planetsize

Planetsize is a measure of the physical size of a planet, typically expressed as its radius (R_p). In practice, sizes are often stated in Earth radii (R⊕) or Jupiter radii (R_J). The size, together with mass, determines density and provides clues about a planet's composition and internal structure.

Measurements: Transit observations yield the planet-to-star radius ratio, from which R_p follows if the stellar radius is known. Radial velocity and transit timing variations provide mass, enabling density when combined with radius. Direct imaging and gravitational microlensing can also reveal size in some cases; each method has its own uncertainties.

Size ranges and categories: Earth-sized around 0.8–1.25 R⊕; super-Earth 1.25–2 R⊕; sub-Neptunes or mini-Neptunes 2–4 R⊕; Neptune-like about 3–5 R⊕; gas giants exceed roughly 6–8 R⊕, with Jupiter around 11.2 R⊕. The observed radius distribution of planets shows features such as the Fulton gap between about 1.5 and 2 R⊕.

Examples: In the Solar System, Mercury ≈0.38 R⊕, Earth ≈1 R⊕, Uranus ≈4 R⊕, Neptune ≈3.88 R⊕, Jupiter ≈11.2 R⊕.

Impact and limitations: Planetsize informs gravity, atmosphere retention, and climate, but size alone does not uniquely determine interior composition or history; combinations with mass, density, and atmospheric data are required for a fuller picture.