In astrology, a ruling planet is a planet of the solar system that is associated more strongly with a particular sign of the Zodiac than other planets are thought to be. The ruling planet for a given sign affects that sign even when the ruling planet is not in the sign or in an astrological aspect related to the sign.
The assignments of the ruling planets appear to be based upon the Northern Hemisphere seasons, as the Sun and the Moon, the principal bearers of light and heat, were awarded to Leo and Cancer, respectively, since the months the sun passed through these signs were the warmest and had the longest days.
Conversely, Saturn, the most distant and coldest of the planets known to ancients, was accorded the rulership of Aquarius and Capricorn, the signs opposite Leo and Cancer, respectively.
Jupiter, being next furthest away, was given the signs on either side of Aquarius and Capricorn (Pisces and Sagittarius), and Mars, next in order, received the next two (Aries and Scorpio).
Since Mercury never appears more than one sign from the sun in either direction, it was deemed to rule the two signs on either side of Leo and Cancer (Virgo and Gemini), and since Venus can never be found more than two signs from the Sun, it obtained the rulership of Libra and Taurus.
The discovery of the three "new" planets in modern times provided a dilemma for astrologers, which most eventually resolved by a general consensus declaring Uranus to be Aquarius' ruling planet, while assigning Neptune to Pisces and later, Pluto was given to Scorpio (with some modern astrologers still insisting Pluto rules Aries and not Scorpio).
Classical astrologers whose methods are rooted in traditional or classical methods do not use any of the outer planets (Uranus, Neptune, and Pluto) as the ruling planets of any of the 12 signs.
On the other hand, most modern, psychologically oriented astrologers do in fact use Uranus, Neptune, and Pluto as ruling planets (or co-rulers).