asterroc: (Astro - H-alpha)
[personal profile] asterroc
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 another 5 hours for the signal to get here.

Among the amazing discoveries so far, we already have that Pluto is larger than previously thought. We knew that Eris was the most massive Kuiper Belt Object - mass is easy to get when an object has moons, and Pluto has five (Charon, Nix, Hydra, Styx, Kerberos) and Eris one (Dysnomia).

Diameter is really tough from Earth b/c they’re so far away that they look really small, just like dots. So we usually make a guess by measuring its actual measured brightness here on Earth (or at the Hubble Space Telescope), then factoring in its distance, then making a guess on how reflective its surface is (astronomers call this albedo), and from this they calculate the radius. For Eris this was done in 2005 using the HST and again in 2007 using Spitzer, but we got a lucky break in 2011. Eris crossed in front of a background star, occulting it (like a solar eclipse but instead of our Moon blocking our view to our Sun, Eris blocked our view to a faraway sun), and the amount of time this took along with the speed that Eris was moving got us a better radius (or diameter) estimate.

Pluto though, our numbers were (heh) fuzzier despite having occultation data since 1988, and this was partially because of Pluto’s (outgassing) atmosphere. Now however, even before New Horizons flew by Pluto it took pictures, and by knowing how far we were from Pluto, the power of the camera, and the size of the image in the picture, we now know Pluto’s size.

And what’s exciting is that Pluto is larger (radius or diameter) than Eris, even while remaining less massive as we thought. Mass is more important to astronomers, so if you hear anywhere that “Eris is the biggest KBO” we’re being lazy and using the generic English word “bigger” to mean the science description “more massive” rather than radius. But what this means is that everything we thought we knew about the inside of Pluto is wrong - density is king when talking about planets, and density is basically saying “okay if we’ve got an object of mass X, what radius is it going to be?” We guessed wrong on the radius, so that means the density is wrong, so the internal composition is different from what we thought.

How exciting is that!?

Originally posted on Dreamwidth. comment count unavailable comments there. Comment here or there.

Date: 2015-07-19 02:32 pm (UTC)
From: [identity profile] blahblahboy.livejournal.com
Is it big enough to promote it from dwarf planet back to planet or does lack of atmosphere kill that?

Date: 2015-07-19 05:32 pm (UTC)
From: [identity profile] zandperl.livejournal.com
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.

Date: 2015-07-19 05:40 pm (UTC)
From: [identity profile] zandperl.livejournal.com
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.

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