Haumea – the Ringed Dwarf Planet

Haumea – the Ringed Dwarf Planet
Drawing of Haumea and its moons

The dwarf planet Haumea [how.MAY.ah] is a TNO (trans-Neptunian object) – that means it's located in the outer Solar System beyond the orbit of Neptune. The third largest of the TNOs discovered so far, its mass is about a third that of Pluto. It's also a bizarre little object whose discovery was controversial, which has the shortest day of any known Solar System body, and is one of a family of objects produced by a massive collision in the early Solar System.

Where is Haumea?
Haumea is so far away from the Sun that its year is nearly 285 Earth years long. The closest it gets is 35 AU (astronomical units). One AU equals the Earth-Sun distance, so at its nearest, the dwarf planet is 35 times farther from the Sun than Earth is. At its most distant, as it was early in 1992, it's 51 AU from the Sun.

The Solar System planets orbit the Sun in the ecliptic plane. Imagine the ecliptic as if it were a flat surface stretching out into the Solar System from the Sun's equator. This diagram of the Solar System shows the planets in this plane, and Pluto's orbit tilted to it. But look at Haumea's orbit. It's tilted by 28°, and takes the dwarf planet well out of the ecliptic.

In a dizzy spin
If a body has enough mass, gravity pulls it into a spheroid. Such bodies aren't all completely spherical, especially if they rotate quickly. The faster the rotation, the more the poles flatten and the equator bulges. Venus, for example, rotates extremely slowly and is spherical. Earth is nearly spherical, with the difference between the equatorial and polar radii only about 22 km (13 mi).

Haumea is much smaller than Earth and has a day scarcely four hours long. It spins faster than any other known Solar System body. In fact, if it went much faster, it would break apart. As it is, Haumea is shaped like an egg rather than a globe. The radius at the equator is twice that at the poles. The header image isn't to scale – the moons are farther from Haumea than shown in the drawing.

Bright and ringed
From infrared and other observations Haumea appears to be a rocky body covered with a layer of crystalline ice that's highly reflective, easily as bright as fresh snow. An amateur astronomer with a good telescope could see it. The icy covering is a puzzle for astronomers, because it suggests resurfacing, but no one has come up with a plausible mechanism for how this could have occurred.

In 2009 an infrared observation showed a large area of dark red against the white surface. Astronomers aren't certain what that would actually look like in visible light, or what caused it. Perhaps it's richer in minerals or organics. Maybe the scar from an impact containing material left by the impactor. However, astronomers are certain of a uniquely strange feature: Haumea has a ring, the first ever discovered for a trans-Neptunian object.

Haumea has two moons, Hi'laka and Namaka. They were discovered in 2005 by Mike Brown's Caltech team observing in Hawaii. Hi'laka is small, around 310 km (190 miles) in diameter and about 0.05% of the mass of Haumea – it circles the dwarf planet's equator every 49 days. Namaka is even smaller, with only 10% of the mass of Hi'laka. It orbits Haumea in 18 days in an orbit that's very elongated and tilted 13° from Hi'laka's orbit. Usually, two small moons would be expected to have circular orbits due to tidal effects. This is yet another mystery.

Hit and run victim
Haumea is small now, but was once part of a larger body. In the early days of the Solar System a massive collision broke up the original object, leaving the dwarf planet and its moons, plus a number of other trans-Neptunian objects. It's also likely that this collision was what set Haumea spinning.

Whose discovery?
The discovery of Haumea is controversial. It's obviously been discovered, but who did it? Two teams claimed the discovery, one led by Mike Brown in the USA, and the other by José Ortiz at the Sierra Nevada Observatory in Spain. This left the IAU (International Astronomical Union) in a quandary for a few years. Finally, the chosen name was that proposed by the US team, and the place and date of discovery was listed as that of the Spanish observatory. The discoverers weren't named.

The name game
The competing discovery teams each proposed a name for the dwarf planet listed as 2003 EL61. The Sierra Nevada Observatory proposed the name Ataecina, a goddess of the ancient Iberian peninsula associated with Persephone, the queen of the underworld in Greek mythology. But the IAU has guidelines for naming various classes of objects, and the new dwarf planet fell into a category for which the name of a creation deity was required, not one related to the underworld.

The Caltech team won the day with Haumea, Hawaiian goddess of fertility and childbirth. She had many children, including Pele, the goddess of fire and volcanos, as well as some that had sprung from different parts of her body. This echoes creation of Haumea's moons and other bodies that broke off from a parent object. The goddesses Ki'laka and Namaka were two more of Haumea's children. The Caltech team had made the Hawaiian connection because they had discovered the moons when observing with the Keck telescope in Hawaii.

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