Astronomical Unit (AU) is the average distance between Earth and the Sun, about 93 million miles or 149 million kilometers.
Cometary plasma is gas that has escaped from a comet (or Pluto) and become ionized by ultraviolet sunlight.
Density is the amount of mass per unit volume of matter, and is usually given as grams per cubic centimeter or kilograms per cubic meter. Water at room temperature has a density of 1 g/cm3 (or 1000 kg/m3), rocks are about 3 g/cm3, and iron has a density of 7.9 g/cm3. For convenience, you can say "Pluto has a density of about 2," meaning it is about twice as dense as water.
Differentiation occurs when a dense material (such as rock) sinks into the middle of a planet and leaves a lighter material (such as ice) on the outer layer. Differentiation generally requires that the interior of the object become warm enough to melt and allow materials to separate—a bit like chocolate chips moving to the bottom of a tub of melting ice cream.
Eccentricity is the departure of an elliptical orbit from a circle. A circle has an eccentricity of 0; a very elongated orbit has an eccentricity approaching 1. The eccentricity of Pluto's orbit has the largest value of all the planets (0.248).
The Ecliptic is the plane of the Earth's orbit. Most of the planetary orbits are close to this plane. Pluto's orbit is inclined at an angle of 17.14 degrees to the ecliptic plane—the largest deviation of any planet.
Frost occurs when a minority constituent condenses out from an atmosphere, such as water on freezing out of the Earth's atmosphere or (probably) methane on Pluto. For both Earth and Pluto, the main constituent is molecular nitrogen (N2). When the atmosphere itself condenses, as in N2 on Pluto, it forms ice, not frost. The difference is significant, both from the surface (very different surface textures result from the two processes) and from the atmosphere, where frost formation is limited by the ability of the minority species to move through the majority component, while condensation of ice is only limited by the ability of the atmosphere to cool.
Infrared is the part of the electromagnetic spectrum next to visible light. Wavelengths of infrared light are longer than those of visible light.
Ionosphere is the region in a planet's upper atmosphere where sunlight ionizes gases.
Jupiter Family Comets have orbits that are strongly influenced by Jupiter's gravity and usually take less than 20 years to make a single trip around the Sun.
Kuiper Belt is a disk of thousands of icy objects (known as Kuiper Belt objects [KBOs]) outside Neptune's orbit, about 30–50 astronomical units (AU) from the Sun.
Magnetosphere is the region where a planet's magnetic field or ionosphere shields the planet from the solar wind.
Occultation occurs when an object moves behind another object. In a stellar occultation, for example, a star moves behind a planetary object.
Oort Cloud Most comets come from the Oort Cloud, a roughly spherical-shaped region between 10,000 and 100,000 astronomical units (AU) from the Sun, and typically have orbital periods of about a million years.
Photochemistry is when sunlight stimulates a chemical reaction, such as changing methane into more complex hydrocarbon molecules.
Photomosaic combines separate images covering neighboring or nearby regions into a single image.
Pick-up ions are newly charged particles in the outer regions of an atmosphere; once they become ionized, they are usually "picked up" and carried away in the solar wind.
PROM is the onboard computer's Programmable Read-Only Memory.
Proto-Triton was the object that later became Triton, before it was captured by the planet Neptune.
Radiolysis occurs when energetic particles (such as those from the Sun or cosmic rays from beyond the solar system) bombard a planetary object and break up molecules in the top layers of the surface. When the molecules rejoin, they often form larger molecules. Thus, under the influence of radiolysis, the simplest hydrocarbon, such as methane, can be turned into complex organic materials.
Reflectivity —the fraction of light that is reflected, rather than absorbed—is an important property of a planet's surface. Reflectivity factors into surface temperatures; high reflectivity (such as white) means little absorption of sunlight and a cooler surface, similar to the advantages of wearing white clothes in summer. Conversely, a very dark surface absorbs lots of sunlight and warms up.
Refraction is the bending of light due to a gradient in density, such as the bending of light around the limb of a planet's atmosphere.
Resonance There are many forms of Resonance between planetary objects, but the simplest is when one object orbits the Sun (or a satellite orbits a planet) in exact proportion to another object's orbit. For example, Pluto orbits the Sun twice for every three times Neptune orbits, putting them in a 2:3 resonance.
Retrograde Most of the objects in the solar system spin and orbit the Sun in the same direction —counterclockwise looking down on the solar system from above. Retrograde objects have suffered collisions that caused them to spin or orbit in the opposite direction.
Solar Wind is the streams of charged particles that speed from the Sun into space.
Spectroscopy is the study of light coming from an object by examining the intensity at different wavelengths—and a very powerful tool for investigating the composition of surfaces and atmospheres.
Sublimation is the transformation of a solid into a gas. You see it on Earth with theatrical fog machines.
Terrestrial Planets are also known as the "rocky" planets; they include Mercury, Venus, Earth, and Mars.