Well technically, *any* planet could be further from us, if its orbit takes it around the other side of the sun relative to where we are, right? So as it starts to move around the part of the sun we're facing, it gets further and further away (more than 93,000,000 miles) until it is behind the sun (at which point I suppose radio contact would be impossible). Also, all the planets are not on precisely the same orbital plane, are they?
...into the light of a dark black night.
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