system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we know so much about far away planets (what they're made of, their atmospheres, possibilities of water, if they are exoplanets etc) just by looking at them through a telescope?
gpt
We can tell if a planet is an exoplanet because if it was orbiting our sun it would move across the sky. Exoplanets can't actually be looked at through a telescope, they're too faint. They have to be indirectly observed - for example, if a star's light dips for a long period then returns to normal that probably means a big planet of its own just passed in front of it.

For finding out about the composition of far away planets, we can use spectroscopy. Every element on the periodic table has a unique pattern of light that it absorbs, meaning the elements can be worked out by splitting up the light from the planet (like when you shine a white light through a prism to make a rainbow) and comparing the pattern you get with the known patterns of elements.

For working out possibilities of water, astronomers look at several factors, like the planet's distance from its star and the density and temperature of its atmosphere. Liquid water requires a thick atmosphere so that it doesn't just evaporate into space, and it needs to be close enough to its star that it doesn't freeze.

An exoplanet's mass and size can be calculated using the wobble it causes to its star or the amount of light it blocks when passing in front of its star.

But the truth is, that's about all we can know about exoplanets using telescopes. The only reason we know so much about the planets in our own solar system is because we've sent probes to all of them. Before New Horizons arrived at Pluto, we didn't even know for sure how big it was, or what its surface was made of.