Michael Guillen: Why is Pluto no longer a planet? The answer may surprise you (here’s why it also must change)
Pluto was a planet in good standing for seventy-six years when in 2006, out of the blue, it was demoted and booted from our solar system’s family of planets. The stunning event – still hotly debated among astronomers – reveals more about astronomy’s messy subjectivity than Pluto’s stature.
In 1915 Percival Lowell predicted the existence of a planet beyond Neptune. In 1930 Clyde Tombaugh – using the Lowell Observatory in Arizona – actually found it!
Years ago I interviewed Tombaugh for ABC News; he was a warm, sweet man. Mercifully he died in 1997, well before Pluto’s scientific character assassination.
The drama traces back to 1992 when astronomers found a region of our solar system beyond Neptune that teems with small, icy worlds. It’s named after Gerald Kuiper, father of modern planetary science.
Pluto exists within this Kuiper Belt and resembles its icy neighbors in certain ways, but is much bigger. In 2003, however, astronomer Mike Brown and others discovered a beltway denizen that rivaled Pluto in size. It was named Eris.
The find made headlines and triggered an internal debate about what actually constitutes a planet. Unbelievably, it was a question that scientists for more than two thousand years had never formally settled.
Yes, it <em>does</em> matter what we call things. Science is all about classifying nature.
The debate boiled down to this: Should Eris be invited into our solar system’s exclusive country club of planets? Or should Pluto be shown the door?
In 2006 astronomers gathered in Prague for the triennial general assembly of the International Astronomical Union (IAU). On the assembly’s final day attendees voted to approve an unprecedented definition of planet; one clearly designed to exclude Pluto.
A legitimate planet, the IAU declared: 1) orbits the Sun, 2) has a “nearly round shape,” and 3) “has cleared the neighbourhood around its orbit.” Pluto fails the third criterion.
Presumably, because it knew how unpopular its definition was going to be, the IAU also voted to approve a booby prize – a category of second-class citizens called “dwarf planets,” which did not satisfy the killer third criterion. Just like that, Pluto became a dwarf planet!
Why would the IAU do such a thing?
Mainly because a more open-minded definition would allow – God forbid – hordes of celestial objects into the ultra-exclusive country club of planets. One group of astronomers recently proposed that planets are simply “round objects in space that are smaller than stars.” If so, our solar system has 110 planets.
My favorite of all proposed definitions is this: a planet has a radius of at least 1,000 kilometers (about 621 miles). That makes it gravitationally hefty enough to assume a round shape – exactly how we picture a planet. This solution implies our solar system has ten planets, including Pluto and Eris.
Options aside, here are three big reasons why the IAU’s decision, not Pluto, should be ditched.
One: “If you take the IAU’s definition strictly, no object in the solar system is a planet,” explains NASA planetary scientist Alan Stern. “No object in the solar system has entirely cleared its zone.” Even Earth’s neighborhood is cluttered with more than 20,000 asteroids – Near-Earth Objects (NEOs) – routinely threatening to hit us.
Two: the IAU definition is ambiguous, completely arbitrary – not based on scientific precedent – and specifically intended to discriminate against planetary diversity. There is not an objective, scientific reason to lock out worlds such as Pluto simply because they’re different than the eight country clubbers.
The truly impartial thing to do is admit that planets – like stars and galaxies – come in a far greater variety of sizes, appearances, and behaviors than we ever imagined.
Three: the IAU ratification process was not legitimate. The votes were taken during the assembly’s closing ceremonies, ****after*** most of its 2,500-plus attendees had vamoosed. Only 424 astronomers ended up voting, out of IAU’s 10,000 total members.
Recently, when some astronomers refused to abandon Pluto’s cause, Mike Brown – Eris’s co-discoverer, you’ll recall – tweeted, “Oh god the stupid Pluto stories are back. Yes, someone has proposed making Pluto a planet again.”
His colleague Konstantin Batygin agreed, tweeting, “let’s all remember that astrophysical bodies are characterized by mass, radius, orbit, etc. These quantities are what’s important, not what we call them.”
But if Batygin really believes categories don’t matter, then why have he and Brown spent so much energy fighting against planetary diversity and for Pluto’s categorical downgrading – and boasting about it? Brown’s Twitter handle is @PlutoKiller.
It’s because they know it does matter what we call things. Science is all about classifying nature.
Sure, Pluto is a small, cold oddball. But thanks to the ongoing analysis of eye-popping photos and data from the New Horizons spacecraft that flew past it in 2015, we’re now learning that Pluto, with its five moons, is one of the most spectacular worlds in our solar system. It even looks to have key attributes possibly conducive to life, including water and organic chemicals.
“It’s more dynamic and alive than Mars,” says University of Central Florida planetary scientist Philip Metzger. “The only planet that has more complex geology is the Earth.”
For all of these reasons and others, I believe it’s high time for us to restore Pluto’s rightful place in the solar system and call it what it is: a planet!
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