Pluto

I just can't help writing about our solar system. It's such an interesting place! The most recent news about Pluto was its fall from planethood. What was once the 9th planet in our solar system has been reclassified to a dwarf-planet, much to the chagrin of Pluto lovers everywhere. But let's be fair, Pluto isn't even the most massive dwarf-planet in our solar system, it's beaten by Eris. I'm not going to say that Pluto isn't cool enough to be a planet, but its reclassification was correct.

Pluto

Pluto is small, only slightly more than 1/5 the mass of Earth. It is so small, in fact, that the barycenter (the point at which two masses orbit each other) between it and its largest moon, Charon, isn't even inside Pluto. This has led to some people thinking of Pluto as actually being a binary planetary system with Charon as the other "planet". Even the moons of other planets are larger than Pluto. The seven largest moons in the solar system, Ganymede, Titan, Callisto, Io, Earth's moon, Europa and Triton are all more massive than pluto.

Pluto compared to Earth

Pluto does have four moons, or satellites, of its own that we can see. The three most massive of these are Charon, Nix, and Hydra, with S/2011 P1 being the smallest. Charon is more than half the mass of Pluto while Nix and Hydra are only 50 or 60 kilometers in diameter. Of all the space that Pluto has gravitational control over, these moons are in the closest 3%. This system is therefore very compact. It is possible that Pluto may have other bits of debris or rings orbiting in the rest of that space, but we have not yet observed it.

Pluto and satellites

Pluto is very far away and very small, so it is hard for us to image it. The best pictures that we can get of it's surface are pixelated and blurry, but they do offer us a good idea of what may be down there. The surface of the planet is very interesting, with a great deal of contrast between different surface colorations and features. This would suggest that it has a very interesting topography. The surface is also fairly colorful with a mix of white, grey, black, and orange. Spectroscopic analysis of Pluto shows us that the surface is nearly 98% nitrogen ice. Pluto does have an atmosphere, but it is very thin, mostly composed of methane and dissolved nitrogen. The atmospheric pressure on the planet can change as its orbit brings it closer or farther from the sun because gas will vaporize from the surface or freeze out of the atmosphere. The core of the planet is composed of rock and ice, in fact, it may be as much as 50% ice. Through heating from radioactive decay, it is likely that this ice has separated from the rocky part of the core to form a mantle, meaning that the center of the planet would be rock surrounded by ice.

Graphical rendering of Pluto's surface

Pluto as seen from Hubble

Surface of Pluto

Pluto, like Uranus, has a tilted axis of rotation. Its rotation is offset at almost 120 degrees from the other planets, but what is really interesting is its orbital path. Unlike most of the other planets that orbit on the same plane, Pluto's orbit is tilted almost 17 degrees. Another artifact of its orbit means that it will periodically come closer to the sun than Neptune. This has obviously brought up the question as to whether or not Pluto will ever collide with Neptune. As far as we can tell, the answer to this question is no. One reason is that Pluto's orbit is tilted, making it unlikely that it will hit Neptune. Another reason is that their orbital resonances are off (3:2). Neptune will orbit the sun 3 times for every 2 times Pluto orbits the sun. The whole system works like clock work, meaning that the two bodies should never hit each other.

Planetary orbits seen from the top

Planetary orbits with Pluto's tilt shown