Astronomers have found a ring of particles surrounding a small, distant space rock at the edge of our Solar System. The ring encircles a strange world called Haumea, a dwarf planet that’s shaped a bit like a squashed egg. Haumea is one of just five officially recognized dwarf planets in the Solar System, but it’s the only one we know of to have its very own ring.
Although Haumea is unique among its peers, this isn’t the first time a ring has been found around a small body like this in our Solar System. In 2014, this same group of astronomers said that they had found two thin rings around a smaller, minor planet called Chariklo that orbits between Jupiter and Neptune. The discovery completely surprised the astronomy community. Up until that point, only the gas giants in our Solar System — Saturn, Jupiter, Neptune, and Uranus — were known to have rings.
But now that rings have been found around another small, distant object, it’s possible that even more bodies far out in our Solar System have rings, too. That poses a puzzle for astronomers: how are these rings forming? Most explanations for ring formation have focused on the biggest planets in our cosmic neighborhood. But now, researchers are going to need to come up with ways to explain how rings are forming around these tiny objects — and how the rings are staying there. “I think that where the rings are coming from, how they’re forming essentially, is going to be a big topic of research,” Amanda Sickafoose, a planetary astronomer at MIT who wrote a Nature editorial on the discovery, tells The Verge.
Astronomers serendipitously found Haumea’s ring, described today in Nature, when they watched the dwarf planet briefly pass in front of a background star, blocking out the star’s light. Such a passing causes a momentary eclipse known as an occultation. The intention was to learn more about Haumea. By observing this quick dip in light, astronomers can gather quite a lot of information about the foreground object, such as its size, shape, and whether or not rings are present.
On January 21st, the astronomy team observed Haumea’s occultation of a distant star — eloquently named URAT1 533-182543 — with 12 different telescopes across Europe. The telescopes helped the astronomers constrain Haumea’s size, shape, and density. And they also observed the star’s light blink out on opposite sides of Haumea, indicating the existence of a ring. “Because they saw that from a number of different sites, they were able to trace that there has to be a ring of material going all the way around Haumea,” says Sickafoose.
Using the data from the 12 different telescopes, the astronomers determined that the ring was about 43.5 miles wide, with a radius of nearly 1,500 miles. It also seems to spin somewhat slowly around Haumea; in the time it takes the ring to do one complete revolution around the dwarf planet, Haumea spins around its own axis three times.
As for how this ring got there, astronomers aren’t quite sure yet. Many of the mechanisms that are thought to have formed rings around the giant planets wouldn’t explain the rings around Haumea and Chariklo. Part of Saturn’s rings, for instance, are made of material spewing from one of the planet’s moons, Enceladus. While Haumea has two moons of its own, they’re too small and too far from the dwarf planet to contribute to the ring, says Sickafoose. Additionally, astronomers believe that the gas giants’ rings may be leftovers from asteroids or other objects from outside the Solar System that got drawn in by the planets and then got torn apart by gravity or collisions. But that explanation also doesn’t work for Haumea and Chariklo. “These small bodies wouldn’t be able to do that,” says Sickafoose.
The likeliest explanation for Haumea’s ring is a large collision of some kind — and there’s some evidence that happened in the dwarf planet’s past. Haumea shares a unique water-ice signature with a handful of other objects in the outer Solar System. That’s a sign that Haumea and these other objects were actually one body in the past, and some collision with another rock broke them apart. It’s possible this same collision also caused a ring of debris. “Because we know there was a collision and we know there’s a ring, there’s a high likelihood those things are tied together,” says Sickafoose.
But even if astronomers come up with an explanation for the ring formation, there’s another riddle to solve: what is keeping the ring there? “Rings are thought to kind of dissipate fairly quickly,” says Sickafoose. Over time, rings are thought to lose energy and fall apart. Also highly energetic particles streaming from the Sun can push on the particles in a ring, causing the debris to fall inward. If Haumea’s ring was formed by a collision, it probably happened hundreds of millions — maybe even billions — of years ago. So something must be preventing the ring from going away.
Above all, this discovery opens up a lot of questions about rings, and Sickafoose thinks many researchers will be trying to answer them in the coming months and years. “How did this form, where did it come from in the first place, and why do some objects have [rings] and others don’t?”