Second ‘Impossible’ Ring Found Around Distant Dwarf Planet

Scientists still do not fully understand how dust and gas in the early solar system coalesced into moons and planets.

Like the first ring around Quaoar, announced by a team of astronomers in February, the second ring lies beyond what is known as the Roche limit. Material orbiting closer than this distance tends to be pulled apart by tidal forces. Thus, a ring within the Roche limit would tend to remain a ring, while a ring of debris outside the Roche limit would usually coalesce into a moon.

For Quaoar, the Roche limit is calculated to be 1,100 miles. The second ring, at a distance of 1,500 miles from the center of Quaoar, is closer than the ring announced in February, which has a radius of about 2,500 miles.

Quaoar (pronounced KWA-wahr, and the name of the creator god for the Indigenous Tongva people who live around Los Angeles) orbits the sun in the Kuiper belt, a region of frozen debris beyond Neptune that includes Pluto.

The ring is not visible in telescope images. Rather, astronomers found it indirectly, when distant stars happened to pass behind Quaoar, blocking the starlight. From 2018 to 2021, Quaoar passed in front of four stars, and astronomers on Earth were able to observe the shadow of the eclipses, also known as stellar occultations.

They also observed some dimming of the starlight before and after the star blinked out, which pointed to the existence of the first ring.

Another occultation occurred on Aug. 9 last year, and astronomers again pointed telescopes, large and small, at Quaoar in hopes of learning more about the ring.

The new observations revealed more details including a dense, narrow core in the ring just a few miles wide, which is surrounded by an envelope of material that is more dispersed. The observations also revealed the second ring.

Another occultation will occur on May 13, visible to telescopes in the United States and Canada.

“This event involves a bright star and will be useful to constrain the Quaoar shape better, in addition to being a good opportunity to obtain more details about these two remarkable rings,” Mr. Pereira said.

A potential explanation for Quaoar’s distant rings is the presence of a moon, Weywot. The moon may have created gravitational disturbances that prevented the ring’s particles from accreting into additional moons. Both rings occur in locations near what are known as resonances with Weywot, and the resonances may turn out to be more important than the Roche limit for determining whether rings turn into moons or remain as rings.

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