Pluto New Horizons

[zandperl]

Post copypasted from Tumblr b/c I'm just that excited.OMFG I’m so excited! New Horizons flew by Pluto about 2.5 hours ago. It’s going to remain pointed at Pluto for another 5 hours or so to get as much data as possible, and then the craft is going to slew around and spit back all the images and other data to Earth, and then it’s going to take

Comments

[zandperl]

Atmosphere isn't relevant to the definition of planet - Mercury doesn't have an atmosphere, and Pluto actually has more of one than Mercury does. And size itself doesn't define planet either - Jupiter's moon Ganymede is larger than Mercury (which is larger than the Moon, which is larger than Pluto). The key to Pluto's non-planethood is that it's not the only thing in that region of the Solar System.

The story of Pluto's demotion is the same as that of the first asteroid (and dwarf planet) Ceres. When it was discovered, people called it a planet, but then they discovered Pallas, and Juno, and Vesta, and tends of thousands more, so it was clear that Ceres was just part of a bunch of stuff out there. We are in the same process with Pluto right now - first we discovered Pluto, then QB1, then Ixion, then Quaoar, then Eris, and now we're up to a few hundred more, so it's clear that Pluto is just part of a bunch of stuff out there.

So it's not size (diameter), it's not mass, it's not atmosphere that makes Pluto not a planet, its the fact that there's many many other large things in similar orbits. If we discovered hundreds of other large objects in orbit with the Earth then we'd demote the Earth itself.