Haumea's mysterious moons


Does the dwarf planet host tiny satellites – and if so, where are they? Belinda Smith reports.


An artist's concept of Haumea and its moons, Hi'aka and Namaka.
NASA
Dwarf planet Haumea is a strange little object. Like its cousin Pluto, it has more than one moon and is around the same size. But Haumea is elongated, not spherical, and spins faster than almost any other large body in the Solar System.

And now, its moons don't seem to follow suit either.

A trio of planetary scientists led by Luke Burkhart from Yale University searched for small moons orbiting Haumea, aside from the two medium-sized moons already detected.

But they didn't turn up a single one – in stark contrast to Pluto, which is accompanied by a bevy of four small moons alongside its big moon Charon.

This means the moons of icy dwarf planets, such as Pluto and Haumea, formed in different ways, says Darin Ragozzine from Florida Institute of Technology and co-author of the paper, published on the preprint online Arxiv: "There is no self-consistent formation hypothesis for either set of satellites."

Haumea resides in the Kuiper Belt – the frozen reaches of the Solar System beyond Neptune. But, being so far away, it was only discovered in 2003, and its moons Hi'aka and Namaka discovered two years later.

As it's locked away in the deep freeze of the Kuiper Belt, it and other large bodies, such Makemake and Eris, can give planetary scientists clues to the formation of the Solar System 4.6 billion years ago.

This image shows a comparison of the four icy dwarf planets and their moons, with all objects to scale. These large bodies in the outer Solar System share many similarities, but one difference is that only Pluto has a collection of tiny moons (shown near the centre).
D. RAGOZZINE (FIT) / NASA / JHU / SWRI
The New Horizons spacecraft, which flew by Pluto in July last year, snapped images of Pluto's tiny moons Hydra, Styx, Nix and Kerberos. Does Haumea have a similar entourage of tiny moons?

This is the question Burkhart, Ragozzine and Michael Brown from Caltech decided to explore. Instead of building and waiting for a probe to reach the dwarf planet, they analysed photos taken by the Hubble Space Telescope in 2010.

Individual photos weren't going to cut it. The tiny moons, should they exist, would be too small, and moving too fast, to be captured in a single frame.

So they used a technique called "non-linear shift-and-stack" which combines multiple images to bring previously invisible objects to light. But none showed up.

This isn't to say Haumea doesn't have tiny moons. They may well be too small, too dull or travelling too fast for even the shift-and-stack method.

But for the moment, how it and its Kuiper Belt counterparts formed remains shrouded in mystery.

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