Once it seemed Saturn was rather unique with its magnificent ring system. But we’ve since found it is not alone – and now another world has revealed that it too is a ring-bearer.
That world is Haumea, a dwarf planet located 50 times further from the Sun than Earth, beyond the orbit of Neptune. In this position, it is known as a trans-Neptunian object (TNO), one of four dwarf planets in this region we know to exist. It's the first dwarf planet or TNO we've found with a ring.
The discovery, published in Nature and led by Jose Ortiz from the Institute of Astrophysics of Andalusia in Granada, Spain, suggests rings are more commonplace in the Solar System than we thought. We’d already discovered rings on Centaurs (Chariklo was the first known to have one), which are bodies that orbit between Jupiter and Neptune. Now we’ve found one much further out around a TNO.
"It now appears that rings might be common in the outer Solar System," Ortiz told IFLScience. "We think that this ring is probably made of rock and water ice, with some organic material as well."
The ring has a diameter of more than 4,500 kilometers (2,800 miles), but is just 70 kilometers (45 miles) wide. It is located about 1,000 kilometers (620 miles) from the surface of the dwarf planet.
Finding the ring was no mean feat. On January 21, 2017, a team of astronomers used 12 telescopes in 10 locations to watch as the dwarf planet passed in front of a distant star, called URAT1 533-182543. As the star’s light was blocked out, the existence of the ring became apparent.
They found that the ring is on a 3:1 resonance with Haumea, which means that Haumea rotates three times (one revolution takes four Earth hours) in the time it takes a particle in the ring to go around once.
From their observations, the team was also able to get a better handle on the size of Haumea. We already knew the dwarf planet was egg-shaped, but they found its longest axis was a bit longer than thought, at least 2,322 kilometers (1,443 miles).
Studying the light blocked by the dwarf planet revealed it had no atmosphere, while they also found it had a similar density to Pluto.
But it’s the ring that is the most exciting finding. Haumea and its two moons may have formed from the break-up of a larger body, and learning more about their origins may also reveal more about how this ring came to be.
It's possible that an impact on Haumea could have thrown material into the dwarf planet's orbit. Or perhaps a moon broke apart in orbit, giving rise to the ring.
We won't be able to visibly see the ring any time soon, because it's too close to Haumea and much less bright (even Hubble cannot see it). However, Ortiz noted they were already looking through old data to try and find more information, and they were planning new observations to try and study it further.
"If the rings are the result of collisions, we can get important information on collision rates and other characteristics of the Solar System," he said.