Oct 29 2019

Hygiea May Be a Dwarf Planet

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Hygiea is the fourth largest asteroid in the asteroid belt, after Ceres, Vesta and Pallas. Recent observations of Hygiea are now challenging the distinction between an asteroid and a dwarf planet.

For some context, Ceres is the largest asteroid and is also the only one that is unambiguously a dwarf planet, by current definitions. Actually, when first discovered in 1801 it was considered a planet. It was the first asteroid discovered in the asteroid belt between Mars and Jupiter, but when more were discovered it became increasingly obvious that Ceres was part of a swarm of objects in a similar orbit. In the 1850s it was then demoted to an asteroid, although was the king of the asteroids, if that is any consolation.

Then in 2006 the International Astronomical Union (IAU) created the new category of dwarf planet and redefined the threshold for being a planet. According to the IAU, in order to be a planet you have to be large enough for gravity to pull your shape into a rough sphere (called hydrostatic equilibrium), not be a moon, orbit the sun, and dominate your orbit. A dwarf planet, rather, is an object with the first three criteria, but has failed to clear out its orbit of other objects. That last bit is what caused Pluto to famously be demoted to a dwarf planet, because its largest “moon”, Charon, was considered to be too large to be just a moon. Pluto and Charon orbit each other around a center of gravity (barycenter) that is outside both worlds.

However, the Pluto-Charon situation shows what a mess the classification system is. For now the five official dwarf planets are Pluto, Ceres, Eris, Makemake, and Haumea. There are many candidate dwarf planets that await further characterization. Charon is controversial. Some astronomers argue¬† that Pluto-Charon should be considered a double dwarf planet system, largely because the barycenter is outside both worlds. However, other argue that the location of the barycenter is not a strict criterion, because it depends on distance. For example, the barycenter of the Jupiter-Sun system is outside the surface of the sun, because of Jupiter’s distance. For now Charon is a moon, and its “promotion” to dwarf planet is in limbo. Also, the New Horizon data has called into question whether or not Charon is in hydrostatic equilibrium, but that is a separate issue.

Let’s get back to Hygiea. Hygiea has a diameter of 430 km. Ceres is currently the smallest dwarf planet at 950 km, while Pluto is the largest at 2380 km. Hygiea shares many attributes with Ceres – not a moon, orbits the sun, and has not cleared out its orbit. The question is, is it in hydrostatic equilibrium. This is the new information – Astronomers using ESO’s SPHERE instrument at the Very Large Telescope (VLT) have made new observations of Hygiea that show it is a rough sphere. It may, therefore, be in hydrostatic equilibrium. If so, that would make it a candidate for the 6th dwarf planet, and by far the smallest (perhaps close to the smallest possible).

This reveals, however, how tricky this criterion can be. Whether or not a world is a sphere depends on more than its size. It also depends on what it is made from. A world of solid iron can be a lot larger without being a sphere than an object of softer material. But it also depends on its history. Did it collide in its past in such a way to alter its shape? Hygiea is the product of a massive collision about 2 billion years ago, creating the current asteroid plus about a thousand smaller asteroids. This collision happened to leave behind a rough sphere.

Let’s compare this to the two larger non-dwarf planet asteroids – Vesta and Pallas. Vesta has a diameter of 530 km. Vesta is larger than Hygiea, but is not considered a candidate for a dwarf planet because of its irregular shape (it’s considered an oblate spheroid, and too oblate to be in hydrostatic equilibrium). Pallas is a little smaller, at 512 km, and is also irregular. But what if we chipped Vesta into a rough sphere? Would it become a dwarf planet? Does it have to be a sphere from gravity alone, or does only its current shape matter? If we took a bowling ball and put it in orbit around the sun, would it be a dwarf planet?

Let’s also compare them to dwarf planet Haumea to add another layer of complexity. Haumea is very oblate, more so than Vesta and Pallas. Haumea is 2300 km along its long axis, but only 996 km along its short axis. However, Haumea is also the fastest spinning large object known in the solar system, rotating once every four hours. This rapid rotation is stretching out the dwarf planet along its equator. So even though it departs significantly from a sphere, it is still considered to be in hydrostatic equilibrium when its rotation is taken into consideration.

The bottom line is that there are lots of objects in the solar system with every possible configuration, and with features that vary along a continuum. Whatever rules we come up with to sort them into neat categories are likely to be broken by some object out there. This is similar to our attempts at categorizing life according to morphology. There are weird creatures out there to challenge the limits of any taxonomical category. This is why biologists are shifting to a cladistic system, which categorizes life according to its strict evolutionary relationship. This is why, for example, birds are now considered a subclade of dinosaurs.

Is there something similar with planetary astronomy? In other words, taxonomy shifted from basing membership in a category on physical characteristics to something fundamental about origins. In astronomy we are stuck basing our categories for planets, dwarf planets, moons, planetesimals, and asteroids on physical characteristics, which includes relationships to other objects. Will it be possible one day to replace this system with one based on the origins of the various types of objects? Perhaps not. The history of objects in the solar system is a lot of chaos. Does the status of our moon depend on whether or not it was captured or the result of a collision, for example?

This is a situation where there is no right or wrong answer. We like for categories to reflect underlying reality, and our scientific understanding of how things work and where they came from. But sometimes they are also arbitrary and based on what things look like to us. It may help us organize and communicate a lot of complex information, but there may be no way to avoid arbitrary criteria.


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