How big is a dwarf planet?

An artist’s impression of dwarf planet, Eris

As the name ‘dwarf planet’ indicates, size has a lot to do with the designation. To be a dwarf planet and not an asteroid or other small Solar System body, an object has to be big enough to have sufficient gravity to pull the object into a stable spheroid (Haumea, with its ellipsoid shape, is an exception because it’s considered stable). This can’t be defined by a specific measurement, because it varies depending on both the object’s composition and its history. For example, astronomer Mike Brown believes that rockier bodies reach hydrostatic equilibrium at about 900 kilometres (559 miles) and icier ones, between 200 and 400 kilometres (124 and 249 miles).

Compounding the issue is the fact that it can be difficult to measure the size of distant objects. We estimate sizes of Solar System objects by measuring their absolute magnitude (brightness), as well as their albedo (reflectivity). Absolute magnitude allows astronomers to measure the brightness of Solar System objects as if they were all the same distance from the Sun and the Earth and at the same angle. A negative absolute magnitude indicates a bright object, while positive numbers indicate dimmer objects. Albedo is a ratio of reflected sunlight, so an albedo of 1 would be a perfect reflection of a white surface and zero would be no reflection of a perfectly dark surface. The presence of satellites or other objects around it also helps determine an object’s mass. Yet all of the measurements are estimates, with varying margins of error.

Initially the IAU did not establish limits for dwarf planet size. Later, it clarified that dwarf planets must have an absolute magnitude brighter than +1. This means that its diameter will be greater than 838 kilometres (521 miles), assuming an albedo greater than or equal to 1.

The New Horizons spacecraft is currently on its way to Pluto

Estimates of Pluto’s diameter have varied by as much as 70 kilometres (44 miles) depending on the instrument used and the haze in its atmosphere. When Eris was discovered, its diameter was estimated to be 2,397 kilometres (1,500 miles) – making it larger than Pluto. Later it was revised, and given the margins of error, these dwarf planets are considered to be roughly the same diameter. Eris is the more massive of the two, with a mass about 0.27 per cent that of the Earth’s mass. It also has an albedo of 0.96, one of the highest in the Solar System, and an absolute magnitude of -1.19.

When it comes to dwarf planet candidates, there is some controversy. Since the IAU has decreed that a dwarf planet must have an absolute magnitude brighter than +1, which potentially rules out some otherwise good candidates if you consider Mike Brown’s list of possible dwarf planets. For example, dwarf planet candidate Sedna has an absolute magnitude of 1.8 and the latest measurements estimate a diameter of 995 kilometres (618 miles), give or take about 80 kilometres (50 miles). This is large enough to be spherical. The largest estimated unnamed object in the Solar System is a trans- Neptunian object currently named 2007 OR10. It has a very reddish surface and an estimated absolute magnitude of 2, but a diameter between 1,070 and 1,490 kilometres (665 and 926 miles). The object is just too far away to get a better measurement and to be sure of hydrostatic equilibrium.

So, although there are just five officially confirmed dwarf planets currently, many astronomers and planetary scientists agree that in reality far more objects should probably be classified as more than just KBOs. It’s likely a matter of getting better measurements of the candidates as well as the IAU making some more changes.

Image Credit: NASA