system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how we detect water in the atmosphere of a planet 110 light years away.
gpt
We use super-powerful telescopes that can look out into space in a wide range of spectrums of light.   


So when we start looking for potentially habitable planets, what we often start with is finding planets that are orbiting around a star in the "Goldilocks Zone". This term is a play on the Goldilocks story, where the perfect bowl of porridge is "not too hot, not too cold". So when a planet is too close to its sun, it's too hot for life to exist (as far as we know) and so hot that any water it might have had would just evaporate away. And when it's too far from the sun, it's too cold for life (as far as we know) and any water it might have would be frozen solid. So the Goldilocks Zone is where a planet is just the right distance away from the sun to have 1. Liquid water, and 2. Potential life..  

So once we find a planet at the right distance from the sun, we can look at it through these powerful telescopes. Now the specific planet we're talking about here is much too far away for us to see it in detail, so through a telescope it kind of just looks like a little dot. But what we CAN see is the sun going dim each time the planet passes in front of it as the planet orbits. It's almost like seeing a little mini solar eclipse. Every time the telescope sees the sun go dark for a second, we know the planet has just passed in front of it.  


Ever more cool is that the scientists can then look at the color of the sun's light as it passes through that planet's atmosphere. Like imagine if you had a pink balloon and you filled it with blue water. If you just hold the balloon in your hand all you see is the pink rubber. But if you hold the balloon up in front of a really powerful lightbulb, that light will pass through the balloon and shine through the blue water, and we can see the blue through the thin pink skin of the balloon.  


So when we watch the planet, and we see the light of the sun filtering through its atmosphere, we can tell what that atmosphere is made of, because different gasses and substances show up as different colors. And by analyzing the colors we can tell that there's a great deal of water vapor in that atmosphere. We can't tell exactly how much water, but we can see that it's there, and that's a big deal.  


Now, just because a planet is in the Goldilocks Zone and it has water, that doesn't automatically  mean that we could just land on it and live normal lives. It's too far away for us to see the surface so it could be covered in evil man-eating slime monsters, or it could be a planet that has water but absolutely no life at all, or there could be weird gasses that would kill any life form we know of.  But finding water is a good start, and as technology advances we'll be able to see more and more.