The dwarf planet Haumea orbits the sun in the deep, dark reaches of the outer solar system. Astronomers spotted this object in 2004, and it helped usher in the formal definition a “dwarf planet,” resulting in the demotion of Pluto. The longer astronomers observe Haumea, the more interesting it becomes. Not only does this object have two moons, but it also has a ring system. Haumea is too distant to observe the rings directly, but researchers with the São Paulo Research Foundation have now worked out the characteristics of Haumea’s rings with a simulation.
Haumea is 43 times more distant from the Sun than Earth, so direct observation is difficult. From Earth, astronomers can make out Haumea itself and the moons known as Hi?iaka and Namaka (detected in 2005). Scientists speculate that the Haumea system is what’s left after a collision between two larger trans-Neptunian objects (TNOs). Haumea has a diameter of 904 miles (1,456 kilometers), but it’s an oblong object twice as wide as it is tall. It’s currently the third largest known TNO after Eris and Pluto.
That was all enough to make Haumea an interesting TNO, but then a chance orbital encounter in 2017 made Haumea downright fascinating. Haumea passed in front of a bright star called URAT1 533-182543. This process, known as an occultation, allowed astronomers to measure Haumea’s size, shape, and density via the dips in light from the star. It’s a process similar to the way we detect exoplanets. In addition to Haumea’s characteristics, the team also realized it had rings.
Haumea and its moons as seen by Hubble.
Haumea is too distant to image subtle features like this directly, but the Brazilian team used a computational model to work out the mechanics of Haumea’s rings. From the occultation data, we know that Haumea and the rings have an orbital resonance of 1:3. So, Haumea rotates three times for every orbit of ring particles. The simulation shows that the ring can’t be perfectly circular if that’s true.
The model devised at the São Paulo Research Foundation shows the 1:3 arrangement would have resulted in highly eccentric orbits. The model did find islands of stability at lower trajectories (less eccentric) that do match observations. Therefore, the team concludes that Haumea’s rings don’t have a true 1:3 resonance, but they orbit periodically on different paths that average out to be close to the observed 1:3 resonance. Clearly, we have a lot still to learn about ring systems around dwarf planets.