Alan Stern, leading investigator of NASA’s New Horizons mission to Pluto, along with five colleagues, has written a manifesto where they proposed a new definition of planets. The key change in their definition is that worlds no longer need to be orbiting the sun to be considered planets. Among the other conclusions, this proposal could add more than 100 new planets to our Solar System, including Pluto.
Current definition accepted by the International Astronomical Union (IAU) describes planet as a celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and has cleared the neighborhood around its orbit. Thanks to the last condition for planet status, Pluto was expelled from the list due to its uncleared neighborhood around the orbit. The IAU decided to classify Pluto and other small, fringe-dwelling bodies in a new category as dwarf planets.
Stern’s definition said in simple words is that the planet represents any round object smaller than a star. The object that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape regardless of its orbital parameters – scientists propose the definition focused on the body itself, not on its interaction with external objects.
Now, if the IAU accept new definition, this will not include only Pluto as a planet. Also, the satellites such as our Moon, or Titan, Ganymede, Europe and Enceladus could be added to the planet’s list.
The scientists believe that the debate will be successful, as this definition expelled flaws from current one such as necessity orbiting around the sun, the requirement of zone-clearing, emphasizing that no planet in our Solar System can satisfy these criteria. The team is also relying on the public support, which already in 2014 recalled IUA decision made in 2006.