if there is a planet on the exact same orbital plane as this earth and its the exact opposite side of the sun so we could never see it
Yeah loogical, WE (observers on Earth) couldn't see it, but there have been enough space probes that would have found it, unless the civilisation that live in the centre of the Earth has loaned them the cloacking technology
Before finding intelligent life, perhaps we should find a planet which could support intelligent life. So far we've found gas giants larger than Jupiter. SETI has never reported any response to any radio signals.
Sorry Big Tex, the reason we can't detect smaller planets than the gas giants we can detect is *drumroll* that they are... smaller. Better technolgy should enable us to resolve planets in the Earth size range. But remember, just as we can't live by deep sea volcanic vents, but there are beasties that can, just because it's not an Earth-like planet, doesn't mean there is no life.
As light travels at only about 189,000 miles per second, and as we have had say a hundred years of radio technology, even if aliens were listening to the radio signals from the direction of Earth, our signals would have only gone 596,030,400,000,000 miles, and we could only expect a reply from those recepients within 298,015,200,000,000 miles, as the message needs to return at the same speed. 100 light years (or 50 light years) is simultaneously very far and not far. Very far to us, 0.1% (or 0.05%) of the galactic diameter
If you get the technolgy sorted out, it's far better to communicate using entagled particles that allow communication at FTL speeds. We know that that might be possible, if it is, then we might be trying the equivalent of using jungle drums to signal to people using radios. As has been pointed out, there could be races with civilisations hundreds of thousands or even millions of years old. If they communicate using the same technology as we do now, it would be like us using runners to carry all our messages.
Alternately, some theorise that this galaxy is just at the age when you'd realistically expect intelligent life to have a chance of evolving. It's an arguement based on the amount of heavy elements available, which in turn is based on solar lifecycles; we are stardust, billion year old carbon.
But we don't know.
Logically, if aliens are here and have travelled at sub-luminal speeds, then we might know about it as their technology would be to some extent comprehensable by us (like a steam engineer could get his head round a internal combustion engine). They'd be likely to have 'screens' (ECM), sensors or weapons much the same as ours. And if they've gone to all the trouble of taking decades on centuries to get here, we can bet they have good reasons above and beyond boldly going, so some sort of parity in technology could be handy. They'd come mob-handed in generation/hibernation ships, and be here to stay, not set up trade.
If they travel FTL, then we could only really know about it if they wanted us to, or if they had a 'mechanical' problem. We would have extreme problems with their technolgy (say the same problem a guy who made a windmill would have understanding a fusion tokamak), and if they weren't friendly, we'd be toast/slaves/food/whatever they wanted. BUT, they might just be boldly going, and have gone past the sociological stage we are at (and any sub-luminal technology could also be at) when violenece is still quite easy, and actually be "coming in peace for all I'eji-uhj-e'poqo-kind".
In view of the above and the lack of any real proof, all UFO sighting are exactly that; either Unidentified Flying Objects or Unaccountable Fantasy Objects, but not aliens. Remember that a few hundred years ago it was as common for people to see great battles and huge divine figures in the sky as it is for people to sight UFO's, and with similar cycles of frequency. It's like witch burning. One decade it's popular, the next it isn't.
As anyone saying they saw the Archangel Gabriel fighting a huge serpant in the sky would be considered rather mad, the ft the things people see in the sky are more credible (to modern tastes) is no surprise.