A demotion for Pluto

Published: Saturday 30 September 2006

A demotion for Pluto

-- "The match was declared closed at the loss of one planet." This could well have been the closing statement of anyone doing a running commentary on the proceedings of the 26th General Assembly of the International Astronomical Union (iau). The meeting held at the Czech capital, Prague between August 14 and 25, 2006, saw members voting for a new definition of planets. According to this definition, Pluto will no longer be called a planet: it will be called a dwarf planet instead.

The iau defines a planet as, "A celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid forces so that it assumes a hydrostatic equilibrium (nearly round shape), and has cleared the neighbourhood around its orbit." A dwarf planet has all but one of these properties: it has not cleared the neighbourhood around its orbit. It isn't a satellite as well.

A day before this definition was formulated, astronomers would have said, "The Solar System has nine planets, their 130 satellites and more than 10,000 asteroids or minor bodies -- most of them orbiting between Mars and Jupiter -- and many more objects not yet discovered." Five centuries ago, our ancestors knew of five planets: Mercury, Venus, Mars, Jupiter and Saturn. After 1600, it was found that the Earth was not the centre of the universe, it orbited around the Sun, like the five other planets. So, the solar system now had six planets. Two more were added to the list after the telescope was discovered: Uranus in 1781 and Neptune in 1846. Pluto was added to this list in 1930.

The latest entrant was, however, a little too small: smaller than our Moon. Also, while all other planets orbit the Sun in near circular orbit, Pluto's orbit is long, elliptical and it crosses Neptune's orbit. No other planet does that. So, there were question marks over Pluto's status as a planet.
Kuiper Belt objects In 1951, Gerard Kuiper, a Dutch-American astronomer, predicted lots of small objects in a ring-like region beyond Neptune. This ring is called Kuiper Belt and the objects, Kuiper Belt Objects (kbos). By 1992, the list of such ring-like regions had expanded to more than a dozen. It began to be argued that Pluto was a kbo. Today astronomers believe that there are about 70,000 such ring-like regions.

On January 5, 2005, a team of astronomers discovered a celestial object, somewhat larger than Pluto. It was catalogued as 2003 ub 313. The discovery set off a dilemma: was the 2003 ub 313, the tenth planet or should one catalogue it differently.

A committee of seven astronomers was set up to define a planet. Their first proposal at Prague met with much opposition: it would have increased the number of planets to 12. This number was sure to go up. The second proposal -- that was passed -- divested Pluto of its status as a planet. Pluto and all other celestial objects like it would now be referred to collectively as Dwarf Planets.

This is not the first time a celestial body has been stripped of planethood. On January 1, 1801, the Sicilian astronomer Giusseppe Piazzi discovered an object that was first thought to be a comet. It was, however, quite unlike comets. The object was found orbiting between Mars and Jupiter. It was initially called a planet. But it was found to be too small for this designation. It was less than 1,000 km in diameter. Soon similar objects were discovered. These were called asteroids or minor planets.

Barring any drastic occurrence, the eight-planet Solar System looks set to be the norm for some time now. In the words of Bill Arnett, host of a popular site The Nine Planets, "It's not important how we classify objects in our Solar System. What is important is that we learn about their physical nature and histories."

Arvind Paranjpye is with the Inter-University Centre for Astronomy and Astrophysics, Pune

Subscribe to Daily Newsletter :

Comments are moderated and will be published only after the site moderator’s approval. Please use a genuine email ID and provide your name. Selected comments may also be used in the ‘Letters’ section of the Down To Earth print edition.