NASA Astronomers: “Pluto is a Planet, So is Jupiter’s ‘Ocean’ Moon Europa”
Johns Hopkins University scientist Kirby Runyon wants to make one thing clear: Regardless of what one prestigious scientific organization says to the contrary, Pluto is a planet. So, he says, is Europa, commonly known as a moon of Jupiter, and so is the Earth’s moon, and so are more than 100 other celestial bodies in our solar system that are denied this status under the prevailing definition of “planet.”
Runyon and his co-authors argue for a definition of “planet” that focuses on the intrinsic qualities of the body itself, rather than external factors such as its orbit or other objects around it.
This definition differs from the three-element IAU definition in that it makes no reference to the celestial body’s surroundings. That portion of IAU’s 2006 formula – which required that a planet and its satellites move alone through their orbit – excluded Pluto. Otherwise, Pluto fit the IAU definition: It orbits the sun and it is massive enough that the forces of gravity have made it round.