An international team of astronomers has found a new class of objects in our Solar System. Space rocks that blur the line between asteroids and comets, spewing out jets of dust and gas that are completely invisible to telescopes on Earth. Thanks to these jets, these objects gain speed, accelerating in ways that cannot be explained by taking gravitational forces alone into account. The discovery was published in ‘The Planetary Science Journal’.

Many comets are known to accelerate by periodically emitting bursts of gas and dust as the Sun melts their icy surfaces. Events of this type have been measured in which some comets were able to emit up to 10 kg of dust per second.

The reflection of the sun’s rays on this icy dust causes it to glow, forming the characteristic cometary tails. But asteroids don’t have as much ice available. They tend to be much rockier and don’t produce such emissions.

They look like asteroids, but…

In their article, the researchers report having found a series of objects that, at first glance, look like asteroids, but at the same time they are ‘firing’ jets of dust and gas. These objects, says the article, were discovered by observing how, as they approached the Sun, they periodically suffered “jumps” in their speed, accelerations that could not be attributed to other known factors. In the words of Davide Farnocchia of NASA’s Jet Propulsion Laboratory and co-author of the paper, “there has clearly been an acceleration.”

Observed objects are small. Most, in fact, are no more than a few tens of meters in diameter and all orbit relatively close to Earth. The most interesting thing, however, is that they all ‘shot’ jets similar to those of comets, but with much less intensity, around 0.0001 grams per second. Therefore, the emissions are not visible to telescopes and it is only possible to detect them by the acceleration they cause in the rocks. Farnocchia and his team called them ‘dark comets‘.

“The reason we haven’t found them before,” says Aster Taylor of the University of Chicago and co-author of the study, “is that they are very small objects and we needed many observations for these accelerations to be meaningful. You have to wait months or years to see the acceleration.” In any case, no one has detected anything similar so far. The study therefore opens up the possibility that an entire population of these low-activity ‘inert comets’ could exist hidden in the Solar system.

‘Oumuamua’s secret?

Invisible emissions, mysterious accelerations… Does that ring a bell? Because, according to Farnocchia, those dark comets could hold the key that solves the mystery of the strange acceleration of Oumuamua, the object from another solar system that passed by ours in 2017, leaving more questions than answers. “What happens to these dark comets – risks the researcher – could be the same thing that happened to ‘Oumuamua”.

In any case, in the coming years we will leave doubts. One of the new objects, in fact, about 30 meters in diameter and classified as 1988 KY26, will receive a visit from a Japanese spacecraft in 2031, which will confirm, or refute, the ideas of Farnocchia and his team. At this point, says the researcher, “perhaps we can test our predictions. And we may have to revise our definition of what is and is not a comet.”