Schmuklore

Hi. I'm new to this game. I've searched this forum extensivly and cannot find an answer to this problem. When I have a colonising ship floating about in space, I select it, then right click on what appears to be a hospitable planet (it looks 'earth-like'), yet the colonising ship gets close to the planet, then stops. It does not colonise it. What am I doing wrong? This is driving me crazy. Thankyou for your kind help. Cheers, Schmuklore