Kirby Runyon, an astronomer from Johns Hopkins University, is striving to get Pluto its planetary status back by proposing a new definition.
According to Runyon, Pluto is a planet and so are over 100 celestial bodies including Europa and Earth’s Moon. Runyon is rooting for Pluto as a planet as he believes that the change that was approved in 2006 doesn’t make any sense as Pluto has all the things on its surface that one would associate with things on a planet. Pluto is a rocky celestial object and was the smallest of the nine planets when it still had the status. Pluto’s diameter is under three-quarters that of the moon and nearly a fifth of Earth.
In 2006, the International Astronomical Union demoted Pluto to “non-planet,” thus dropping the consensus number of planets in our solar system from nine to eight. Per the IAU, Pluto didn’t fit the definition of a planet and hence it was demoted.
Runyon says there’s “nothing non-planet about it”. Runyon is leading a group of six astronomers from five institutions and has drafted a new definition of “planet,” and a justification for that definition. Runyon proposes that the new draft definition be used for classification of celestial bodies.
All the authors are science team members on the New Horizons mission to Pluto, operated for NASA by the John Hopkins University Applied Physics Laboratory. 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.
They define a planet as “a sub-stellar mass body that has never undergone nuclear fusion” and that has enough gravitational heft to maintain a roughly round shape.
This definition differs from the IAU definition in that it makes no reference to the celestial body’s surroundings.
Researchers have argued in the past that the IAU definition also excludes Earth, Mars, Jupiter and Neptune, which share their orbits with asteroids.
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.
The proposed new geophysical definition omits stars, black holes, asteroids and meteorites, but includes much of everything else in our solar system. It would expand the number of planets from eight to approximately 110.