A dwarf planet has an unusual ring around it, like Saturn's, that shouldn't be there

Credit image: ESA - Artist impression of Quaoar and its ring
Credit image: ESA - Artist impression of Quaoar and its ring

Article by: Andacs Robert Eugen, on 19 February 2023, at 09:13 am PST

Quaoar, which orbits the Sun in the distant Kuiper Belt, is the latest small object to be shown to have a ring like those around Saturn. A small icy world far beyond Neptune possesses a ring like those around Saturn.

Paradoxically, the ring is at a distance where simple gravitational calculations suggest it shouldn't exist. "It's very strange," said Bruno Morgado, a professor at the Federal University of Rio de Janeiro in Brazil. Dr. Morgado is the lead author of a paper recently published in the journal Nature that describes the ring surrounding Quaoar, a planetary body with a diameter of about 700 miles orbiting the Sun at a distance of about four billion miles.

Quaoar is a little less than half the diameter of Pluto and about one-third the diameter of Earth's Moon. It is probably large enough to qualify as a dwarf planet, "shaped" by its gravity into a round object. But no one can say this for sure, because images captured by even the most powerful telescopes have shown Quaoar to be just an indistinct spot. This small spot also has a moon, Weywot.

The ring is not visible in the images obtained by the telescope. Rather, astronomers discovered it indirectly, when distant stars happened to pass behind Quaoar, blocking the light. From 2018 to 2021, Quaoar passed in front of four stars, and astronomers on Earth could observe the shadow of the eclipses.

However, they also observed some dimming of the starlight before and after the star blinked. This indicated a ring that obscured some of the light, an international team of astronomers concluded in their paper published in the journal Nature.

The ring appears to be uneven. In some places, it would be very thin, a few kilometers wide, while in other places, it could be several hundreds of kilometers wide. The ring particles, if collected, would form a moon about five kilometers wide, Dr. Morgado said.

For a long time, astronomers believed that asteroids and other small bodies were too small to have companions like moons and rings. But in recent decades, they have discovered moons around many asteroids and Kuiper belt objects. Then they noticed rings-essentially satellites that failed to coalesce around smaller objects.

In 2013, astronomers discovered several rings around Chariklo, a body orbiting the Sun between Saturn and Uranus. In 2017, a ring was discovered around another Kuiper belt object, Haumea. One possible explanation for Quaoar's distant ring is that the body's moon created gravitational perturbations that prevented ring particles from accreting to another moon. At temperatures in the outer solar system, ice particles are also more consistent and less likely to stick together when they collide.

Michael E. Brown, an astronomer at the California Institute of Technology who was part of the team that discovered Quaoar in 2002, said he was very surprised by the discovery of the ring. "If the data hadn't been so compelling, I would have insisted it wasn't real," he said.

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