Oddball dwarf planet Haumea has a ring

Haumea can do the hula-hoop. The egg-shaped dwarf planet is the first object beyond Neptune to be spotted sporting a ring of particles.

“It now appears that rings can be common in the outer solar system,” says Jose-Luis Ortiz of the Institute of Astrophysics of Andalusia in Granada, Spain.

On January 21, Ortiz and colleagues used 12 telescopes at 10 observatories to peer into the Kuiper Belt, a region of icy objects beyond the orbit of Neptune, and watch Haumea block the light of a distant star. That tiny eclipse let the team measure the dwarf planet’s size, shape and surrounding environment more accurately than ever before.

Haumea turned out to be larger — its long axis stretches at least 2,322 kilometers, a bit more than half the width of the contiguous United States — and less dense than previously thought, the team reports October 11 in Nature. To their surprise, the researchers also saw the background star flicker before and after its light was blocked by Haumea itself. That flicker is consistent with a 70-kilometer-wide ring about 1,000 kilometers above the dwarf planet’s surface.

The ring is probably made of rock and ice, Ortiz says, but more observations are needed to know for sure. It could be debris kicked up by impacts from small stray space rocks — or even just from the dwarf planet’s spinning. Haumea twirls unusually fast, completing a rotation once every 3.9 hours, which could help fling particles into orbit.

Until recently, the four giant planets — Jupiter, Saturn, Uranus and Neptune — were the only solar system bodies known to have rings. Then in 2014 and 2015, astronomers spotted rings around the tiny planetoids 10199 Chariklo (SN: 5/3/14, p. 10) and 2060 Chiron, suggesting that small bodies could hold on to rings, too.

Both of those small worlds are known as centaurs, objects whose orbits take them between Jupiter and Neptune, although they may be interlopers from the more distant Kuiper Belt. Since recent searches for rings around Pluto came up empty (SN Online: 10/4/17), no object farther away than Neptune seemed to have rings. Some astronomers speculated that something about the Kuiper Belt disrupted rings around small worlds there, or that the centaurs got their rings as part of the process that kicked them into their current orbits. Haumea’s ring suggests that such structures can form and survive at the solar system’s fringes after all.

“This discovery does disrupt that tidy narrative,” says Matthew Tiscareno of the SETI Institute in Mountain View, Calif., who studies Saturn’s rings but was not involved in the new study. “Reality is more complicated — that is, interesting.”