Washington. The small, faraway world called Quaoar, named after a god of creation from Native American mythology, is offering astronomers a few surprises as it orbits beyond Pluto in the frigid reaches of our solar system.
Researchers said Wednesday they have detected a ring around Quaoar similar to the one around the planet Saturn. But the one around Quaoar defies current knowledge of where such rings can form, since it lies much further away from it than current scientific knowledge would allow.
The ring’s distance from Quaoar puts it in a place where scientists believe the particles should easily coalesce around a celestial body to form a moon, rather than remain as separate components in a disk of ring material.
“This is the discovery of a ring located in a place that should not be possible,” said astronomer Bruno Morgado, from the Valongo Observatory and the Federal University of Rio de Janeiro in Brazil, lead author of the study published in the journal Nature.
Discovered in 2002, Quaoar is currently defined as a minor planet and proposed as a dwarf planet, although the International Astronomical Union, the scientific body that deals with such questions, has yet to formally grant it that status.
Its diameter, about 1,110 km, is about one-third that of Earth’s Moon and half that of the dwarf planet Pluto. It has a small moon named Weywot, the son of Quaoar in mythology, with a diameter of 170 kilometers that orbits beyond the ring.
Inhabiting a distant region called the Kuiper belt populated by several icy bodies, Quaoar orbits about 43 times farther than the distance from Earth to the Sun. For comparison, Neptune, the farthest planet, orbits about 30 times farther than Earth. distance from Earth to the Sun, and Pluto about 39 times farther.
Quaoar’s ring was observed with the European Space Agency’s (ESA) Cheops orbital telescope, whose main objective is to study the planets located beyond our solar system, as well as with ground-based telescopes.
The ring, a lumpy disk made up of ice-covered particles, lies about 4,100 km from the center of Quaoar, with a diameter of about 8,200 km.
“The ring systems may be due to remains from the same formation process that originated the central body or to material resulting from a collision with another body and captured by the central body. At the moment we have no clues about how Quaoar’s ring was formed” , says astronomer and co-author of the study Isabella Pagano, director of the Catania Astrophysical Observatory of the Italian research institute INAF.
Unlike any other known ring around a celestial body, Quaoar’s lies outside the so-called Roche limit. This refers to the distance from any celestial body that possesses an appreciable gravitational field into which an approaching object would be pulled. Material orbiting outside the Roche limit should form a moon.
Saturn has the largest ring system in our solar system. The other large gas planets – Jupiter, Uranus and Neptune – have rings, though less impressive, as do the non-planetary bodies Chariklo and Haumea. All are within the Roche limit.
But how can Quaoar break this rule?
“We considered a few possible explanations: a ring made of debris, the result of a putative perturbation impact on a Quaoar moon, would survive for a very short time – but the probability of observing that is extremely low,” Pagano said.
“Another possibility is that icy particle aggregation theories need to be revised, and that particles do not always aggregate into larger bodies as quickly as might be expected.”