Haumea

Haumea is one of the strangest large worlds in the solar system: a dwarf planet in the Kuiper Belt, out beyond Neptune, that has been spun so fast it is no longer round. Instead of a sphere, it is a stretched triaxial ellipsoid — roughly the shape of a rugby ball or an American football — measuring some 2,100 km along its longest axis but only around half that from top to bottom. Its mean radius of about 780 km makes it one of the largest bodies past Neptune, yet its most defining feature is motion rather than size.

Haumea completes a full rotation in under four hours, faster than any other known large body in the solar system. That blistering spin, a collisional past that scattered a whole family of icy fragments across the outer solar system, two small moons, and a slender ring make Haumea a genuine oddity — a dwarf planet that behaves less like a quiet frozen rock and more like the aftermath of a cosmic crash.

A world spun into an egg

Most large bodies in the solar system are pulled into spheres by their own gravity. Haumea is the striking exception. It turns on its axis roughly once every 3.9 hours — a day shorter than a single work shift on Earth — and that rapid rotation flings its equator outward, stretching the whole body into an elongated shape. Measurements from stellar occultations, where Haumea passes in front of a distant star and briefly blocks its light, give dimensions of about 2,100 km by 1,700 km by 1,000 km along its three axes.

This is thought to be an equilibrium shape: Haumea is spinning so fast that the egg-like form is the balance point between gravity trying to round it off and centrifugal effects trying to tear it apart. Its relatively high density, near that of solid rock, suggests a rocky interior wrapped in a bright icy shell rather than a body of ice all the way through.

Cousins born from a collision

Haumea's fast spin is almost certainly a scar from its past. Astronomers believe that at least a billion years ago Haumea suffered a massive impact that stripped away much of its icy mantle, set it spinning, and flung debris into surrounding space. Those fragments make up the Haumea collisional family — the only such family yet identified in the Kuiper Belt.

The family members share Haumea's tell-tale signature: surfaces coated in nearly pure crystalline water ice, unusually bright and blue-white compared with the reddish tint of most distant objects. Larger fragments such as 2002 TX300 are a few hundred kilometres across. Haumea's own surface is 66 to 80 percent crystalline water ice, and a darker, redder patch spotted in 2009 hints at a region richer in minerals and carbon-bearing compounds.

Two moons and a ring

Haumea is orbited by two small moons, both discovered in 2005 and named — like the dwarf planet itself — from Hawaiian mythology. Hiʻiaka, the outer and larger, is roughly 310 km across and circles Haumea every 49 days. Namaka, the inner moon, is smaller at about 150 km and loops around in an eccentric 18-day orbit that is noticeably tugged and warped by its larger sibling. Both are named for daughters of the goddess Haumea.

In 2017, astronomers watching Haumea pass in front of a background star found something no one had seen at a body this far out: a ring. The ring sits about 2,287 km from Haumea's centre, is roughly 70 km wide, and orbits in a 3:1 resonance with the dwarf planet's spin — ring particles complete one lap for every three of Haumea's rotations. It was the first ring system ever discovered around a trans-Neptunian object, and it upended the assumption that rings belong only to the giant planets.

A distant, tilted orbit

Haumea circles the Sun at an average distance of about 43 astronomical units — some 43 times farther from the Sun than Earth — deep in the Kuiper Belt. A single orbit takes roughly 283 Earth years, so Haumea has not completed even one lap since telescopes powerful enough to find it existed. Sunlight, which reaches Earth in about eight minutes, takes around six hours to travel out to Haumea.

Its orbit is both eccentric and steeply tilted, inclined about 28 degrees to the plane of the planets, carrying it from roughly 35 AU at its closest to over 51 AU at its most distant. Out there, temperatures never climb far above 50 kelvin — some 220 degrees below zero Celsius — cold enough to keep its water ice locked hard as rock.

A disputed discovery

Haumea's discovery is one of the more contentious episodes in modern astronomy. A team led by Mike Brown at Caltech identified it in images in December 2004 and gave it the informal nickname 'Santa'. Before they published, a Spanish team led by José Luis Ortiz at the Sierra Nevada Observatory announced the object in July 2005, having found it in older precovery images from 2003. The controversy deepened when it emerged that a computer at the Spanish group's institution had accessed Brown's telescope observation logs shortly before their announcement.

The International Astronomical Union settled the matter with a compromise: it credited the Sierra Nevada Observatory as the discovery location but adopted the Caltech team's proposed name, Haumea, after the Hawaiian goddess of childbirth and fertility. Haumea was formally recognised as a dwarf planet on 17 September 2008 — the fifth to receive that designation, and one of only a handful that also carry moons.

Observing Haumea

Haumea is far beyond naked-eye visibility. At around 17th magnitude it is roughly a million times fainter than the dimmest stars a person can see without help, so glimpsing it requires a large amateur telescope, a dark sky, precise coordinates from an ephemeris, and patient star-hopping to catch a faint, slowly moving point of light — in recent years it has tracked through the constellation Boötes. No spacecraft has ever visited it; everything we know comes from telescopes on and near Earth, especially the stellar occultations that revealed its shape and its ring. A dedicated Haumea mission has been proposed but not yet flown, leaving this spinning egg among the least-explored large worlds in the solar system.