Some idle speculation for you all:
Let us hypothesize that there exists a second planet in Earth’s orbit, which stays at exactly the opposite point in the orbit (i.e. half a year around the orbit). Let us further say that this planet matches exactly Earth’s size, mass and albedo, and - what the heck - let’s say it’s got a similar sized moon.
What evidence would we have that it was there? Given our elliptical orbit would we be able to see it at cetain points of the year? Would it have an effect on Earth at all? Or other planets?
(Do not Need Answer Fast)