new nomenclature resembles calling animals without stripes “unstriped zebras”. We all know of many animals that could fall into this category. Light sails or spacecraft propelled by engines that do not release exhaust gas would also be cataloged as dark comets, in the same way that elephants can be called unstriped zebras.
It is also clear that if the modeling of the motion of objects did not include an important non-gravitational effect or relevant post-Newtonian corrections offered by Einstein’s General Relativity, then the experts would deduce the existence of dark comets. In this case, the anomalous acceleration has nothing to do with the rocket effect due to an invisible cometary evaporation. Instead, it reflects a residual acceleration that was not accounted for in describing the dynamics of the object. Some of the dark comets exhibit residual accelerations with a fractional amplitude comparable to the square of their speed relative to the speed of light, (v/c)², as expected from General Relativistic corrections to Newtonian dynamics.
Before Einstein offered General Relativity, the anomalous precession of the perihelion of Mercury was recognized in 1859 as a problem in Newtonian mechanics by Urbain Le Verrier, who suggested that another hypothetical planet might exist.
The brilliant playwright Josh Ravetch told me that the actress Stefanie Powers, who was once married to the actor William Holden, continues to manage the William Holden Wildlife Foundation in Kenya, which hosts an albino zebra. I noted that one can still see the faint stripes in an albino zebra, so indeed it should count as a zebra. But in the case of dark comets, there is no visible coma which is the defining signature of comets, so why is it justified to label them as comets? A more accurate label would be “anomalously accelerating objects”.
Of course, cataloging a new class of unidentified objects as `anomalous’ would suggest that comet experts may have missed something entirely new. Labeling them dark comets provides psychological peace of mind, implying that these anomalous objects are actually familiar. Anomalies create a cognitive dissonance for experts. The mental tension is relieved by associating the anomalous entities with a new category of familiar entities.
An expert on solar system rocks told me after the anomalous interstellar object `Oumuamua was discovered: “`Oumuamua is so weird; I wish it never existed.” I was shocked to hear that, because a curious scientist should be thrilled to learn about anomalous objects — as they offer us an opportunity to learn something new. We should welcome the rare privilege of expanding our knowledge base by admitting humbly that nature could be more imaginative than we are. There are numerous examples from the history of science that reflect this state of affairs, including the discovery of quasicrystals in 1982 by Dan Shechtman which earned him the Nobel Prize. Quasicrystals were considered impossible in traditional solid-state textbooks.
Cataloging `Oumuamua as a member of a class of dark comets makes it sound familiar. The only problem is that `Oumuamua exhibited a non-gravitational acceleration which was in the range of 150–500,000 times larger than the Solar system objects in the new class of dark comets. If any of these objects are comets with low evaporation rates — in accordance with their weak non-gravitational accelerations compared to `Oumuamua, the question remains whether their coma would stay invisible if their evaporation rate was enhanced by factors of 150–500,000 to match `Oumuamua’s acceleration. Clearly, the albino zebra would look like a normal zebra if its stripes were darkened.
There is no doubt that psychological biases shape the practice of science, especially in the face of anomalies. In the early decades of studying dark matter, it was popular to argue that dark matter is composed of dark objects which are made of ordinary matter, such as stellar remnants. This option was ruled out by microlensing surveys like MACHO Project in search for Massive Compact Halo Objects (dubbed as MACHOs), which could be traced as gravitational lenses that brighten temporarily background stars. Based on new data from these surveys, stellar remnants were ruled out as the source of dark matter in the halo of the Milky-Way galaxy.
As this dark matter example illustrates, the mainstream tendency to label anomalous entities as a new category of known entities does not get rid of these anomalies if new data rules out conventional interpretations. As Shakespeare reasoned in the play Romeo and Juliet: “A rose by any other name would smell as sweet.” In the context of science, this statement can be rephrased as follows: “An anomaly by any other name would remain as intriguing.”
In order to overcome the smoke screen of labels invented by experts to hide anomalies as familiar objects, we need better data. Gladly, additional interstellar objects from the class of `Oumuamua could be discovered by the new 3.2 gigapixel camera of the Rubin observatory in Chile, starting in 2025. Here’s hoping that some of the exquisitely monitored objects will exceed the limited imagination of comet experts. A wealth of scientific data offers the privilege of learning something truly new.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
