Pluto fascinates, whatever we call it

Pluto fascinates, whatever we call it

We are less than a month away from the climax of one of NASA’s most exciting space missions, New Horizons’ close flyby of distant Pluto.

Pluto images
Some of the latest images received between May 29 and June 2 from New Horizons Long Range Reconnaissance Imager (LORRI). Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Already we have been getting tantalising views of this remote and mysterious world, with pictures sent back that reveal contrasting light and dark regions on its surface. 

And New Horizons, the fastest spacecraft ever to race out across the Solar System, promises to bring us images in much greater detail of Pluto and its five satellites as it zips past them on July 14, 2015.

Scientists hope to learn much more about the nature of icy bodies circling the Sun in the faraway Kuiper Belt, because Pluto stands as a kind of cosmic gatekeeper to a whole new unexplored outlying region of our star’s domain.

You may notice that I have got this far into this week’s blog without giving Pluto an official label. But the fact is that 76 years after its discovery, in 1930 by Claude Tombaugh, a period during which it enjoyed the status of a planet, Pluto was unceremoniously kicked out of that premier league within the Sun’s family.

Actually there was an element of ceremony about it, because it was the result of a vote by the International Astronomical Union, the official arbiters of matters celestial. But what a controversial decision it proved to be.

The organisation, meeting in Prague in the Czech Republic, produced a new definition of a planet with conditions that Pluto did not meet. It was therefore demoted to a new class of object called a dwarf planet (the continued use there of the word planet adding another level of confusion). Ceres, the largest of the asteroids, was at the same time promoted to this new class.

The decision was swiftly challenged, with one of the leaders of the rebellion being Alan Stern, who happens to be the principal investigator on the New Horizons mission that had launched on its long journey just seven months before the IAU vote.

One can imagine the annoyance in discovering that one’s ongoing mission to a planet has become a trip to a body considered less significant in the grand order of things. But the rebels argued that the IAU’s new criteria for what constituted a planet did not meet fundamental scientific standards.

For example, they said, one strand to the definition, declaring that a planet must have cleared its neighbourhood of other objects, would mean that Earth was not a planet either if strictly applied, since asteroids still cross our orbit.

The other definitions in the IAU’s resolution, by the way, declared a planet to be a celestial body that is in orbit around the Sun, and has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.

There was special anger within the United States over Pluto’s demotion, and the State of New Mexico even passed a resolution declaring it still to be a planet. Perhaps some of that was down to national pride, as Pluto had been the only planet discovered from that part of the world. Tombaugh found it on photographic plates taken at the Lowell Observatory in Flagstaff, Arizona.

Some of the backlash, dare I suggest, was down to sentiment, from a generation that had learned to love Disney’s loveable cartoon pooch. In fact, Pluto was named after the Roman god of the underworld, by Venetia Burney, an 11-year-old girl in the UK. 

Interestingly, there was some debate over whether Pluto was really a planet right back when it was discovered in 1930. The discovery telegram simply termed it a “trans-Neptunian body”. And a contemporary report of the find, in the June issue of Popular Science Monthly, includes the line: “It behaves differently from other planets. Its orbit is so far from a perfect circle, and tilted so askew, that astronomers hesitate to declare it a planet with certainty.”

The writing was finally on the wall for Pluto early in the new millennium, when other icy bodies were discovered in the Kuiper Belt, that remote zone that Solar System modelling had predicted would exist half a century earlier. In particular, it was found to be home to three new worlds—Eris, Haumea and Makemake—that also gained the status of dwarf planets because, like Pluto, they are big enough to have become round in shape under their own gravitation. They are also known as Plutoids. Mike Brown, a leading discoverer of these new worlds, believes there are dozens more dwarf planets out there that have not yet been classified.

I expect that the controversy over Pluto’s status will become a talking point again over the next few weeks, as NASA’s first visit to its neighbourhood stirs international interest in that distant world. I just hope that the debate does not get in the way of the fantastic scientific discoveries that we can expect, much as an ill-chosen shirt diverted attention from ESA’s stunning encounter with a comet in November.

Our understanding and perception of the Solar System has been constantly changing throughout history. And as Shakespeare put it in Romeo and Juliet: 

“What’s in a name? that which we call a rose

By any other name would smell as sweet.”

Meanwhile Pluto is still there, the same object and just as interesting as before, regardless of how it is labelled. So I, for one, am going to sit back and enjoy the ride as our knowledge of this fascinating object gets a massive boost.

(This article originally appeared on my regular blog for

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