How would you detect signs of life on an alien world hundreds of light years from Earth?
One way would be to look for molecules in its atmosphere that indicate the presence of life. Say, oxygen, methane, ozone, and so on. That wouldn’t prove that aliens live there, but it would certainly be interesting.
It’s difficult to make such an observation, but Nature provides. If the planet’s orbit appears edge-on from Earth we see the planet passing directly in front of its star; what we call a transit. The solid part of the planet blocks all the light, but the atmosphere of the planet allows some light through. That’s important, because molecules in that atmosphere absorb very specific colors of light; this acts like a fingerprint allowing astronomers to ID the molecule.
Ozone, for example, absorbs light in the near-ultraviolet, just bluer of what our eyes can see (and protects us from even shorter wavelength UV which can damage our skin and eyes), as well as light in the blue-green part of the spectrum, too.
We don’t have telescopes that can do quite this with planets orbiting distant stars just yet, but we can test the idea much closer to ...