Brown Dwarf:

A huge ball of gas.  It is too big to be a planet and too small to be a star. They radiate more energy than they absorb.  The excess radiation to space may be due to gravitational energy converted to thermal energy, or to a very low rate of fusion occurring in the center of the brown dwarf. (by Alberto Monteiro)


Gas giant:

A planet mainly composed of gases, like H2, He, NH3, CH4, H2O.  They are usually quite large compared to terrestrial planets.   A gas giant does not have a solid surface that is visible from orbit.  The core of a gas giant will be under very high pressure.  It is therefore possible that some gas giants have cores made of metallic Hydrogen. (by Alberto Monteiro)


Green house planet:

For any planet we can compute an expected temperature based on the thermal radiation it receives from the star, and the index of reflection (albedo) for the planet.  However, some planets are hotter than what is expected.  This is because of gases in the atmosphere that trap the heat.  Heat eventually flows out until an equilibrium is reached, but at a higher equilibrium temperature than one would expect based on radiation and reflection alone.  This is called the "green house effect."   Humans first observed the green house effect on Venus.  Like any planet with an atmosphere, Earth also has a green house effect, but it is relatively mild.  On inhabited planets the green house effect can be increased by some sapient-made gases, such as clorofluorcarbons, methane, and carbon dioxide.  (by Alberto Monteiro)


Terrestrial planet (contrast with "gas giant"):

A planet composed of mainly of heavy elements, with a liquid or solid surface. (by Alberto Monteiro)



The time it takes for a body in orbit around a star to complete a revolution around the star (this movement is called translation).  The definition of what is a complete revolution creates different definitions of a year:

* Sidereal Year is the time it takes to return to the same direction in space.

* If the orbit is elliptical, we can define an Anomalistic Year: the time between two passes at the perihelion [or peri-<star name>, in the case of bodies around a star that is not Sol].  If the planet rotates around itself, and the rotational axis is inclined relative to the plane of the orbit, then the planet has seasons.

* If a planet has seasons, a Tropic Year is the time between the beginning of one season and the next occurance of same season [for example, the time from the beginning of one Winter and the start of the next Winter].

These three kinds of years are usually very close to one another.  Also note that the duration of the year in days changes with time, due to the tidal friction.  Due to the large size of Earth's moon, rotational deceleration has been particularly pronounced on Earth. (See bibliography; by Alberto Monteiro)

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