A Caltech-led study has discovered that majority of the 3,500 confirmed exoplanets found till date fall in two main categories: rocky "super-Earths" up to 1.75 times the size of Earth, and gaseous "mini-Neptunes" 2-3.5 times Earth's size. Researchers aim to understand why majority of exoplanets have sizes between Earth and Neptune and why they don't exist in our Solar System.