Australasian Science: Australia's authority on science since 1938

Dwarf Planets Are Not So Rare

By Stephen Luntz

The number of dwarf planets in the solar system may be ten times higher than previously thought.

The point at which objects in the solar system become dwarf planets has been wrongly estimated, a new study has found, potentially increasing the number of these objects in the solar system.
When Pluto was demoted from its status as a planet in 2006, a new category was created know as dwarf planets. These are objects that circle the Sun directly and are large enough that their gravity makes them spherical (allowing for flattening at the poles for faster-spinning members). However, they are not large enough to have cleared the region neighbouring their orbit of planetesimals, which is necessary for them to attain planetary status.
Popular outrage at the demotion of Pluto was somewhat assuaged by the fact that the new category was in some sense even more exclusive than its old one. At the time there were eight planets and only five dwarf planets, although several other possibilities had been found.

Dr Charles Lineweaver of the Australian National University’s Planetary Science Institute was intrigued by images of icy moons in the outer solar system. Noticing that the larger ones were round and the smaller ones looked more like potatoes, he pondered the point at which the transformation would occur.

“Whether the self-gravity of an object is strong enough to make the object round depends on the strength of its material,” Lineweaver says. “That is why strong rocky objects need to have a radius of roughly 300 km before they turn from lumpy, potato-shaped bodies into spheres, while weaker icy objects can be spheres with a radius of only roughly 200 km.”

Lineweaver did the calculations to find these figures, and then checked them against observations of asteroids and icy moons. However, when he reviewed the literature he found that astronomers were assuming that the size required to make an object circular was 400 km.

“It seems to have come from a conservative handwaving estimate based mostly on rocky objects. The most recent paper we can find comes from 2006, but they didn’t calculate it themselves – they repeated something which had been informally handed down,” Lineweaver says.

Since almost all objects in the outer solar system are icy, Lineweaver’s work suggests that many objects already known to be comfortably larger than 200 km across should be classified as dwarf planets, possibly expanding the members of the category by a factor of ten.