All the planets in our solar system, by the current scientific classification system, must satisfy three basic criteria:

1. A planet must be in hydrostatic equilibrium—a balance between the inward pull of gravity and the outward push of the supporting structure. Objects in this kind of equilibrium are almost always spherical or very close to it.
2. A planet’s primary orbit must be around the Sun. That means objects like the Moon, Titan, or Ganymede, are not planets, even though they are round owing to hydrostatic equilibrium, because their primary orbit is around a planet.
3. A planet must have cleared out other, smaller objects in its orbital path, and thus must be by far the largest object in its orbital neighborhood. This means that Pluto is not a planet, even though it meets the other two criteria; there are thousands of Plutinos in the orbital path of Pluto, and it crosses the orbit of Neptune, which is a much larger and more massive object.

The eight objects in our solar system that meet all three criteria are Neptune, Uranus, Saturn, Jupiter, Mars, Earth, Venus, and Mercury.