Add discovery data for Solar System planets

Unless there's some reason for excluding it that I'm missing, there are applications where it would be helpful to have discovery data (year and method) for Solar System planets. Some of values may be approximate (e.g. for the earliest discoveries) or a bit of a stretch (was Earth discovered by 'imaging'); but for most uses these should suffice and in any case should do no harm (no one should be using the absence of a discovery method, for example, as his or her test for a Solar System planet).