Pluto gets to stay a planet, but 3 additional objects get an upgrade to planetary status. Under a proposal by the International Astronomical Union an attempt is being made to define what exactly is a planet. Until now it has been a touch and go classification. But now astronomers are trying to use gravity as the basis of planetary qualification.
- The object must be in orbit around a star, but must not itself be a star. In other words; no moons, no matter how large.
- It must have enough mass for the body’s own gravity to pull it into a nearly spherical shape. That means no oblong asteroids; an object must be larger than about 500 miles in diameter to assume a spherical shape.
Say hello to your new planets, Ceres, Charon and 2003 UB313 aka Xena. Ceres is the largest object in the asteroid belt, with a diameter of 512 miles. Pluto and its moon Charon are roughly the same size and will become the solar system’s first double planet, and 2003 UB313 is even larger than Pluto.
What this does not do is recognize large moons like our own, Saturn’s Titan or the Galilean Moons of Jupiter as planets. Just does seem fair that barren little Ceres is a planet while great big honking Ganymede or Titan, both with diameters of roughly 3200 miles, are denied worldly status. But the definition is explicit; Ganymede and Titan orbit their parent worlds, not the Sun, and thus they are moons.