The notion that a grahas are astronomical planets is not entirely accurate. In Sanskrit a Graha literally means, ‘To catch or hold’ and it’s an entity that influences both the animate and inanimate on Earth. For example Rahu & Ketu are mass-less, they’re mathematical points called as the North node and the South node (http://en.wikipedia.org/wiki/Lunar_node)
Grahas are like transponder stations, where they receive signal from the distant stars (Nakshaktras) and relays back to Earth. Each Graha has it’s own vibratory frequency and personality. It’s also interesting to note that Jyotisa does not take the far-away Grahas – Uranus, Neptune & Pluto.
Sage Vyasa has identified Uranus , Neptune and Pluto some 5000 years ago, where he names them as Sweta (greenish white) for Uranus, Syama (bluish white) Neptune and Teekshana (volatile) Pluto. This can be traced in the great epic Mahabharata.