- Scientists say Pluto can no longer be considered a planet
- It has an erratic orbit, due to lower gravitiy, so it is not a classical planet
- Our solar system is now reduced to eight planets
Millions of textbooks and wall-charts across the world will have to be revised, after astronomers yesterday declared that Pluto could no longer be considered a planet.
Under historic new guidelines, the solar system has shrunk from nine planets to eight. Scientists decided that tiny Pluto was too puny for its own gravity to pull its orbit into a nearly spherical shape - the orbit is erratic - and so could not be considered a classical planet.
The world's leading astronomers yesterday voted to reduce the size of the solar system by stripping Pluto of its status as a planet.
The decision, taken by 2,500 scientists at the International Astronomical Union meeting in Prague, will force an urgent rewrite of the textbooks to include only eight planets in the solar system, with three newly-defined "dwarf planets", of which Pluto is now one.
The move was taken to solve an embarrassing 76-year-old fudge that has until now been papered over. When scientists at the Lowell Observatory announced the discovery of Pluto in 1930, they claimed it was several times larger than Earth, ensuring its prompt labelling as the ninth planet. It turned out to be a runt substantially smaller than the moon.
Until last year, the astronomy community turned a blind eye to Pluto's apparent crashing of the planetary party. But last year, Mike Brown, an astronomer at the California Institute of Technology, discovered a fly in the ointment - another celestial body larger than Pluto, with an orbit stretching beyond Neptune. The Hubble space telescope measured the object at about 1,490 miles in diameter - roughly 70 miles more than Pluto. While it is officially known as 2003 UB313, Professor Brown named the rock Xena, after the Warrior Princess television series, and claimed it as the 10th planet.
The discovery of Xena brought the crisis over Pluto, and the definition of the word "planet", to a head as it dawned on scientists that tens of other rocks hurtling around the sun could also qualify for the title.
By a majority vote, the IAU decided on a definition of planet as a body that orbits the sun, is so large its own gravity makes it roughly spherical, and, crucially, also dominates its region of the solar system.
The definition admits Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune and Uranus, but excludes Pluto because it is not big enough to clear smaller bodies close to it. Pluto, along with Xena and Ceres, an asteroid that lies between Mars and Jupiter, are now officially dwarf planets.
The IAU's decision to demote Pluto represents a U-turn on a proposal put before the scientists last week, which would have seen the solar system expand to 12 planets, with four, including Pluto, being classed as new objects called plutons. The proposal irked geologists, who have already bagged the word pluton to describe common rock formations, but the proposal was voted down because it was still too vague.
Although yesterday's decision was reached by a majority vote, some of the most senior scientists involved are dismayed. "We now have dwarf planets which are in fact not planets. I consider this a linguistic catastrophe. I think the union is going to get a lot of flak for this, in doing it in such a muddy way," said Owen Gingerich, chairman of the IAU's official planet definition committee.
"It'll cause a rewrite of the textbooks. I think a lot of the astronomers at the meeting were really just trying to correct the mistake made in 1930," he added.
But Iwan Williams, president of the IAU's planetary systems sciences and an astronomer at Queen Mary, University of London, was happy with the decision. "It was a clear majority and Pluto is still an interesting body to study," he said.
In addition to the categories of "planet" and "dwarf planet", the definition creates a third category to encompass all other objects, except satellites, to be known as small solar system bodies.
The astronomer Patrick Moore said: "They've made it far too complex. What is a dwarf planet? I agree that Pluto is not a planet, but why not just call it a Kuiper belt object or a large planetoid? In the end, I don't suppose it matters too much. It's just a name."
Source: The Guradian, Scotsman 
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