How many planets in your solar system? (Image: JPL / NASA)
HOW many planets are in the solar system? The official answer is eight - unless you happen to live in Illinois. Earlier this year, defiant Illinois state governors declared that Pluto had been unfairly demoted by the International Astronomical Union, the authority that sets the rules on all matters planetary.
Three years ago, the IAU decided to draw up the first scientific definition of the term planet. After days of stormy arguments at its general assembly in Prague, the delegates voted for a definition that excluded Pluto, downgrading it to the new category of dwarf planet.
The decision caused outrage among many members of the public who had grown up with nine planets, and among some astronomers who pointed out that only 4 per cent of the IAU's 10,000 members took part in the vote. The governors of Illinois saw the decision as a snub to Pluto's discoverer, Clyde Tombaugh, who was born in the state.
Next week the IAU's general assembly will convene for the first time since Pluto was axed from the list of planets. Surprisingly, IAU chief Karel van der Hucht does not expect anyone to challenge the ruling made in Prague, but Pluto fans can take heart: resistance remains strong.
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Please do not blindly accept the controversial IAU demotion of Pluto and/or their statement that our solar system has only eight planets as "official." That gives the IAU an authority they have not earned. Their decision was opposed by hundreds of professional astronomers in a formal petition led by Dr. Alan Stern, Principal Investigator of NASA's New Horizons mission to Pluto. Stern and like-minded scientists favor a broader planet definition that includes any non-self-luminous spheroidal body orbiting a star. The spherical part is important because objects become spherical when they are large enough to be pulled into a round shape by their own gravity. This is a characteristic of planets, not of shapeless asteroids. Using this criterion, our solar system has 13 planets: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris.
ReplyDeleteYes, resistance and rejection of the IAU decision remain strong regardless of what happens in Rio. Many of the scientists who reject the IAU decision have chosen to simply ignore the IAU and boycott the General Assembly altogether. The IAU definition should not be viewed as more legitimate or official than the opposing view, which is supported by an equal number of planetary scientists.