Ketu

Just like Rahu, Ketu is also an imaginary planet, a node. The two nodes, Rahu and Ketu are positioned where the earth's trajectory around the Sun and the Moon's trajectory around the Earth meet. According to Indian mythology, Ketu represents the tail of demon(just like Rahu represents the head). The two planets are the parts of the same danava(demon) but are 180 degree apart from each other.

When Ketu, Sun and Moon lie in the same zodiacal longitude lunar eclipse takes place. Ketu in certain way resembles Mars. But many times, it activates only the negative side of Mars. This is why people ruled by Ketu are secret adversaries. It also influences people in pharmaceutical industries, doctors, and astrologers.

The two imaginary nodes, Rahu and Ketu are the mysterious forces and show in the birth chart both karmic and spiritual influences. Like Rahu, Ketu is also not a real luminary and therefore doesn't rule any zodiac sign.