What are “black comets”, these strange space debris discovered recently ?

© European Southern Observatory/M. Kornmesser

The discovery in 2017 of Oumuamua (photo at the top of the article), this interstellar object with an elongated shape, had already shaken up the way we categorize celestial objects. Initially classified as an asteroid, it nevertheless displayed an acceleration characteristic of comets as it moved away from the Sun. This behavioral duality would become the signature of a new class of objects: dark comets, remnants of the early days of our solar system, composed of primitive and very dark materials, which orbit in the coldest and most distant regions of our star.

A celestial population with dual characteristics

The work published this week in the journal Proceedings of the National Academy of Sciences reports seven new dark comets, bringing their total number to fourteen. These enigmatic objects are actually separated into two distinct populations.

The External Black Comets, the first category identified, are characterized by highly elliptical orbits that extend well beyond the main asteroid belt. Their trajectory, similar to that of Jupiter-family comets, suggests an origin in the colder regions of the solar system. These objects have prolonged orbital periods, sometimes several Earth years, and their aphelion (the point of the orbit furthest from the Sun) can reach into Jupiter's gravitational zone of influence. This orbital characteristic profoundly influences their composition: periodically exposed to extreme thermal variations, these celestial bodies manifest a subtle but measurable cometary activity.

On the other hand, inner black cometsdescribe near-circular orbits confined to the inner regions of the Solar System. Their more modest size—typically two to three times smaller than their outer counterparts—suggests a distinct evolutionary history. The latter maintain a relatively constant distance from the Sun, implying more stable thermal conditions. Their trajectories frequently place them close to terrestrial planets, including Mars and Earth, making their study particularly relevant for understanding primitive interactions between celestial bodies in the inner Solar System.

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This orbital and dimensional dichotomy is not accidental. As Darryl Seligman, a postdoctoral researcher in physics at Michigan State University and lead author of the study, explains, it likely reflects distinct formation and evolution processes.

Preliminary spectroscopic observations suggest slightly different compositions between the two populations, although their hybrid nature—between an asteroid and a comet—remains their common characteristic. This differentiation could result from complex dynamical processes involving gravitational resonances with giant planets for outer comets, and more subtle interactions with terrestrial planets for inner comets.

The enigma of black comets

The history of these objects begins in 2016 with the asteroid 2003 RM, whose trajectory showed unusual anomalies. Davide Farnocchia, a researcher at NASA's Jet Propulsion Laboratory, recalls: “These disturbances suggested the presence of outgassing characteristic of comets, but no cometary tail was visible. The object appeared as a simple point of light, like a classic asteroid “.

The nature of these orbital disturbances is particularly intriguing to astronomers. Traditionally, such deviations are thought to evoke the characteristic outgassing of comets, a process by which the sublimation of ice creates a thrust that alters the trajectory of the object. However, as Farnocchia noted for 2003 RM, this was not the case. This duality comet behavior in an asteroid body is therefore the distinctive signature of this new class of objects.

Their hybrid nature makes them potential vehicles for the transport of complex molecules. As Seligman points out, these objects may have played a key role in providing the prebiotic compounds essential to the emergence of life on Earth. Further research will one day complete the picture to unravel the mysteries of these black comets. In the long term, space missions could even be considered to study them more closely, or even to bring samples back to Earth and analyze them.

• Black comets, hybrids between asteroids and comets, are primitive vestiges of the solar system.
• The outer comets follow elliptical orbits influenced by the gas giants, while the inner ones orbit in a quasi-circular fashion in the inner solar system.
• These comets could have transported complex molecules essential to the emergence of life on Earth.

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