The most effective way of discovering planets beyond our Solar System has been through observing occultations when a body passes in front of its host star. Astronomers have now used that method to get a better look at a little-known dwarf planet much closer to home: Haumea, which orbits the Sun beyond Neptune. The observations not only gave scientists a better understanding of the planet's size, shape and density, but it allowed them to make a surprising new discovery: Haumea has a rocky ring around it.

Discovered in 2004, Haumea is an icy, egg-shaped dwarf planet about which not much is known because it's so far away – up to 50 times further from the Sun than Earth is. A few years ago astronomers peered at fellow trans-Neptunian object Makemake when it passed in front of a distant star, and now the same thing has been done to study Haumea in more detail.

The occultation occurred on January 21 this year, and it was observed by 10 facilities in six European countries. The data gathered was then studied by another 50 institutions to better pin down Haumea's size, shape and density.

"Since its discovery more than ten years ago we knew that Haumea is exotic, but its properties were only poorly known," says Thomas Müller, lead researcher on one of the teams from the Max Planck Institute. "This is why we pushed for more observations by the most powerful telescopes on Earth and in space. It is fantastic to see that a coordinated campaign of observers with small to medium-sized telescopes has now made the most spectacular contribution to our understanding of Haumea and the world of icy dwarf planets beyond Neptune."

Because of its strange, squashed shape, a static shot isn't enough to get a good understanding of the dwarf planet. So, the researchers watched it for a few days before and after January 21 to measure its rotational light curve, or how the amount of light it blocks changes over time. These observations revealed that its longest side measures at least 2,322 km (1,443 mi), which is larger than previously thought, with its other axes measuring about 1,138 km (707 mi) and 1,704 km (1,059 mi).

But the light curves also revealed another intriguing feature. The light from the distant star dims briefly both before and after the deep drops that indicate Haumea itself, which points to the presence of a dusty ring like that sported by Saturn.

"We were very surprised to see these secondary events, which hinted at an additional structure around the dwarf planet," says Ulrich Hopp, another researcher on the study. "The analysis shows that these dips are explained by a narrow and dense ring around Haumea that absorbed about half of the incoming stellar flux."

Further study revealed the ring to be about 70 km (43.5 mi) wide, forming a circle around Haumea with a radius of about 2,287 km (1,421 mi). The ring lines up with the dwarf planet's equator, and likely contains one of its two previously-known moons, Hi'iaka.

"We now know more about the shape and build-up of Haumea and the discovery of a ring around this dwarf planet, with completely different dynamical mass and more distant than other objects with rings, means that rings are probably much more common than we thought previously," says Müller. "This is a new, unexpected and exciting research field where small and medium-sized telescopes can make fantastic contributions."

Among the other details discovered were its density, which was found to be about 1,885 kg per m3 (117.7 lb/cu ft) – close to that of Pluto. The scientists also detected no trace of a nitrogen- or methane-based atmosphere.

The research was published in the journal Nature.

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