Someone school me, pl0x. How is it that planets or objects go unnoticed for a long time yet we can find things that are farther than these objects. Are they too small to be seen and need different equipment?
Things have to emit lightwaves to be detected. Distant stars and galaxies can be detected because they are emitting a lot of light that we can collect and analyze. Exoplanets around these stars can also be detected because they are partially blocking out the light the stars emit and from that we can determine roughly the size and orbit of these exoplanets.
These planets aren't emitting any light themselves and are so far away and small that the amount of lightwaves they would reflect from the sun are impossible to detect with our current astronomical instruments. Only way currently to know they are there is by detecting changes in the orbits of objects that we know exist, which is what these astronomers have done.