Condensation  -  The process in which the vapor (gas) of some compound changes into liquid.

Phase  -  Liquid, solid, and gas are the three 'phases' of matter.

Comets  -  Comets appear like 'dirty snowballs,' and are remnants of the formation of our solar system. When comets get close to the Sun, they vaporize and leave a tail of gas and dust visible to the eye.

Elliptical orbit  -  Orbit in the shape of an ellipse. An ellipse can be constructed by finding one point at which the summation of its distance from two fixed points, called foci, is constant.

Newtonian  -  Attributed to the British physicist (1643-1727) who invented modern mechanics and wrote the law of gravity, as still used today in most applications. Read the SparkNote on Newton.

Spectrum  -  The summation of the light of all different wavelengths coming from a given light source.

Internal energy  -  The internal energy of a planet comes in the form of heat and gravitation. A contracting planet releases its potential energy, transforming it into the heat of its interior. The difference in temperature between the planet's interior and outer space induces an outflow of heat from the planet. The process in which a denser compound sinks to the center of planet also releases gravitational energy.

Outflow  -  The difference in temperature between the planet's interior and outer space induces an outflow of heat from the planet.

Oblateness  -  A measure of how much a spheroid is flattened at the Poles.

Silicates  -  Minerals that contain silicon oxide and metals, for example Fe₂Si₄.

Solar nebula  -  The cloud of gas and dust associated with the formation of the primordial solar system.

Planetesimal  -  The current theory of the formation of planets involves the collision of smaller bodies, called planetesimals, which formed the planets we observe today. Such collisions would have been common in the very first 500 million years of our solar system's history.

Solar wind  -  The hot wind of particles emitted by the Sun.

Asteroids  -  Small planetoids distributed widely throughout the solar system. Their biggest concentration is found between Mars and Jupiter.

Resurfaced  -  This term is used to define the process by which the external crust of a moon is completely reformed through the gradual destruction of the older surface.

Zenith  -  The zenith is the point of the celestial sphere directly over the observer, and perpendicular to any point on the observer's horizon.

William Herschel  -  William Herschel discovered Uranus. A British astronomer, he also catalogued many astronomical objects, such as nebulae and comets, over the course of his career.