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Planet Pluto's demotion and promotion: does size really matter?

Photo: coulartsandleisure.blogspot
Many moons ago – in January 2006 to be precise – NASA sent a probe, called New Horizons, on a ten-year mission to search for the edge of our solar system. It went very close to planet Pluto, our solar system’s smallest planet.

On August 24, 2006, planet Pluto was stripped of its title. Pluto was demoted from planet status to dwarf planet status. The demotion was a blow to Pluto. There was much debate on its demotion with many scientists seeking its promotion to its original status. Size isn’t everything, they said. A Chihuahua is still a dog, they said. They said that it was obvious that objects of this size, thousands of miles in diameter, were planets. They argued that it had a crust and a core (the attributes of planets) and atmospheres and moons (the attributes of planets) and seasons too (also the attribute of a planet). Full planethood was assured, they said. But no, it remains to this day a demoted planet.

Nine years (and 3 billion miles) later, the New Horizons probe is getting close to Pluto again – on the outer edge of our solar system. And, in the next few months, people on Earth will see photographs of pretty Pluto. New Horizons was estimated to reach Pluto in July 2015, but we may see photographs sooner than that. New Horizons will be the first spacecraft to photograph Pluto with the aim of learning more about it – how it evolved, what its moons are like, and what type of geology it has.
To keep New Horizons going, scientists powered down a lot of onboard systems – they basically turned it off. Now that it is getting close to its target, all systems are up and running again, in readiness to take the photographs. What if, when the whole of planet Earth sees the photographs of Pluto, scientists have second thoughts about their 2006 decision? Could Pluto be on its way to a promotion?

Here’s a reminder of why Pluto was demoted in the first place. The International Astronomical Union (IAU) voted to demote it. American astronomer Clyde W. Tombaugh first discovered Pluto in 1930 from the Lowell Observatory in Flagstaff, Arizona. He had observed it for about a year – with its 5 moons – before he finally declared that he had found planet X (which scientists had predicted might be in our solar system). Tombaugh and his team called it Pluto. In 1978 scientists worked out its mass and size (2,400 kms or 1,500 miles across). It was a bit smaller than planet Mercury (4,880 kms or 3,032 miles across).

The problem was that in a place in space called the Kuiper Belt (where Pluto is), astronomers were finding other large objects with the same composition as Pluto. So some scientists said that Pluto was “just another Kuiper Belt object.” And then in 2005 an astronomer found an object about the same size as Pluto (called UB313 first before it was called Eris). Eris is about 2,600 kms or 1,600 miles across with 25% more mass than Pluto. What were astronomers going to classify Eris as – a planet or a just a Kuiper Belt object? At the 2006 IAU conference scientists debated the definition of a planet. Some said both Pluto and Eris were planets (and maybe so was the large asteroid Ceres). IAU concluded that a planet was defined as: (1) an object in orbit around the Sun; (2) having sufficient gravity to hold its spherical shape; and (3) having “cleared the neighbourhood” of its orbit. Big objects in space push away smaller objects in its own orbit. For example, Earth has 1.7 million times the mass of other objects in its own orbit and therefore can push them away. But Pluo is only 0.7 times the mass of other objects in its orbit and therefore is not very strong. The final vote was that Pluto did not meet the third requirement. So it came down to strength, not size!

The New Horizon spacecraft will not only take photographs of Pluto, but it may also determine any difference in its size or strength (to fulfil the third requirement). Will Pluto’s demotion become a promotion? Well, it will it terms of promoting the beautiful photographs and Pluto’s public relations campaign, but will it be enough to promote Pluto to full planet status?

That’s why it will be exciting to see the planetary photographs. We wait with great anticipation. I’m on the side of a promotion!

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