You had an extra } in there. Also, the second IF can be an ELSEIF for efficiency.

Code:
on *:CONNECT: {
  if ($network == Engine) {
    nick username
    msg NickServ IDENTIFY password
    join #Cars,#Fast,#Help......
  }
  elseif ($network == Earth) {
    nick username
    msg NickServ IDENTIFY password
    join #Planet,......
  }
}

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