The orbits of planets and other bodies in our solar system are carefully studied and modeled. We need to be very good at this to for example launch something from Earth and hit a specific point on Mars with a probe.
Everything with mass has its own gravity and pulls on everything else. If there was a planet in the L3 point then it would be pulling on everything else in the solar system and those effects would be easily spotted. It would introduce an unexplained element in the calculations and someone would very quickly figure out what must be causing it. We have also launched probes which could directly observe the area as well.
So unless this planet is invisible and massless we can be pretty sure it doesn’t exist.
Latest Answers