I walked past "Neptune" just last Sunday doing this wonderful walk: http://www.walkingclub.org.uk/book_1/walk_23/index.shtml

It's beautiful countryside there - I highly recommend it. There's also a circular walk in that area which can be more convenient if you're driving: http://www.walkingclub.org.uk/book_1/walk_43/

Both walks take you past parts of the Otford solar system.