From BBC news
What makes a planet?
Long time back, if you can detect it on a telescope, it’s fine.
This was how Pluto became our solar system’s ninth planet after Clyde Tombaugh discovered it.
Now with much more powerful viewing devices, it’s become more important to define what exactly a planet is. Otherwise far off objects bigger than Pluto will have to be added to the original list of nine planets orbiting the Sun.
Astronomers have decided on the following criteria, a planet must:
– be in orbit around the Sun
– be large enough that it takes on a nearly round shape
– has cleared its orbit of other objects
In a historic and very likely highly emotional meeting, the International Astronomical Union (IAU) reclassifed Pluto as a dwarf planet. A brave rational move. Pluto is now classified as a dwarf planet.
So now there are eight. Well its an auspicious number.