How did scientists work out the size of planets?
For planets in our solar system, the angular diameter can be measured visually through a telescope. Then, knowing how far away the planet is, you can calculate its diameter. The distances to the other planets were calculated first using parallax and Kepler’s laws, and later with radar and other techniqes to a high degree of accuracy. Of course, we have also sent probes to orbit several of the planets, so we’ve had a good close look at those. For extrasolar planets, often all they have is a mass estimate, from the amount of wobble the planet causes in the star, and some assumptions about density. They’ve been able to make a few size estimates on extrasolar planets by seeing how much of its star’s light was blocked when the planet transited in front of the star.
