The planets featured on the shirts correspond to the five planets that were visible to ancient Greek astrologers with the naked eye - Saturn, Jupiter, Mars, Mercury, and Venus - plus the Sun and the Moon. These planets represent different aspects of basic drives, dimensions of experience, or urges in the unconscious mind.  

The Sun (Soleil / Sunday) represents the conscious ego. Corresponding to creativity, vitality, and power, it embodies the cumulative life force.

The Moon (Lune / Monday) represents the unconscious ego, hopes and wishes. it's reflective of one's interior mental landscape, memory, and intuition.  

 

Mars (Mars / Tuesday) represents physical activity, sexuality, and energy. It is the planet of passionate impulse and action.

Mercury (Mercure / Wednesday) represents communication of all kinds, rationality, curiosity, and reasoning.  

Jupiter (Jupiter / Thursday) is associated with principles of growth, prosperity, and good fortune.  It represents the urge for freedom, exploration, and expansion.

Venus (Venus / Friday) represents beauty, harmony, and pleasure.  It governs romantic relationships, sex, and social life.  

Saturn (Saturne / Saturday) represents structures and boundaries which are associated with heavy responsibility, and the discipline necessary to produce meaningful achievements. 

From its inception, astrology was dominated by two series of signs - the signs of the seven planets (see above), and the twelve signs of the zodiac.  Each of the twelve zodiacal signs (ram, bull, twins, lion, crab, virgin, scales, scorpion, archer, goat, water-carrier, and fish) corresponds to and governs a specific part of the body.  This figure depicts this intimate connection between astral and anatomical alliances.