Pluto was booted off the list of planets by the International Astronomical Union in 2006, a controversy that still smarts for some skywatchers.
But Pluto should be added – along with more than 100 other celestial bodies in our solar system, according to a new planetary definition proposed by a group of scientists.
The new definition would include moons, spherical orbiting bodies and other phenomena that are distinctly different than the currently-accepted eight planets, according to the paper, to be presented at the forthcoming Lunar and Planetary Institute’s annual conference.
“In keeping with both sound scientific classification and people’s intuition, we proposed a geophysically-based definition of ‘planet’ that importantly emphasizes a body’s intrinsic physical properties over its extrinsic orbital properties,” they write.
“A simple paraphrase of our planet definition – especially suitable for elementary school students – could be, ‘round objects in space that are smaller than stars,” they add.
The IAU definition had two factors that essentially eliminated Pluto in 2006: the planet must be able to gravitationally hold its own in orbit, and clear the zone around itself, and it must be big enough to do so at progressive distances outward from the sun.
Six authors from five institutions were involved in the work: Kirby Runyon of Johns Hopkins University, S. Alan Stern and Kelsi Singer of the South Research Institution in Boulder, Tod Lauer of the National Optical Astronomy Observatory in Tucson, Will Grundy of the Lowell Observatory in Flagstaff, and Michael Summer of George Mason University.
Stern, who was the principal investigator for the successful New Horizons mission which buzzed by Pluto at a distance of several thousand miles back in 2015, has argued that the IAU definition itself eliminates some of the accepted eight planets – including the Mars, Jupiter, Neptune, and even the Earth itself.
By adding all the spherical objects in the solar system, the total number of planets would increase from eight to 110. The increase would make understanding the solar system more difficult – but ultimate more rewarding, the astronomers said.
“Certainly 110 planets is more than students should be expected to memorize, and indeed they ought not,” they conclude. “Instead, students should learn only a few (9? 12? 25?) planets of interest. For an analogy, there are 88 officials constellation and ~94 naturally-occurring elements, yet most people are content to learn only a few. So it should be with planets.”