Dark comets, a new class of stars

It is increasingly clear that there are comets, with very specific characteristics, that are not like the others. This minority class of comets is characterized above all because, unlike the more common comets, the dark ones, as their name suggests, do not shine at all. These dark comets look like asteroids but move like comets. Now, a new study points to the existence of two subtypes of dark comets.

The study is the work of a team that includes, among others, Darryl Seligman, from Michigan State University, and Davide Farnocchia, from NASA’s Jet Propulsion Laboratory (JPL), both institutions in the United States.

The first dark comet was detected a few years ago. Not long after, six others were found.

In the new study, another seven have been discovered, thus doubling the number of known dark comets. The authors of the study have also realized that dark comets belong to one or the other of two populations of them: that of large ones (hundreds of meters or more from end to end) that reside on the periphery of the solar system and that of the small ones (tens of meters) that reside in the central region of the solar system (the orbital strip that includes Mercury, Venus, Earth and Mars).

The first indication of the existence of dark comets was obtained when observing, in a 2016 study, that the trajectory of the “asteroid” 2003 RM had deviated from its predicted orbit without any object having disturbed it with its gravity. That deviation could not be explained by the non-gravitational forces that typically affect asteroids, such as the Yarkovsky effect. This effect is the small push provided by the Sun when heating an asteroid, which absorbs its light and re-emits it as heat. Over time, this effect can influence the asteroid’s trajectory.

The deviation observed in the 2003 RM orbit was in line with what we would expect from a comet, with volatile material expelled in the form of gas from its surface, giving it a thrust considerably greater than that received by asteroids from the Yarkovsky effect.

In the new study, Seligman, Farnocchia and their colleagues have determined that the subtype of dark comets characterized by large size and residing in the periphery of the solar system has characteristics similar to those of comets of the Jupiter family: the main one is that both have very elliptical orbits.

Dark comets move like comets, but due to their low brightness they are more similar to asteroids than comets. (Image: NASA)

As for the subtype of dark comets characterized by a small size and residing in the central area of ​​the solar system, they have verified that their orbits are almost perfectly circular.

The study is titled “Two distinct populations of dark comets delineated by orbits and sizes.” And it has been published in the academic journal Proceedings of the National Academy of Sciences (PNAS). (Fountain: NCYT by Amazings)