Planetoid 127 by Edgar Wallace

A short story, kind of boring sometimes but fascinating in a way.
A science-fiction story with a bit of mystery. The story is: young Tim Lensman visits his old professor Colson, and is very surprised when he tells him he wants him to be his successor in his work. Colson is very rich, over the years he has gained money over money playing the market as if he could know the future. Now Hildreth, a very wealthy con-man, is angry at hi because he can't get control of Colson's source of news. Hildreth tries to buy Colson, then threatens him, then sends a thief that kills Colson, than tries again to find Colson's manuscript with all the explanations, but this time Tim manages to get it back, and can read it. Colson had found a new planet and a way to communicate with it, but Tim knows now that it is lost forever because both the Colson in this and in that world are dead.  This Neo is a planet just like Earth, same size, more or less same people and same events, with the same orbit around the sun. More precisely, around our sun. Planet Neo is invisible to us because it's exactly behind the sun, moving as Earth moves, so that they can never be visible to each other, with the sun always between them.
It was published in 1924, I read, and I guess the world was so different back then, but it is still a fascinating idea, in a nice, simple, very little short story. :-)