When the distant dwarf planet Haumea briefly slipped in front of a star last January, astronomers found more than they expected.
Observers across Europe had two years' notice that the dwarf planet 136108 Haumea, which orbits in the realm beyond Neptune, would briefly cover an 18th-magnitude star in Boötes, and they jumped at the chance to watch. After all, Haumea was already a well-known oddball: It's among the largest objects in the Kuiper Belt, has a highly elongated shape, spins rapidly, and possesses two moons.
The occultation of that star, designated URAT1 533–182543, came and went in the wee hours of January 17, 2017, and observers at a network of 10 observatories across central Europe successfully recorded a brief coverup. But they also found something unexpected: there's a dense, narrow ring surrounding Haumea.
As José Luis Ortiz (Instituto de Astrofísica de Andalucia, Spain) and 92 colleagues detail in October 12th's Nature, several observing teams clearly detected the ring as secondary dips in the star's light roughly 2 minutes before and after the main event. This ring is dark, narrow (70 km wide), and dense enough to block about half the star's light — so it's much like those surrounding Uranus and Neptune. The ring's orbit has a radius of about 2,287 km (1,421 miles) — that's too close to Haumea itself, well inside a gravitational threshold called the Roche limit, to be able to collect into a single body.
How the ring came about isn't clear. Haumea isn't the only outer-solar-system body with one. Occultations by 10199 Chariklo and 2060 Chiron, both in 2013, also turned up rings; these two objects are Centaurs, meaning they occupy elongated orbits the drift among those of the outer planets. But Haumea is much farther out, moving along a 284-year orbit that ranges between 35 and 51½ astronomical units from the Sun. It's also a fast spinner (once every 3.9 hours) and the biggest fragment of a larger Kuiper Belt object that broke apart in the distant past.
The ring appears to lie in Haumea's equatorial plane, which is also shared by the larger, outer moon Hi'iaka. It's also positioned just a few kilometers from a 3:1 spin-orbit resonance, a gravitationally unique location at which an object would circle around once in the time Haumea takes to complete three rotations.
Meanwhile, Ortiz and his team used the occultation records to reconstruct Haumea's silhouette. Prior telescopic observations had shown that Haumea has the shape of a triaxial ellipsoid, bloated through its equator and squashed through its poles — a consequence of the rapid spin. The values derived from January's cover-up, 1,404 km long and 1,138 wide, aren't the dwarf planet's true overall dimensions. In fact, they note, "It turns out that Haumea was at its absolute brightness minimum" during the occultation, and the true size must be substantially larger: 2,322 km through its longest axis and 1,026 through its shortest. Among all Kuiper Belt objects, only Pluto and Eris are larger.
The new, larger size means that Haumea's average density must be less than previously thought, dropping to about 1.9 g/cm3. That's a close match to Pluto's mean density, and it suggests a composition with roughly equal portions of rock and ice. The larger size also means that Haumea reflects about half the sunlight that strikes its water-ice-dominated surface, substantially lower than the albedos of 70% to 80% that had been derived from infrared observations with the Spitzer and Herschel space telescopes.
However, Haumea's size and characteristics aside, it's the just-discovered ring that has dynamicists buzzing. One implication, the authors conclude, is that such rings might be common in the trans-Neptunian region from which Centaurs like Charon and Chariklo originate.
Incidentally, the discovery and naming of Haumea were mired in controversy. Observing teams led by Ortiz and by Michael Brown (Caltech) both claimed to have spotted it first. Ortiz even offered the name Ataecina, an Iberian goddess of the underworld. Then, in September 2008, the two International Astronomical Union panels charged with naming dwarf panels announced their decisions, each decided by one vote, in favor of Brown's team.