Directions for teachers: Ask students to read the online Science News article “The Kuiper Belt’s dwarf planet Quaoar hosts an impossible ring” and have them answer the following questions. A version of the article, “This dwarf planet hosts an odd ring,” appears in the March 11, 2023 issue of Science News.

1. What and where is Quaoar? Can you also name the galaxy in which it resides?

Quaoar is a dwarf planet on the edge of our solar system in the Kuiper Belt. Quaoar is in the Milky Way galaxy.

2. Explain when “stellar occultation” occurs and why scientists use the phenomenon to study planets and other objects in space.

Stellar occultation occurs when the light from a distant star is blocked by a planet passing in front of the star as viewed from Earth. When the planet blocks the star, the planet is silhouetted, and light from the star can be seen around the edge of the planet. The changes in the patterns of the starlight around the planet help astronomers learn about a planet’s atmosphere and whether it has rings.

3. What did scientists studying Quaoar discover using stellar occultation?

The scientists discovered that Quaoar appears to have no atmosphere, and it has a ring.

4. If a planet has a ring, where is the ring supposed to be relative to the planet, according to current scientific thinking? What makes the Quaoar discovery extraordinary?

In current thinking, rings are expected to be close to a mathematically determined distance from the planet they are near. This distance is called the Roche limit. The ring around Quaoar is far beyond where scientists would have expected to see a ring.

5. In current scientific thinking, what would scientists expect to happen in rings that are far beyond a planet’s Roche limit? How is the ring around Quaoar unusual?

Beyond the limit, rings are expected to be unstable as a result of the lower gravitational force from the planet, and the objects in the rings are expected to clump to form one or more moons. Because the ring around Quaoar was found far beyond the planet’s Roche limit, scientists would have expected the ring to clump to form one or more moons. However, the ring around Quaoar, so far, has remained a ring.

6. What has this recent discovery done to scientists’ thinking about rings around planets, and what will they need to look for to better understand the Quaoar ring finding?

The finding has forced scientists to re-examine what they thought they understood about the rings around bodies in space. The scientists need to continue to observe Quaoar’s ring and make more observations of other rings in the solar system. In particular, the researchers need to find other rings that are far beyond the Roche limit of the planets they surround to see if they behave the same way the Quaoar ring behaves.