Do other planets like Mercury, Venus, Mars, etc. exist? And if so, are they also flat?

Some people - such as Socrates, Bill Nye (the science guy) and Ricky Ponting - have put out interesting theories about the other planets not existing at all. However, this is all up for debate until the day we manage to actually (don't believe all those pesky conspiracy theories out there) send a man into outer space. Thank you for your time.