In recent years, astronomers have discovered thousands of planets around other stars, thanks to missions like Kepler, K2, and TESS. When the sizes of these planets are plotted, an interesting pattern appears: there are two main groups. The first group contains planets slightly larger than Earth, known as super-Earths. These planets are mostly rocky, like Earth, but a bit bigger. The second group contains planets about two to three times the size of Earth, known as sub-Neptunes. These planets have thick gaseous atmospheres. Between these two groups, at around 1.5–2.0 Earth radii, there are fewer planets than expected. This drop is called the radius valley. It is not an actual physical boundary, but rather a statistical feature in the data. Studying this valley helps us understand why some planets keep their atmospheres, while others lose them.
