system
Given the following post, respond as if you're commenting on Reddit:
human
With our current capabilities, would we be able to tell that our own system contained a habitable planet if we were a number of light years away? From how far out would be be able to identify this?

What would we be able to tell about our own system and its planets if we were in another system? Would we be able to identify the number of planets or would we only see the effects of the gas giants?
gpt
This has been the subject of multiple spacecraft investigations.

Both the Galileo mission and the EPOXI mission observed Earth to determine if they could detect signatures of life from only spectra of our planet to determine its chemical make up.

A few things stand out strongly among those:

- Free Oxygen in the atmosphere. It's very rare to find this, since oxygen is incredibly reactive with just about everything. One would expect it to quickly decay, but the abundance in our atmosphere suggests some very interesting process must be producing it.

- Free Methane in our atmosphere. Methane on its own isn't all that uncommon (the outer planets all have a good deal) but it simply shouldn't exist in the presence of free oxygen without some process actively producing it to keep the planet in "chemical disequilibrium".

- The Red Edge. Chlorophyll produces a distinct spectral feature in the red part of the spectrum. This isn't the strongest signature of life (other chemicals can produce similar features), but when combined with free oxygen, is a very strong indicator of plant life.

The red edge is probably the easiest feature to detect, but also the weakest evidence of life of the three. Exoplanet spectra are pretty ratty right now, and generally not good enough to pick up anything but the red edge (particularly for Earth-sized planets), but give it 10-15 years. Once the James Webb Space Telescope and the ELT come online, we should be able to pick up more of these definitive spectral features on exoplanets at least a few hundred light-years away.