Is Pluto a planet or not? Depending the age of whom you ask, and depending they keep up with the latest new, they might answer yes or now. Apparently, it also depends whether you’re asking an astronomer or a planetary scientists. At least that’s the sentiment that NASA scientists, led by New Horizon principal investigator Alan Stern, is giving off in proposing a redefinition of what a planet is, which would re-induct Pluto to the club once again. Curiously enough, it would also add 100 new planets to our solar system alone.
Stern is hardly one you would consider unbiased in this context. After all, New Horizons helped put space science in the limelight again. It gave Pluto a level of prominence that would be far more befitting a planet than the dwarf planet it had become in the past decade. Stern has been very vocal about the newly accepted definition of a planet, formulated by an astronomer, one who studies celestial objects in general, rather than a planetary scientists that specializes in, well, planets.
Of course, being scientists, they also have technical arguments against the current definition of planets. The International Astronomical Union (IAU), which has almost godlike powers in determining what is and isn’t a planet, settled on three criteria for a planet: it must orbit the sun, it must be (nearly) spherical, and it must have cleared the zone around its orbit. Pluto only fails the last requirement.
Stern et al. finds the definition rather problematic for a few reasons. For one, it technically doesn’t recognize other planets that orbit around other stars or the rare “rogue planets” that don’t orbit anything. The third criteria is also problematic as not only does it require a mathematical model of what a planet’s zone is, it also requires “clearing” those zones. If you consider the number of cosmic bodies that fly through orbits, like comets, none of the planets in our solar system, including our very own Earth, would qualify.
NASA scientists propose that the definition of planets should be defined by their intrinsic physical characteristics rather than their orbital relationship with suns. Their proposed re-definition is deceptively simple: a sub-stellar mass that hasn’t undergone nuclear fusion and has enough self-gravitation to be almost a sphere. This definition immediately disqualifies stars, black holes, white dwarfs, etc. Almost ironically, it does qualify our Moon as well as the moon of all other planets, and maybe a few spherical asteroids as well. Of course, it’s still up for debate and will require the judgment of the IAU, which will probably be the quickest turn around for the re-definition of a planet.
VIA: Science Alert