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

[blahblahboy]

Is it big enough to promote it from dwarf planet back to planet or does lack of atmosphere kill that?

[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

[zandperl]

Oh, and for what it's worth mass is slightly relevant to the definition of planet, but radius only tangentially. To be a planet it has to

1) orbit the Sun and not another body (this is why Ganymede is not a planet),
2) be massive enough to be round (basically if something has a large enough mass then gravity rounds it out); and
3) have cleared its orbit.

Dwarf planets only have to pass criteria 1 and 2, that's why Pluto and Ceres are dwarf planets, they're both orbiting the Sun (more-or-less) and are spherical.

Mass is the most fundamental thing in astronomy, mass rules everything including planet formation, how long a star will burn for and at what temperature, and even the overall curvature of the universe and its eventual fate. Density is second most important, tells us what things are made of. Radius (what laypeople mean by "size") is only a result of mass and density.