Is Pluto back?

New suggestions for defining a planet would put Pluto and many other objects back on the list.

Ask planetary scientist Mark Sykes where NASA’s Dawn spacecraft is headed, and he will say it is on its way to the largest asteroid and the smallest planet.A planet in the solar system, the IAU says, must:-

orbit the sun; have enough gravity to make it nearly round; and have gobbled up or sent packing any objects found in its orbit. 

A dwarf planet, under IAU rules, is not a planet. The IAU says a dwarf planet orbits the sun, is not a satellite, has enough mass to make itself nearly round and has not booted objects from its orbit.

But how can a dwarf of something not be considered one of that thing? Sykes asked.

That sentiment was expressed again and again by many scientists at the conference. “It is grammatically and logically weird that a dwarf planet is not a planet. That rule is unacceptable and violates laws of logic and grammar,” said planetary scientist David Morrison of the NASA Ames Research Center in Moffett Field, Calif. And that is what Sykes is doing, he said — at least partially. He is selecting the part of the IAU definition that he finds useful, arguing that a planet is anything that orbits a star, doesn’t fuse elements in its core and has enough internal gravity to be nearly round.

Those criteria would make Ceres a planet. It would remake Pluto one too. There would be at least 13 planets in the solar system with many more, possibly thousands to come, he said. 

The thousands would lie in the Kuiper Belt, the ring of planet-like chunks of rock and ice in Pluto’s neighborhoodStern countered by saying that his concept of a definition — one “based on the physical, the intrinsic properties of a planet” — is how he defines a planet. It also pushes the bounds of what a planet is. When, or if, there is ever a consensus, he thinks the definition of planet should fall between his “radical” definition and the more restrictive, dynamics-based IAU definition.

At any rate, when Dawn gets to Vesta and then Ceres, and NASA’s New Horizon mission gets to Pluto and other Kuiper Belt objects in the 2010s, the information gathered is going to be important, whether or not the objects are planets, said planetary scientist Hal Levison of the Southwest Research Institute in Boulder,Colo.