There's a potato-shaped dwarf planet in our solar system, and scientists have just discovered that it's got its very own ring.
Haumea, named after a Hawaiian goddess, orbits just past Neptune and has two moons, called Hiʻiaka and Namaka. The term "dwarf planet" probably brings to mind bodies like Pluto – perfect spheres that only just miss out on true planethood. But Haumea is a bit different. This dwarf planet is a third of the size of Pluto, has a weird, elongated shape, and rotates once every four hours. Aside from that, scientists didn't know a whole lot about it – until now.
New research published today in the journal Nature and led by Jose Ortiz from the Sierra Nevada Observatory in Granada, Spain, adds some new information to the dwarf planet's fact file. We now know it has a 70km-wide ring orbiting around its equator, 2,000km away from the planet itself. We know the dwarf planet itself reflects about half the sunlight that shines on it, and that it doesn't have an atmosphere. And we know it's shaped like an ellipsoid, and how dense it is – which tells us more about what it's like on the inside too.
"Prior to our work, researchers thought that Haumea was made mostly of rock and that it had a relatively thin crust of water ice," Ortiz told BuzzFeed News. "After our work, we can say that Haumea is far less rocky and it can have an interior more similar to that of Pluto." Pluto has a rocky core but is mostly made up of ice.
Haumea's discovery in 2005 was contentious. Two separate teams of astronomers – one led by Ortiz at the Sierra Nevada Observatory, the other led by Mike Brown at Caltech in the US – claimed to have discovered it in close proximity to each other, leading to a dispute that delayed its official naming. It wasn't until 2008 that the International Astronomical Union (IAU) officially classified it as the fifth dwarf planet, gave it the name Haumea – a suggestion that came from the US team – and left the name of its discoverer blank.
Hauma was also one of a handful of objects that actually led the IAU to rethink the definition of a planet altogether, and reclassify Pluto as a dwarf planet. The total number of officially recognised dwarf planets is now five, but some astronomers argue there could be hundreds of objects in the solar system that fit the definition.
To learn more about this particular dwarf planet, Ortiz and his colleagues worked out that on 12 January this year Haumea would pass in front of a distant star, known by the catchy title URAT1 533-182543, giving them the perfect opportunity to study it in more detail. They got 10 Earth-based observatories ready, and on that night all pointed their telescopes towards the same patch of sky to learn as much as they could.
Part of what Ortiz and his colleagues found might throw a spanner in the works for Haumea's classification as a dwarf planet, though. "The authors’ results suggest that Haumea might not be in hydrostatic equilibrium, and this touches on the still-sensitive topic of how planets and dwarf planets should be defined," writes Amanda Sickafoose, an astronomer at MIT, in an accompanying article also published in Nature today.
Hydrostatic equilibrium is the scientific way of saying that a lump of rock is so big its own gravity has pulled it all in and made it spherical, or at least an ellipsoid. And it's what makes a dwarf planet a planet (according to the definition the IAU came up with), and not just a "small solar system body". But it's too soon to say for sure whether Haumea really doesn't match up to this criteria.
The rings could be key to figuring out Haumea's history. We think that ring systems can form in different ways: They could be cobbled together from material left over from a planet's own formation, formed when a passing lump of rock is captured and broken up, or even, in the case of Saturn's E-ring, constantly replenished from ice spewing out of an orbiting moon.
Haumea's ring is less reflective than the dwarf planet's bright water ice surface, suggesting it is made up of a mixture of rock and ice. But as for how it formed, we don't yet know. And we don't yet know how common features like it are either. Ortiz estimates that about a quarter of bodies in the outer solar system might have rings around them, although he stresses that this is still "pure speculation" for now.
