Look a little into astrology and you will meet the phrase “the ruling planet of this sign is such-and-such.” A ruling planet is the planet thought to suit a sign best and to “govern” its character. Just as the same light takes on a different hue through a different lamp, the same sign is seen to shift in tone depending on the planet that lights it.
The pairings handed down run roughly like this. Aries belongs to Mars; Taurus and Libra to Venus; Gemini and Virgo to Mercury; Cancer to the Moon; Leo to the Sun; Scorpio to Mars (and, in modern terms, Pluto); Sagittarius to Jupiter; Capricorn to Saturn; Aquarius to Saturn (and, in modern terms, Uranus); Pisces to Jupiter (and, in modern terms, Neptune). You will notice that one planet can take charge of two signs together.
Knowing the ruling planet lets you read a sign one layer deeper. For instance, Venus, the “star of love and harmony,” rules Taurus and Libra, which are seen as rich with a care for beauty and relationship; Mercury, the “star of speech and thought,” rules Gemini and Virgo, which are marked by communication and discernment. Instead of looking at the sun sign alone, you also call to mind the colour of the planet that tints it.
It helps, though, to know the difference between “traditional” and “modern.” In the age before telescopes, the twelve signs were divided up among only the seven visible bodies (the Sun and Moon plus five planets), so several planets each took two signs. Then, as Uranus, Neptune and Pluto were discovered, modern astrology added these outer planets as new rulers for Scorpio, Aquarius and Pisces. So it is less that one scheme is correct than that two traditions are used side by side.
So a ruling planet is not a verdict that “this planet sets my fate” but more a lens that lets you savour your sign in fuller dimension. You might call to mind the hue of your sign’s ruling planet and quietly ask, “which quality do I wish to grow more?” As always in FortuneLeaf, this is offered not as a fixed fate but as one small pleasure of seeing yourself in a wider light.