Fans of Pluto returning to its old planetary position rejoice – a recent study by planetary scientists found that the controversial 2006 decision the International Astronomical Union to demote Pluto to dwarf planet status was based on a flawed characteristic which had never been used before to define planets. While removing this characteristic would return Pluto to its ninth planet spot, the scientists also want to add a new characteristic which would open the planetary club to a number of the solar system’s moons. Are we ready for that?
“This might seem like a small change, but it undermined the central idea about planets that had been passed down from Galileo. Planets were no longer defined by virtue of being complex, with active geology and the potential for life and civilization. Instead, they were defined by virtue of being simple, following certain idealized paths around the Sun.”
Dr. Philip Metzger, a planetary scientist at the University of Central Florida’s Florida Space Institute, is the lead author of a new study published in the journal Icarus which presents the idea, outlined in a UCF press release, that new data from modern telescopes and space travel has reignited an interest in planets and moons which has prompted some rethinking back to old thinking about the definition of a planet – old thinking going all the way back to the father of observational astronomy and proponent of heliocentrism … Galileo Galilei. According to Metzger, Galileo defined planets as space objects which were currently or had in the past been geologically active.
That held true until the 1900s, when Metzger claims almanacs like The Old Farmer’s Almanac drew the public to observe – and give credence to – the positions of the planets over their composition. This led to an inclination that size was important in the definition of planets because only large planets with strong gravity could make their own unique traceable path through the sky.
“So, some scientists tried to develop a method to mathematically justify a small number of planets, which was the criterion that a planet has to clear its own orbit. And this was really developed post facto to keep an orderly, small number of planets.”
Metzger objects to a planet’s current trajectory being the defining factor of its nature because a star passing near our solar system could disrupt the path of Jupiter and have it cross orbits with Saturn – would that mean the gas giant is no longer a planet?
“It’s like defining ‘mammals’. They are mammals whether they live on the land or in the sea. It’s not about their location. It’s about the intrinsic characteristics that make them what they are.”
That’s where study co-author Charlene E. Detelich, a geologist and researcher with the Johns Hopkins University Applied Physics Laboratory, joins in and pitches the idea that large geologically-active moons like Jupiter’s Europa should be called planets too.
“For the term planet, myself and most planetary scientists consider round icy moons to be planets. They all have active geologic processes that are driven by a variety of internal processes, as does any world with enough mass to reach hydrostatic equilibrium. As a geologist, it is immensely more useful to divide planets by their intrinsic characteristics than by their orbital dynamics.”
By that definition, Pluto would rejoin the current eight and adding the active moons would push the total to around 200. Detelich says she sympathizes with schoolchildren who would now have to memorize the names of so many planets (not to mention devote a LOT more time to solar system science projects) but she argues that simplicity should not be the sole driving force in planetary definition.
“We present evidence that taxonomical alignment with geological complexity is the most useful scientific taxonomy for planets. It is this complexity of both primary and secondary planets that is a key part of the chain of origins for life in the cosmos.”
Do you agree?