If a planet is too small (< 0.07 x Earth's mass), then its force of gravity is insufficient to hold onto an atmosphere (for example, the Moon). Active geological processes (especially plate tectonics) are required to renew the surface and atmosphere (CO₂), and this suggests an even larger mass (0.25 x Earth's mass) is probably required for internal heating (or external heating by tidal stresses).

If a planet is too large (> 10 x Earth's mass), then its gravity is strong enough to retain its original hydrogen atmosphere, and the planet would become a gas giant (with no solid surface).