system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it that we can "see" and even guess the composition of planets that are hundreds of light years away, yet we are unsure of the existence of other planets in our own solar system?

I've heard lots of talk of other planets that may support life based off of guesses at their composition and atmosphere, and these are often hundreds of light years away. Yet we are still not sure if there are even other planets in our solar system beyond Pluto (Planet nine, for example). Is this due to a difference in how they are detected, or is our knowledge of the potential life-supporting planets heavily exaggerated?
gpt
Planets don't emit light of their own, but only reflect light emitted by the star they orbit. So planets that we try to detect in our solar system can only be seen because of the sunlight they reflect. And small and distant objects (such as the trans-Neptunian objects) are therefore extremely dim and hard to see. They require a powerful telescope to be pointed at the right location to be observed directly. We therefore try to first observe them indirectly, by measuring slight differences in the orbit of more nearby objects that may be caused by the gravity of the unknown body. But even that becomes very hard to do when the objects are so small and so far away.

Exoplanets are discovered in a different way. If while looking at a star a planet moves in front of the star (between telescope and star), the star will appear to be slightly dimmer for the duration of this crossing. By measuring the magnitude of this dimming, an estimate can be made of the volume of the object passing in front of the star.

The mass of an exoplanet can be estimated with help of the Doppler effect. The exoplanet will exert some gravitational force on its star, causing it to move around slightly. When the star is moving away from us due to this effect, its light is Doppler shifted ever so slightly in the direction of the blue side of the spectrum, while when the pull of the planet brings the star a bit closer, the light is slightly redshifted. The size of this effect depends on the mass of the planet.

With volume and mass, density can be computed. The density is then compared with that of planets in our own solar system to make a prediction on whether the planet is a rocky planet, like Earth, or a gas giant, like Jupiter.

The contents of the atmosphere are determined by examining the spectrum of the light coming from the star as the planet is in front of it and some light passes through the atmosphere. Different elements absorb different wavelengths of light and by looking at which wavelengths show a slight drop in intensity as the planet moves in front of the star, we can say something about the contents of the atmosphere.