The heroes first express surprise when studying the parallel Earth, not yet when entering the system... That they'd already be deep in the system is suggested by Farrell trying to have a dialogue with the radio transmitter, thus supposedly at lightspeed.
Might be an entire duplicate Sol system nevertheless, with the Other Earth at the #3 slot as usual. But I'd think the heroes would be more impressed by the Other Saturn, supposedly a rare beauty in the cosmic scale... That is, if they bothered to take a look at the Other #6 at all, and we know they aren't really in the habit of doing so. If they only entered this system to study the mystery SOS, they'd probably fly in wearing the same blinders as Captain Terrell a couple of decades later.
(Interesting that Kirk says they're "hundreds" of lightyears from the real Earth, and thus well outside the colony and outpost regions. Rather direct proof for the small UFP model there!)
Timo Saloniemi