– The Earth’s Moon is a planet, leading scientists announced and by doing so they re-opened a centuries-old debate. Ancient Greek and medieval astronomers all assumed that the Moon was in fact a planet, and several modern astronomers have reached the same conclusion.
Alan Stern, principal investigator of Nasa’s New Horizons mission to Pluto, suggests the definition of what constitutes a planet is outdated.
In his science paper, Stern suggests that the geophysics of a body should determine whether it is a planet and not just whether it orbits the Sun.
It’s not the first time Stern has expressed annoyance over the definitions of celestial bodies. In 2006, the International Astronomical Union [IAU] deemed that Pluto was not a planet and Stern was very angry. When his probe reached its destination, Pluto was a mere “plutoid”, a “trans-Uranian dwarf planet”.
Stern admits that he is fed up with people asking “why did you send New Horizons to Pluto if it’s not a planet anymore?”
“In the mind of the public, the word “planet” carries a significance lacking in other words used to describe planetary bodies… many members of the public assume that alleged “non planets” cease to be interesting enough to warrant scientific exploration,” Stern says.
But Pluto is very interesting and worthy of exploration.
To avoid confusion and misconceptions, Stern and his team propose “a geophysically based definition of planet that emphasizes a body’s intrinsic physical properties over its extrinsic orbital properties.”
Stern suggests that orbiting other planets – not just orbiting the Sun, could be used as a method of determining whether a moon could be upgraded.
The NASA team now proposes that the Moon, along with Europa and Ganymede, which orbit Jupiter, plus Titan and Enceladus, which orbit Saturn, have all the features of planets and should be upgraded as part of a modernization of the entire solar system. The same reorganization would see Pluto regaining its status as a planet.
If Stern’s arguments are accepted by the International Astronomical Union (IAU), it could mean the solar system’s eight planets being joined by more than 100.