Scientists have discovered a ring system around a small object beyond the orbit of Neptune, a surprising discovery in itself. But the observation also comes with a mystery: How is this ring system possible when, by all accounts, it shouldn’t exist?
The ring in question orbits Quaoar, a small dwarf planet - approximately half the size of Pluto - that lies more than 6.5 trillion miles from the sun. That's roughly 44 times the distance between Earth and our star. Detecting a dense ring around such a small, distant object was no easy feat, but what really stunned the international group of researchers who made the discovery was this: The ring appears to orbit Quaoar too far away. At that distance, the dwarf planet’s gravity should be too weak to tug on the individual particles in the ring to keep them from forming into a moon or moons.
Astronomers say its discovery calls into question existing theories about how ring systems are formed. Quaoar's wispy hoop is at a distance of more than seven planetary radii - that's twice as far out as ring systems were thought possible.
To compare, the main rings around Saturn lie within three planetary radii. It was thought they were able to survive because of their close orbit, as the forces created prevent them from breaking away and growing into moons.
Rings are made of chunks of dust, ice, and other materials, orbiting in a disk around a planetary body. For many years, the large, beautiful rings of Saturn, first observed and characterized in the 1600s, were the only ones known to astronomers. It wasn’t until Voyager 1 passed by Jupiter in 1979 that it was determined the largest planet in our solar system also possessed a ring system, albeit less striking than Saturn’s. It’s now known that all of the solar system’s gas giants possess ring systems.
All of these ring systems but Quaoar’s have one thing in common - the rings orbit within what is known as the Roche limit of their planetary body. This is the distance beyond which the gravitational pull of the planetary body can no longer keep the ring material from forming into larger chunks, which would eventually coalesce into a moon. Inside the limit, the varying strength of gravity on ring particles at different altitudes would keep them spread out.
Astronomers are not exactly sure what's causing Quaoar's anomaly, but they are working on it.