Scientists have discovered a ring system around a small object beyond Neptune’s orbit, a surprising discovery in itself. But the observation is linked to a puzzle: How is this ring system possible if it shouldn’t actually exist?
The ring in question orbits Quaoar, a small dwarf planet more than 6.5 trillion miles from the Sun – about 44 times the distance between Earth and our star. Discovering a dense ring around such a small, distant object was no easy task, but what really amazed the international team that made the discovery was this: the ring appears to be orbiting Quaoar too far away. At that distance, the dwarf planet’s gravity should be too weak to pull on the individual particles in the ring, preventing them from forming into one or more moons.
“This ring isn’t where we expected it to be,” says Bruno Morgado, an astronomer at the Federal University of Rio de Janeiro and lead author of a new paper published in the journal Nature told popular science in an email. “This could change our knowledge of how rings are formed.”
As University of Idaho physicist Matthew Hedman put it a comment appeared in the same issue of Nature this places Quaoar’s ring system “contradictory with our current understanding of how such rings are maintained”.
Rings are made of dust, ice, and other materials that orbit in a disk around a planetary body. For many years, Saturn’s large, beautiful rings, first observed and characterized in the 17th century, were the only ones known to astronomers. It wasn’t until Voyager 1 passed Jupiter in 1979 that it was discovered that the largest planet in our solar system also has a ring system, albeit less prominent than Saturn’s. It is now known that all of the gas giants in the solar system have ring systems. Rings have also been found between some distant bodies, such as B. Humea, which like Quaoar—and Eris and Pluto, for that matter—are considered Trans-Neptunian objects.
All of these ring systems except 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 at which the gravitational pull of the planet’s body can no longer prevent the ring material from forming into larger chunks that would eventually coalesce into a moon. Within the limit, the different strengths of gravity on ring particles at different altitudes would keep them distributed.
But Quaoar’s Roche limit is about 1,100 miles from its center. The dense ring orbits 2,500 miles from the center of the dwarf planet. “This means the mutual attraction of water ice chunks [in the ring] should easily overpower fluctuations in Quaoar’s gravitational pull,” writes Hedman. “We therefore need another explanation for why this material has not accumulated into a moon.”
Morgado and his colleagues consider several explanations. One is that the ring material is relatively new, resulting from impacting a moon orbiting Quaoar and simply not having time to rejoin. But that’s unlikely, they write in the paper, as their computer models suggest this material would condense into a new moon within decades.
It’s also possible that the ring material itself is more elastic than the models predict. In this case, each chunk would bounce off another rather than stick, even without the tugging of Quaoar’s gravity to keep them apart.
Another possibility is that the ring system is periodically perturbed by the gravity of another object, such as Quaoar’s moon, Weywot, or another moon yet to be discovered. Finally, it is very difficult to study trans-Neptunian objects. This ring discovery was only possible because the research group brought a wide array of state-of-the-art telescopes to Quaoar. These include the characterizing ExOPlanet satellite of the European Space Agency ESA (Cheops) mission, a space telescope that was able to discover the ring of Quaoar by monitoring the dwarf planet for changes in background starlight intensity.
“Now we need to continue monitoring Quaoar to better define this ring, and also see if (and how) it changes over time,” says Morgado. “More dynamic studies and simulations also need to be done to see under what circumstances a ring so far outside the Roche limit is stable.”
The results of further studies could force astronomers to change their understanding of the Roche limit. It’s also possible that Quaoar is an exception to the rule — but one that allows for a deeper understanding of the orbital dynamics of other large structures, Morgado says, “even other objects like exoplanets, galaxies.”