>>11207988>the atmospheric makeupSee star, record its exact colors, wait for planet to pass in front of star, total amount of light dips (because the planet is in the way) and the color changes slightly, since we know what gasses absorb what colors we can figure out what's in that atmosphere.
>diameter of a planetPlanet passes in front of a star, taking a certain amount of time to do so. Based on how bright the star is, we can determine its mass, because in stars mass is directly proportional to power output. Knowing the mass, and knowing how long it took for the planet to pass in front of the star, we can determine how far away from the star it is (orbital speed corresponds to distance from gravitational source). Knowing hoe far away from the star the planet is, and knowing how much light it blocked, we can figure out the exact cross sectional area of the star being blocked, and from a disk of that size we can determined the radius. Also, we can do some more math to figure out the mass of the planet orbiting that star, which means we can figure out the density of the planet, since we know the radius (and therefore volume) and the mass. This is why we know that some planets are likely water worlds, since they're bigger than Earth but significantly less dense.
