Two kinds of dwarf planets

Between 30 AU and 50 AU from the Sun lies a disk of ancient icy objects known as the Kuiper belt. Most of the objects are just a few tens of kilometers across; they never coalesced with other objects to form planets. But there are exceptions. Astronomers estimate there are a few hundred dwarf planets in the Kuiper belt. Thanks to ground-based observations using adaptive optics and the visit to Pluto last year by New Horizons, the properties of five of the Kuiper belt dwarfs—Eris, Haumea, Orcus, Pluto, and Quaoar—are becoming clearer. In particular, Amy Barr of the Planetary Science Institute in Tucson, Arizona, and Meg Schwamb of Academia Sinica in Taipei, Taiwan, have reexamined each planet’s density and the mass ratio of each to its principal moon. Barr and Schwamb found that the planets fall into two groups. Pluto (shown here) and Orcus each have mean densities below 2 g/cm³ and a large moon. Eris, Haumea, and Quaoar each have mean densities above 2 g/cm³ and a small moon. Given that the dwarf planets formed initially by accreting building blocks made from the same mix of ice and rock, Barr and Schwamb propose that the distinction between the two groups arose from the nature of the collisions that created the planet–moon systems. Pluto and Orcus captured their respective moons in relatively gentle collisions that preserved the bodies’ compositions and sizes. The other systems were created in collisions that were so violent that the planets lost some of their ice and the moons were formed anew from collision ejecta. (A. C. Barr, M. E. Schwamb, Mon. Not. R. Astron. Soc. 460, 1542, 2016.)