Why has the debate about Pluto being a planet been exhumed all over again?
The immediate provocation was a Johns Hopkins University scientist, Kirby Runyon, and his poster last month at a scientific conference, the 48th Lunar and Planetary Science Conference, Texas. His presentation argued that the definition of what constitutes a planet be changed. Dr. Runyon and poster co-authors (including Alan Stern, a senior astronomer who’d vigorously opposed Pluto’s demotion a decade ago) were part of the science team on the New Horizons mission to Pluto, operated for NASA by the Johns Hopkins University Applied Physics Laboratory. In the summer of 2015, the New Horizons spacecraft became the first to fly by Pluto, passing within 8,000 miles and sending back the first close-up images ever of Pluto. These factors combined to whet interest in the revivification of Pluto’s planetary status.
What’s Runyon’s argument and will that once again mean nine planets?
The International Astronomical Union, in 2006, laid down three criteria for a rocky body to be planet: it must orbit the sun, it must be round, the body and its satellites must “orbit in a clear path around the sun”. It’s the last bit that buried Pluto, as many other asteroids and planets, some bigger than Pluto, were found in its orbital neighbourhood. Dr. Runyon and co-authors proposed that the offending third clause be deleted. To be sure, there isn’t a novel scientific argument for Pluto’s case that hasn’t already been made. Pluto being made a planet again, according to him, would mean that “the public would again fall in love with planetary exploration.”
What are the consequences of accepting the modified definition?
Along with Pluto being upgraded from its current “dwarf planet” status, nearly 100 other celestial bodies in the solar system could also become planets. The celestial bodies include Europa, a moon of Jupiter, and our very own moon. It also means that there will be nothing special about the existing eight planets and that, according to critics of Dr. Runyon, would offer a distorted picture of the solar system.
Can there be finality to this debate?
The International Astronomical Union arrived at their decision to demote Pluto after two years of debate and a proposal to a ‘Planet Definition’ sub-committee. This was then put to a vote, with 237 astronomers voting for and 157 against. There’s no report yet of the IAU moving to reconsider their position. Dr. Stern has previously argued that most of these astronomers were experts in stars and galaxies and not ‘planetary scientists’. Those who are convinced, proceed with their science as if Pluto is the planet from pre-2006 textbooks.
