bodies causes their orbital periods to have a ratio expressible in small, whole numbers. For example, for every two times that Neptune orbits the Sun, Pluto revolves three times.



—The point at which a body's orbital motion takes it closest to the Sun.

Phase space

—A multidimensional plot showing the positions and velocities of particles in a dynamical system. The trajectory followed by any particular particle represents the evolution of that particle's dynamics as a function of time.


—The planetary bodies that formed the building blocks of all the solar system's planets and satellites.


Radiogenic heating

—Heating of a celestial body due to the decay of radioactive isotopes.

Retrograde motion

—The rotational or orbital motion of a planetary body opposite to the dominant direction of the orbiting and rotating of the Sun and planets. In our solar system, retrograde motion is clockwise as viewed from the north pole of the ecliptic. Triton's orbit around Neptune and Venus's rotation are both examples of retrograde motion.


Solar nebula

—The cloud of gas and dust from which our Sun, planets, and other bodies in our solar system formed.


—The characteristic emission or absorption of certain electromagnetic frequencies by elements and compounds.


—The region above the troposphere where the atmosphere becomes stably stratified as a result of solar heating.



—The uppermost region of an atmosphere, where the temperature increases with height as a result of strong heating from above and where molecular diffusion of heat plays a major role in vertical heat transport.


—the reddish tarlike organic residue created in simulations of the action of ultraviolet radiation on gases typically found in planetary environments.

Tidal heating

—The internal heating of a planetary body due to friction caused by the differential gravitational effect of an external body on the mass in question.


—The apparent passage of one body across the disk of a larger companion.


—The top of the troposphere and the base of the stratosphere.


—The lowermost portion of a planetary atmosphere, in which temperature decreases with height and thermal convection takes place.

T Tauri star

—A type of irregular variable star whose spectrum shows broad and very intense emission lines. They are believed to be very young stars that have not yet reached the main sequence.



—A pair of deep-space missions launched by NASA to the outer solar system in 1977. Combined, Voyagers 1 and 2 have conducted close-up observations of Jupiter (1979, 1979), Saturn (1980, 1981), Uranus (1986), and Neptune (1989).

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