We would notice the gravitational effects. Like an asteroid going around near this other planet we can’t see, once we find the asteroid again we would notice it got deflected onto a new path.
Also not to mention our own orbit isn’t stable. Everything with mass effects everything else with mass gravitationally. Jupiter, Earth, the Sun, Pluto, they all change how things move throughout the solar system. In the 4.5 billion years our solar system has existed the planets have flung each other around the solar system and where we are now is just what’s survived, but there’s no guarantee it will last forever. If there were another planet opposite the sun, it probably wouldn’t stay there, and while there is a Lagrange point there, it’s an unstable one, so if the planet were nudged slightly out of place (a very likely thing to happen) it would then continue moving away from that point. If something were there, it wouldn’t stay there for long.
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