The little planet that could

The jury of the International Astronomical Union, the astronomers' regulating body, is still out on whether Pluto is a planet or a dwarf planet.

The jury of the International Astronomical Union, the astronomers' regulating body, is still out on whether Pluto is a planet or a dwarf planet.


Hiding behind the cheers and the whoops that accompanied the triumph of Nasa’s New Horizons mission, there was an elephant in the room every bit as large as the world that the spacecraft had been sent to study. Embarrassingly, Pluto is no longer a planet according to the International Astronomical Union, the astronomers’ regulating body.

That didn’t stop the mission’s principal investigator, Alan Stern, referring to it in passing as a planet during the press conference at closest approach.

Nor did it stop Nasa’s chief administrator, Charles Bolden, using the P-word in the days leading up to the flyby. But the reality is that the argument over Pluto’s status has been rancorous, and highlights both the lofty ambition of science and the pettiness of its practice.

When New Horizons left Earth on January 19 2006, Pluto was a planet.
However, little more than six months later it was ignominiously demoted in a spat between astronomers in Prague, where the International Astronomical Union was meeting.

To be a planet, they decided, a celestial body had to be in orbit around the sun and it had to be spherical (more or less). So far, so logical. The problems began with the final clause: it had to have “cleared the neighbourhood of its orbit”.

This was the Pluto-killer because Pluto was diminutive and shared similar orbits with smaller asteroid-like objects. Clearly, it had not cleared out its orbit and so could not be a planet.

The trouble is that the third criterion makes little sense. Jupiter is dogged by asteroids called trojans, Earth has the moon following its every move and Neptune crosses orbits with Pluto.

Until 2006, astronomers had bumbled along quite happily for centuries discussing whether new bodies were planets on a case-by-case basis. It allowed usage and culture to be the final arbiter of planethood. In attempting to make a scientifically binding definition, astronomers simply made things confusing.

There is no way the definition can be easily taught in schools or widely understood because it relies on Byzantine knowledge of solar system dynamics and even then doesn’t actually make much sense.

Pluto is without question a small world. With a diameter of just 2 370km, it is smaller than our moon. Its volume is less than 1% of the Earth’s, meaning that it has a total surface area equivalent to that of Russia. Yet this should not necessarily ban it from being a planet.

To deny Pluto is also to deny the cultural history of astronomy. Its 1930 discovery by Clyde Tombaugh at the Lowell Observatory, Arizona, was a milestone in science that had a tremendous impact on the study of the celestial realms. It focused attention on the solar system’s outermost realms, and we can’t just rewrite this history.

As the flyby images have shown, not only is Pluto an important world in its own right, it is also the herald of the outer reaches where ice mingles with rock to sculpt the worlds we find there. Studying it may provide the very keys we need to understand the formation of our own much larger world.

However you look at this, Pluto deserves to be a planet, and astronomers deserve to put their efforts into understanding it.

It is time to strip back the rules of planethood, reinstate Pluto and accept that we live in a larger, more varied solar system than some would like. – © Guardian News & Media 2015

All you need to know about Nasa’s historic Pluto flyby

Nasa’s spacecraft New Horizons made its flyby of Pluto on Tuesday and scientists are standing by for data from the craft. Here’s a guide to the mission and what happens next.

When did the mission begin?
New Horizons blasted off on January 19 2006, and was the fastest launch recorded, reaching more than 58 000km an hour. The spacecraft passed the moon after nine hours, about eight times quicker than the Apollo programme, and reached Jupiter the following year.

Is Pluto a planet?
Several months after the launch of New Horizons, astronomers at the International Astronomical Union voted to change the definition of the word “planet”, a move that downgraded Pluto to the lesser “dwarf planet”. The fly-by has resurrected the debate, however, and Charles Bolden, Nasa’s chief administrator, said he hoped the official classification would be reconsidered. “I call it a planet, but I’m not the rule-maker,” he said.

Who are the scientists behind the mission?
Alan Stern is the New Horizons mission commander; he closed the hatch on the spacecraft and bid it a final farewell when it launched.

But his cheerleading for the scientific exploration of Pluto dates back to a decade before, when the possibility of Pluto missions were regularly floated in Nasa and then scrapped or put on hold.

“If the Pluto mission was a cat it would be dead long ago because it’s had more than nine lives,” he said.

Stern and other key members of the mission team, such as Alice Bowman, mission operations manager, and Glen Fountain, the project manager, have been quietly preparing for this week out of the public eye for nearly a decade, tracking the probe as it crossed the solar system.

After this week, once the fuss dies down, their work will start in earnest as they begin analysing the data that is slowly being streamed back to Earth from the spacecraft.

Where is New Horizons heading next?
The mission is now speeding on into the Kuiper belt where it will examine one or two of the ancient, icy miniature worlds in the region. The so-called Kuiper belt objects it might fly by include Quaoar, Eris (close in size to Pluto), Makemake, Haumea or Sedna.

In the coming months, scientists will decide the spacecraft’s next target. They will then send signals from Earth to New Horizons to thrust its rockets and tweak its trajectory.

When will the mission end?
The fuel is designed to last until the late 2020s or even beyond. When it runs out of power, astronomers will lose contact with the probe and it will continue to drift out past the Kuiper Belt and eventually leave the solar system. – Hannah Devlin © Guardian News & Media 2015

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