Humanity detects a very suitable planet, lets call it Vaung, that is roughly 80 years away from us with their current technology. A ship is made and sent to colonize it, same old stuff.
The issue is the following. At some point in the journey, after 30-40 years did not decide yet, our astronauts suddenly discover a planet that is even more suitable for human habitat. The ship is fitted with powerful telescopes and whatnot so detecting the planet is not an issue.
The issue is how can we not detect this planet that is like 30-40 years closer to us? Note that humanity spent billions of dollars and over 4 years frantically searching for a suitable planet and deciding on the original chosen planet because of it's proximity.
So. I want a scientific or scientifically based phenomena that can lead the astronauts to be like: That makes sense as to why we did not discover the planet earlier. Guess we better consider setting up shop here because it's even closer.
This is an important point as there are other elements at play here and I want my astronauts to be really oblivious to anything unnatural happening on the planet.
I understand that deviating from the mission should not be accepted but this is not here or there. Nor what is on the planet since I do want to explain this in a normal natural way instead of hand waving it with magic or a super duper advanced alien civilization...etc
What explanation can be plausible?
Edit: Sorry if the distance is not clear. Lets just say planet B is half the distance of planet A. Planet A is the original destination that is discovered earlier, while B is the one we want to be discovered later.