Seven new dark comets discovered in our solar system

Dark comets have become a buzzworthy subject among astronomers since researchers announced the discovery of seven additional examples of these mysterious objects.

In direct observations, they seem to behave like asteroids, but accelerate in ways that make them act like comets. The newfound group doubles the tally of known dark comets to fourteen, causing excitement and questions about what might lurk behind their secretive surfaces.

Dark comets don’t fit the mold

Dark comets first drew attention when a near-Earth object defied the expectations set by ordinary asteroids. In most cases, tiny effects like sunlight heating an asteroid’s surface account for small changes in its path.

However, the unexpected shift these objects displayed hinted at a different story. Scientists kept an eye on them and, in 2017, a major clue appeared in the form of 1I/2017 U1 (‘Oumuamua), which looked like an asteroid but moved as if gas was pushing on it. 

That puzzling find nudged researchers to ask whether they had encountered a new type of comet altogether.

Asteroids that behave like comets

Not long after those early revelations, astronomers noticed a small group of objects that resembled asteroids yet behaved like comets.

Davide Farnocchia of NASA’s Jet Propulsion Laboratory in Southern California described one of these oddities in vivid terms. 

“When you see that kind of perturbation on a celestial object, it usually means it’s a comet, with volatile material outgassing from its surface giving it a little thrust,” said Farnocchia.

He explained that researchers tried to identify a typical comet’s tails as part of the objects, but found no visible signs. Eventually, these mysterious objects began fitting the description of so-called “dark comets.”

Darryl Seligman, a postdoctoral fellow in the Department of Physics at Michigan State University in East Lansing, took a particular interest in their unusual orbits and appearances.

His analysis of their reflectivity revealed that they come in two distinct groups. That simple observation had larger implications for categorizing these cryptic wanderers.

The puzzle of two types

New data suggest that some dark comets hang out in the outer solar system and others live closer in. The ones spinning around the outer region display orbits that are long and eccentric, similar to known Jupiter-family comets.

These comets measure hundreds of yards or more across and have striking similarities to typical ice-rich bodies. Meanwhile, others are located between Mercury and Mars, and follow almost circular paths.

They tend to be much smaller (tens of yards or even less in diameter) and keep a low profile in the inner solar system.

What’s inside these dark comets?

Scientists wonder about the presence of ice within these odd objects. Observations hint that hidden ices might explain their comet-like movements, but telescopes have yet to detect the kind of outgassing commonly associated with classic comets. 

This gap in the data keeps the question open: could these objects still hold pockets of volatile material, or is something else responsible for the boost to their speed?

Big questions about origins

Researchers also ponder how dark comets formed. One theory suggests that they may have come from the same raw materials as regular comets (ice and dust) except that they lost the usual bright halo that comets develop under solar heating. 

Another possibility is that collisions or extreme conditions triggered transformations that hid their comet-like traits from casual observation. 

The truth is still hidden, and ongoing studies aim to shine more light on these elusive wanderers of the solar system.

Clues for life’s ingredients?

“Dark comets are a new potential source for having delivered the materials to Earth that were necessary for the development of life,” said Seligman.

Though these objects appear bland at first glance, Seligman believes they could have contributed to Earth’s building blocks. 

He and his colleagues wonder whether these disguised comets might have ferried water or organic compounds during the solar system’s early days.

This possibility, if confirmed, would reshape how we think about the origin of essential elements that allowed life to emerge.

Paths forward

Scientists intend to keep a close watch on dark comets. Improved imaging methods, more powerful surveys, and interplanetary missions offer ways to analyze them more precisely. 

Some teams propose sending spacecraft to rendezvous with one in order to scoop up material for direct observation.

Others plan to study the objects’ reflective properties from ground-based observatories, in search of subtle clues that might indicate the presence of ices. 

Each new discovery has the potential to refine, or rewrite, what astronomers know about how our solar system has evolved.

What’s next for dark comets?

There is no shortage of mysteries surrounding these objects. Still, the announcement of seven more examples and the identification of two distinct categories represent steps toward a deeper understanding of the boundary between asteroids and comets. 

As astronomers refine their techniques and explore innovative ways to detect concealed ices, the next discovery might reveal even more unexpected members of this quiet but intriguing family.

The study is published in the Proceedings of the National Academy of Sciences (PNAS).

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