I was somewhat disappointed that I couldn't land where-ever I wanted...
I knew that inhabited planets, like Earth, Mars, Achenar and so on would be off-limits. But a majority of uninhabited planets are also off limits.
The easy way to find out where you can land is to go to your Navigation page on the left, then open the Filters and set the filter to "Landfall planets" (or some such filter option, don't have the exact wording as I am at work).
By setting that filter it will only display planets where you can land in your Navigation page.
Really hope they improve on this so we can land on any planet we want to (except planets like gas giants where, well, there is nowhere to land...)