Which of these would you say is a dog: a German Shepherd or a Chihuahua? This is the kind of question put before delegates of the International Astronomical Union (IAU) General Assembly who powwowed in Prague over Pluto's planet-hood. No wonder hardly 300 of the 2,700 astronomers at the meet even bothered to vote for, or against, stripping the ninth rock from the Sun of its planetary status and making it a 'planet dwarf' instead.
The planets in our solar system have been traditionally divided into two main groups. The inner band of relatively small worlds - Mercury to Mars - is followed by a wide gap, where the 'minor planets' or asteroids move. Beyond this is the realm of the four giants - Jupiter, Saturn, Uranus and Neptune.
Pluto's 248-year journey around the Sun is unusual for two reasons. First, its narrow, elliptical orbit edges it closer to the Sun at times than Neptune. Second, if you imagine planetary orbits as discs and look at their edges, they will appear to lie close to each other. Pluto's disc, however, is different, being tilted at an angle of 17 degrees away from Earth's orbit.
This prompts some astronomers to suggest that Pluto is not a 'real' planet, and could even be a former moon of Neptune. With new telescope technologies enabling scientists to find more and more objects orbiting far from the Sun, some of which are similar to Pluto, the clamour for downgrading the planet has become louder.
Under the new IAU guidelines, a celestial object qualifies as a planet only if it orbits a star, without itself being a star, and is large enough for gravity to pull it into a roughly spherical shape. Besides, it must dominate its orbit, clearing away other objects. Pluto's detractors point to its highly elliptical orbit, which overlaps that of Neptune, to relegate it to a new category called 'dwarf planets'.
The problem is, this will obviously also affect Earth, Mars, Jupiter and Neptune, none of which has a clear orbital zone. Earth orbits with 10,000 near-Earth asteroids, while Jupiter's path is littered with over 100,000 'Trojan asteroids' (chunks of rubble left over from the solar system's formation more than four billion years ago).
As for Neptune, if it were to clear its zone, there would be no Pluto in the first place! Astronomers would've been better off wondering what Pluto is like, rather than what it is. For only by learning more about the outer planets can we get to know more about how the inner solar system was formed.