What is the difference between a rocky super-Earth and a mini-Neptune?

A super-Earth is a category of planet with a mass between approximately 2 and 10 times that of Earth. Within this broad classification, the distinction between a rocky super-Earth and a mini-Neptune is primarily based on composition and structure:

• Rocky super-Earths: These are the smaller, denser worlds within the super-Earth category, primarily composed of rock.1
• Mini-Neptunes: Also known as sub-Neptunes, these are the heavier worlds within the super-Earth range. Unlike rocky planets, they are characterized by a significant gaseous envelope, typically consisting of a hydrogen-helium atmosphere surrounding a core of water, methane, and ammonia ices.1

While some astronomers classify mini-Neptunes as a distinct group rather than a subset of super-Earths, the exact mass threshold where a rocky planet transitions into a mini-Neptune remains unknown. Additionally, some mini-Neptunes, known as Hycean worlds, are defined by having a massive watery ocean beneath their hydrogen-helium atmospheres.1