The dwarf planet Haumea resides in the Kuiper Belt beyond the orbit of the outermost planet Neptune.
An artist’s conception of Haumea, its moons Hi’iaka and Namaka, and its ring system; the moons are much more distant than depicted here. Image credit: Pablo Carlos Budassi.
Also known as 2003 EL61, the dwarf planet Haumea is strange in several ways.
Discovered in 2003, it spins faster, by far, than anything else of its size, whirling on its axis in only 4 hours.
Because of its fast spin, it is shaped like a deflated American football instead of a sphere.
Uniquely among large Kuiper Belt objects, it is almost uniformly (>90%) covered in water ice.
Haumea has at least two moons, Hi’aka and Namaka. It takes 285 years for the dwarf planet to make one orbit around our Sun.
“How did something as weird as Haumea and its family come to be?” said Dr. Jessica Noviello, a postdoctoral researcher at NASA’s Goddard Space Flight Center.
“To explain what happened to Haumea forces us to put time limits on all these things that happened when the Solar System was forming, so it starts to connect everything across the Solar System,” added Professor Steve Desch, an astrophysicist at Arizona State University.
Haumea is too far away to measure precisely through an Earth-based telescope, and no space mission has yet visited it, so data are scant.
Thus, to study Haumea and other little-known worlds, scientists use computer models to make predictions that fill in the gaps.
Dr. Noviello, Professor Desch and their colleagues began by feeding only three pieces of information into their models: Haumea’s estimated size and mass, and its rapid four-hour day.
The models spit out a refined prediction of Haumea’s size, its overall density, and the density and size of its core, among other features.
The authors then fed this information into mathematical equations that helped them calculate the amount of ice on Haumea and the dwarf planet’s volume.
Additionally, they calculated how Haumea’s mass is distributed and how that affects its spin.
With this information in hand, they sought to simulate billions of years of evolution to see which combination of features of a baby Haumea would evolve into the mature dwarf planet it is today.
“We wanted to understand Haumea fundamentally before poking back in time,” Dr. Noviello said.
The scientists assumed that baby Haumea was 3% more massive to account for the family members that once were part of it.
They also assumed Haumea likely had a different spin rate and was bigger in volume.
Then they slightly changed one of these features at a time in their models — such as tweaking Haumea’s size up or down — and ran dozens of simulations to see how small changes in its early years would influence Haumea’s evolution.
When the simulations spit out results that resembled today’s Haumea, they knew they had landed on a story that matched reality.
Based on their modeling, the researchers hypothesize that when the planets were first forming and everything was zipping around the Solar System, Haumea collided with another object.
Though this impact would have knocked off pieces, they suggest that those pieces are not the Haumean family we see today, as other scientists have proposed.
Such a powerful impact would have knocked off bits of Haumea into much more scattered orbits than the family members have.
“The Haumean family we see today instead came later, as the dwarf planet’s structure was taking shape: dense, rocky material was settling to the center while lighter density ice was rising to the surface,” Professor Desch said.
“And when you concentrate all the mass towards the axis, it decreases the moment of inertia, so Haumea ended up spinning even faster than it does today.”
Fast enough, scientists calculated, that ice flung off the surface forming the Haumean family.
“Meanwhile, Haumea’s rocks, which, like all rocks, are slightly radioactive, generated heat that melted some ice, creating an ocean below the surface (no longer there),” said Dr. Marc Neveu, a researcher at NASA’s Goddard Space Flight Center.
“Water soaked into the rocky material at the center of Haumea and made it swell into a large core made of clay, which is less dense than rock.”
“The larger core increased the moment of inertia and thus slowed Haumea’s spin to its current rate.”
The results were published in the Planetary Science Journal.
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Jessica L. Noviello et al. 2022. Let It Go: Geophysically Driven Ejection of the Haumea Family Members. Planet. Sci. J 3, 225; doi: 10.3847/PSJ/ac8e03
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