Out in space no one can hear you complain. So whoever is living on Pluto is spared the endless biffo between scientists as to whether that distant ball of rock is in fact a planet. Or not.
This week, physicists from the University of Central Florida published a paper saying it really is a planet, in part because it’s so interesting. We’ll get back to that.
Twelve years ago, most of the world had a good howl when the International Astronomical Union (IAU) declared that Pluto – a fifth the size of Earth, and a mite smaller than our moon – was in fact not a planet because it shared an orbit with bits of icy junk from the Kuiper belt, that mysterious circle of stuff that encircles the outer Solar System.
There was also the matter of Neptune pulling on Pluto like a possessive boyfriend – and therefore not entitled to a life of its own.
The IAU defines a planet as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
Pluto apparently fails the third requirement.
Are we over it?
If you grew up with Pluto as a planet – and retained it as a remnant of easy knowledge from science class – the downgrade to dwarf planet felt like one more disappointment. And then we moved on.
Last year Johns Hopkins University scientist Kirby Runyon said the IAU got it wrong because – and this is the short version – a planet is essentially a body that has enough gravitational heft to maintain a spherical shape. That is, any hefty ball of rock deserves the tag. This included our moon and more than 100 other objects. Suddenly we’d have 110 planets – too many to care about.
Now, Runyon is back, co-author – with UCF planetary scientist Philip Metzger – of a new study published online on Wednesday in the journal Icarus.
They argued the IAU’s reasoning was “sloppy” in the first place – and not supported by the research literature.
Geeks, we have a problem
Dr Metzger, who is lead author on the study, reviewed scientific literature from the past 200 years and found only one publication – from 1802 – that used the clearing-orbit requirement to classify planets, and it was based on since-disproven reasoning.
In a piece on the UCF website, he said moons such as Saturn’s Titan and Jupiter’s Europa have been routinely called planets by planetary scientists since the time of Galileo.
“And that’s not just an arbitrary definition,” Dr Metzger said. “It turns out this is an important milestone in the evolution of a planetary body, because apparently when it happens, it initiates active geology in the body.”
Pluto, for instance, has an underground ocean, a multilayer atmosphere, organic compounds, evidence of ancient lakes and multiple moons, he said.
“It’s more dynamic and alive than Mars,” Dr Metzger said. “The only planet that has more complex geology is the Earth.”
Watch this … space.