Learn Russian: Эта планета интереснее, чем Луна. - This planet is more interesting than the moon.

This is the regular comparative form meaning "more interesting". To make a comparison in Russian, you often drop the adjective's normal ending and add -ее. A great thing about this -ее form is that it never changes for gender, number, or case, so you don't need to make it feminine to match планета.

What is the word чем doing in this sentence?

The word чем means "than" in a comparison. When you use чем, the noun that follows it must be in the Nominative case, which is why Луна is in its regular dictionary form. Do not forget that Russian grammar requires a comma before чем!

Why is the first word Эта and not Это?

Because it means "this planet" and directly modifies the feminine noun планета. You must use the feminine form Эта. You would use Это if you were making the general statement "This is a planet" (Это планета).

Why is Луна capitalized here?

Just like in English, Луна is treated as a proper noun when referring to Earth's specific moon as a celestial body or astronomical object.