Haumea: The Fast-Spinning, Egg-Shaped Dwarf Planet

A four-hour day and a stretched shape

NASA describes Haumea as oval, with a fast spin that distorts it so it looks like a football. NASA gives a rotation of about 4 hours and says that is one of the fastest among large objects in the Solar System. Astronomers at the Institute of Astrophysics of Andalusia (IAA-CSIC) in Spain, who studied the 2017 occultation, give the more exact figure of about 3.9 hours.

The numbers differ because the measurements describe different directions on a lopsided object. The 2017 study found that Haumea is considerably bigger and less reflective than astronomers had thought, and much less dense. Mike Brown of Caltech says its mass is about 32 percent of Pluto's. NASA describes the interior as rock with a coating of ice, and Brown's page says the surface looks like pure ice spread thinly over a rocky body. An AU, or astronomical unit, is the average distance from Earth to the Sun, about 150 million km (93 million miles).

A stretched body may seem an odd fit for a category built around being "nearly round," yet NASA and the International Astronomical Union (IAU) both list Haumea as a dwarf planet. The rule behind the category is explained in what a dwarf planet is.

The ring found by an occultation

An occultation happens when a nearer body passes in front of a more distant star and blocks its light. Telescopes at different places record the star winking out for different lengths of time, and those timings trace the body's outline. Haumea was expected to cross a star on January 21, 2017, and twelve telescopes at ten European observatories watched.

The data showed more than a shape. The IAA-CSIC team found a ring with a radius of about 2,287 km, which lies in Haumea's equatorial plane, the same plane as the moon Hi'iaka. In the published analysis the ring is about 70 km wide and has an opacity of 0.5, meaning it blocks about half the light passing through it. It sits close to a 3:1 resonance with Haumea's spin, which means ring particles circle once for every three turns Haumea makes. NASA says Haumea is the first known Kuiper Belt object with rings. The observation also showed that, unlike Pluto, Haumea has no global atmosphere.

How astronomers use shadows like this to measure small worlds is the subject of stellar occultations explained. A reminder for anyone who takes up sky watching because of stories like this one: never point binoculars or a telescope at the Sun without a proper solar filter made for that purpose.

Hi'iaka and Namaka

Haumea has two known moons, both discovered in 2005 and named for daughters of the goddess. Hi'iaka is the outer and larger one. Brown's team found it on January 28, 2005, using the Keck Observatory in Hawaii with a laser-assisted adaptive optics system, which sharpens the image by correcting for the blurring of Earth's air. Hi'iaka orbits about every 49 days and holds roughly 1 percent of Haumea's mass.

Namaka is the inner moon, fainter and smaller, with roughly 0.2 percent of Haumea's mass. The two orbits are tilted about 40 degrees relative to each other. Astronomers measured Haumea's mass by watching Hi'iaka's orbit for about six months, the same basic trick used to weigh Eris with its moon, described in the Eris article.

A family of icy fragments

Haumea may be the largest survivor of a collision. According to Brown's page, a large Kuiper Belt object struck Haumea about 4.5 billion years ago, stripping off its icy outer layer and spinning it up. A Nature paper described five small icy objects with similar orbits as debris from that impact. A 2012 study modeled the family as Haumea plus at least ten other bodies of about 100 km (62 miles), all with orbits between about 42 and 44.5 AU, and called it the first collisional family found in the trans-Neptunian region, meaning the region beyond Neptune. Because "at least ten" is a lower bound, the full membership may be larger.

Discovery and naming

Who found Haumea is still disputed. NASA says two teams claim credit, citing observations from 2003 and 2004. NASA adds that the IAU's Gazetteer lists March 7, 2003, at Sierra Nevada Observatory in Spain and names no official discoverer. The Planetary Society reported in 2008 that Brown received credit from the IAU, although the Minor Planet Center had first credited three astronomers from the Spanish team and later switched to simply "Sierra Nevada." The article says the dispute likely delayed the name.

The IAU announced the name on September 17, 2008. Haumea is the goddess of childbirth and fertility in Hawaiian mythology, and the IAU said the body would be classified as the fifth dwarf planet. How that fifth place fits into the larger count is in how many dwarf planets there are.

Common mistakes and myths

• Myth: Haumea is round like Pluto. It is stretched. Its longest axis is at least 2,322 km, while another measured dimension is 1,138 km.
• Myth: Haumea's ring is like Saturn's. It is a single narrow ring about 70 km wide, at a distance of about 2,287 km from Haumea's center.
• Myth: a spacecraft photographed the ring. The 2017 results came from telescopes on the ground that tracked a star passing behind Haumea.
• Myth: Haumea has an atmosphere like Pluto's. The occultation showed no global atmosphere.
• Myth: everyone agrees who discovered Haumea. NASA says two teams claim credit, and the credit has been revised more than once.

Frequently asked questions

How fast does Haumea spin?

About once every 3.9 hours, according to the 2017 occultation study. NASA rounds this to about 4 hours.

How big is Haumea?

NASA lists an equatorial diameter of about 1,740 km (1,080 miles). The 2017 occultation found a longest axis of at least 2,322 km (1,443 miles), so the size depends on which direction you measure.

Does Haumea have rings?

Yes. A ring was found in 2017 at a radius of about 2,287 km, and NASA calls Haumea the first known Kuiper Belt object with rings.

How many moons does Haumea have?

Two, as of November 2026: Hi'iaka and Namaka. Both were discovered in 2005.