what is the old and new defination of planets?

Answer 1

Note that they had to make an exception so Neptune remains a planet. Wish they had done that for Pluto as well.


RESOLUTION 5A
The IAU therefore resolves that planets and other bodies in our Solar System, except satellites,be defined into three distinct categories in the following way:

(1) A "planet"1 is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects3, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
2An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

RESOLUTION 5B
Insert the word "classical" before the word "planet" in Resolution 5A, Section (1), and footnote 1. Thus reading:

(1) A classical "planet"1 is a celestial body . . .

and
1The eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Answer 2

Ancient planets were just the one we could say naked eye, moving erratically on sky, except sun and our moon. They were Mercury, Venus, Earth, Mars, Jupiter and Saturn. Then came the telescope, and Uranus, Neptune and Pluto were discovered (Pluto about 1930 decade).
But now Pluto has been lost its planet calification, and is now a "minor body" or "minor planet", similar as some of the bigger asteroids (agreement of the last Astronomical International Convention hold in Praga in 2006).

The new definition has to do with its size, its spherical form, its orbit must be clean up of other objects, in case of a moon the center of masses must be inside its body.

Important: Explanation from "me" (here down") is exact. I add a point to him.