Given advanced enough telescopic technology, would it be possible to detect that a planet in the order of 50 light years away is inhabitable? (i.e. without having to actually go there)
I'm just looking for the basics here - presence of water, enough oxygen in the atmosphere, temperature, and sunlight (perhaps a bit dimmer than on Earth but still bearable) and assume the technology exists to "fine tune" i.e. remove unexpected toxic gasses from the atmosphere, turn the local algae into plant-growing soil, etc.
But here is the hard part, my scenario involves such a planet that people on Earth decided to colonise, but once they got there, they discovered that it wasn't inhabitable at all - lifeless and without atmosphere like the Moon, as well as unbearably cold and as dark as say, here on Earth in the moonlight.
So my question is, what could lead advanced astronomers to make such a mistake despite being really confident in their initial belief?
I'd prefer an answer where the astronomers got it wrong, rather than circumstances changing during the journey (e.g. a Solar flare destroying the atmosphere or something)
Edit after a bit of thinking time
I've been thinking that a huge undetected magnetic cloud made of handwavium particles between us and them could disrupt the light coming to us, messing with frequencies, etc. However, I think this would make the planet appear darker than it really is - I want it to appear brighter than it really is. Is there any kind of lensing effect something like this could have that would make either the local stars (it's a binary system) or the planet's own reflected light seem brighter? Maybe it could make the planet look closer to the star than it really is and they would deduce that the planet is brighter than it actually is?
It is OK if the astronomers involved are not 100% sure that they are right. External pressures will force them to launch the mission anyway. Also, there are no aliens any more advanced than maybe plant life.