Why there are now eight planets and not nine
By MUNGAI KIHANYA
The Sunday Nation
Nairobi,
03 September 2006
What happened to Pluto? Why did the press report that
there are now eight planets and not nine as we learned in school? Will
the schoolbooks have to be rewritten? Several people have asked me these
questions. So here is what happened:
In the entire history of astronomy, there has never
been a commonly agreed definition of the word “planet”. When Pluto was
discovered in 1930, astronomers were quick to call it the ninth planet
even before many of its characteristics were established.
Pluto was initially thought to be quite large
(comparable to the Earth in mass and diameter) but several decades
later, astronomers confirmed that it is a very small body (smaller than
our moon!). Another problem was calling it the
ninth planet.
Pluto’s orbit around the sun is so elliptical that
some times it is nearer the sun than
But the biggest problem was the discovery of other
heavenly bodies orbiting the sun at greater distances than Pluto. Many
of these objects are comparable in size to the so-called ninth planet.
Thus some Astronomers insisted that if, Pluto is a planet, then there
are 13 planets in the solar system.
So last week, the International Astronomical Union
(IAU) met to resolve this confusion. The experts agreed that a planet is
any object that (a) orbits the sun, (b) is massive enough for its own
gravitational force to pull it together into a round shape, and (c) has
cleared the neighbourhood of its orbit.
Only eight objects in the solar system meet the three
conditions, namely Mercury through
Last week’s IAU meeting classified Pluto as a “Dwarf
Planet” together with two other bodies: Ceres (the largest object in the
asteroid belt between Mars and Jupiter) and the unnamed
2003UB-313 (which is further and larger than Pluto).
So what happens to the schoolbooks? New editions must adopt this
development. What about exams – what if the question “name the ninth
planet” is in this year’s KCPE? Well, the best action for the examiners
would be to ignore the question during marking.
***
A few weeks ago, I gave experimental evidence to
prove that cooking gas is more expensive than electricity – up to four
times more costly. Several readers have challenged my results but,
unfortunately, most have not provided observation data to support their
position. It therefore becomes difficult to respond to such objections.
Nevertheless, Jimmy Wanamboe makes a valid point that
the comparison between the kettle and a
sufuria is inappropriate. As
he puts it “we can’t cook Ugali
in a kettle”. True. But we can save a significant amount in energy cost
by boiling the water in a kettle first and then pouring it in a
sufuria to make the
ugali – or any other meal.
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