Astronomical Society of Edinburgh
Journal 45

Mini Planets

In the light of recent discoveries, let's keep Pluto as a major planet, although the mass ratio goes against it. Consider its size, especially the diameter and surface area: you have quite a sizable object still, and Pluto has a satellite. This emphasises a new class of major planet, the low-density ice-planets, a third category after the denser inner terrestrials and the outer gas giants. Pluto is also the first of another differentiation, the first irregular major planet with an eccentric and inclined orbit, which mirrors the satellite systems of the giant planets on a larger scale, so it is expected there will be more irregular planets. I would argue for 1000 km as the upper limit for minor planet status thus giving Ceres the special status of the largest minor planet. Also, let Pluto be the lower limit for major planet status. We then have a new size category for any object intermediate in size between Pluto and Ceres.

And there are at least two, three or four objects in this group known to exist within the Kuiper Belt, which I would like to suggest we refer to as mini-planets. The more of these we discover and the more we learn about Varuna, Ixion and Quaoar, then the more we realise a new intermediate category of small spherical bodies too big to be classed as minor. Perhaps Charon too is one of these rather than just a satellite of Pluto. After all, Pluto-Charon does constitute a double planet. We can expect to find quite a large population of mini-planets in the irregular Kuiper Belt region nested amongst thousands of minor bodies. As more bodies are found, we will find many more of Pluto's wee brothers such as Quaoar, then one day we will find Big Brother : perhaps a Mars-sized/Mercury mass/Pluto-density ice-rock world which will be the real major planet ten, perhaps a second irregular, but the cause of some of the peculiar orbits of smaller Kuiper Belt objects which could only have been placed by an outer planetary perturbation beyond Neptune. Finally, it might be fitting to give our own satellite devolved planetary status given its constitution and unique position the Moon, after all, is the fifth inner terrestrial planet and would easily be visible from Mars.

Graham Young (Dundee Astronomical Society)