Term | Definition |
Comparative planetology | understanding planets by searching for and analyzing contrasts and similarities |
Terrestrial planet | an Earth-like planet – small, dense, rocky and metallic |
Jovian planet | a Jupiter-like planet with a large diameter and low density |
Differentiation | separation of planetary material within a planet into layers according to density |
Mantle | the layer of dense rock and metal oxides that lies between the molten core and Earth’s surface or a similar layer in another planet |
Heavy bombardment | the intense cratering that occurred sometime during the first 0.7 billion years in the history of the solar system |
P wave | a type of seismic wave involving compression and decompression of the material though which it passes |
S wave | a type of seismic wave involving lateral motion of the material through which it passes |
Primary atmosphere | a planet’s first atmosphere |
Secondary atmosphere | a planet’s atmosphere that replaces the primary atmosphere, for example by outgassing, impact of volatile-bearing planetesimals, or biological activity |
Greenhouse effect | the process by which an atmosphere traps hear and raises the temperature of a planetary surface |
Plate tectonics | one plate slides over another, causing volcanism |
Rift valley | forms where continental plates begin to pull apart |
Mid-ocean rises | formed when plates spread apart and magma rises |
Basalt | a rock typical of solidified lava |
Subduction zone | a deep trench where one plate slides under another |
Global warming | the gradual increase in the surface temperature of Earth caused by human modifications to Earth’s atmosphere |
Folded mountain range | can form where plates push against each other |
Mare | one of the lunar lowlands filled by successive flows of dark lave, from the Latin for “sea.” Plural: “maria.” |
Albedo | the ratio of the amount of light reflected from an object to the amount of light received by the object. Equals 0 for perfectly black and 1 for perfectly white |
Ejecta | pulverized rock scattered by meteorite impacts on a planetary surface |
Anorthosite | Aluminium-and calcium-rich silicate rock found in the lunar highlands |
Breccia | rock composed of fragments of older rocks bonded together |
Large-impact hypothesis | hypothesis that the moon formed from debris ejected during a collision between Earth and a large object |
Magma ocean | the exterior of the newborn moon, a shell of molten rock hundreds of kilometers deep |
Multiringed basin | large impact feature (crater) containing two or more concentric rims formed by fracturing of the planetary crust |
Late heavy bombardment | the sudden temporary increase in the cratering rate in our solar system that occurred about 4 billion years ago |
Micrometeorite | meteorite of microscopic size |
Coronae | on Venus, large round geological faults in the crust caused by the intrusion of magma below the crust |
Runaway greenhouse effect | a greenhouse effect so dramatic that it amplifies itself, becoming stronger with time |
Permafrost | permanently frozen soil |
Shield volcano | wide, low-profile volcanic cone produced by highly liquid lava |
Outflow channel | geological features on Mars and Earth caused by flows of vast amounts of water released suddenly |
Valley network | a system of dry drainage channels on Mars that resembles the beds of rivers and tributary streams on Earth |
Oblateness | the flattening of a spherical body , usually caused by rotation |
Liquid metallic hydrogen | a form of liquid hydrogen that is a good electrical conductor, inferred to exist in the interiors of Jupiter and Saturn |
Magnetosphere | the volume of space around a planet within which the motion of charged particles is dominated by the planetary magnetic field rather than the solar wind |
Belt-zone circulation | the atmospheric circulation typical of Jovian planets in which dark belts and bright zones encircle the planet parallel to its equator |
Forward scattering | the optical property of finely divided particles to preferentially direct light in the original direction of the light’s travel |
Roche limit | the minimum distance between a planet and a satellite that can hold itself together by its own gravity |
Tidal heating | the heating of a planet or satellite because of friction caused by stretching due to the gravitational influence of a nearby body |
Shepherd satellites | gravitationally usher straying particles back into the rings |
dwarf planet | a body that orbits the sun, is not a satellite of a planet, is massive enough to pull itself into a spherical shape but not massive enough to clear out other bodies in and near its orbit. for example, pluto, eris ,and ceres |
asteroid | small, rocky world. most orbit mars and jupiter in a belt |
comet | one of the small, icy bodies that orbit the sun an produce tails of gas and dust when they approach the sun |
terrestrial planets | small, dense, rocky worlds with little or no atmosphere (inner four planets) |
jovian planets | large, low-density worlds with thick atmospheres and liquid or ice interiors (outer four planets) |
volatile | easily vaporized |
kuiper belt | the collection of icy objects orbiting in a region from just beyond neptune out to 50 AU or more |
meteor | a small bit of matter heated by friction to incandescent vapor as it falls into Earth's atmosphere |
meteoroid | a meteor in space before it enters earths atmosphere |
meteorite | a meteor that survives its passage through the atmosphere and strikes the ground |
carbonaceous chondrite | stony meteorite that contains small glassy spheres called chondrules and volatiles. these chondrites may be the least-altered remains of the solar nebula still present in the solar system |
widmanstatten pattern | bands in iron meteorite sections due to large metal crystals |
meteor shower | a display of meteors that appear to come from one point in the sky, understood to be cometary debris |
half-life | the time required for half of the radioactive atoms in a sample to decay |
solar nebula theory | the theory that the planets formed from a spinning disk of material around the forming sun |
uncompressed density | the density a planet would have if its gravity did not compress it |
ice line | boundary beyond which vapor could freeze to form ice |
condensation sequence | the sequence in which different materials condense from the osolar nebula depending on distance from the sun |
planetesimal | one of the small bodies that formed from the solar nebula and eventually grew into protoplanets. asteroids and comets are remnants of the planetesimals |
condensation | the growth of a particle by addition of material from surrounding gas, atom by atom |
accretion | the sticking together of solid particle to produce a larger particle |
protoplanet | massive object, destined to become a planet, resulting from the coalescence of planetesimals in the solar nebula |
gravitational collapse | the process by which a forming body such as a planet gravitationally captures gas rapidly from the surrounding nebula |
differentiation | the separation of planetary material inside a planet according to density |
outgassing | the release of gases from a planet's interior |
NEO (near earth object) | a small solar system body (asteroid or comet) with an orbit near enough to earth that it poses some threat of eventual collision |
evolutionary theory | an explanation of a phenomenon involving slow, steady processes of the sort seen happening in the present day |
catastrophic theory | an explanation of a phenomenon involving special, sudden, perhaps violent, events |
heat of formation | in planetology, the heat released by infalling matter during the formation of the planetary body |
oort cloud | the hypothetical source of comets, a swarm of icy bodies understood to lie in a spherical shell extending to 100,000 AU from the sun |
stellar parllax (p) | the small apparent shift in position of a nearby star relative to distant background objects due to earths orbital motion |
parsec (pc) | the distance to a hypothetical star whose parallax is 1 arc second |
intrinsic brightness | a measure of the amount of light a star produces |
absolute visual magnitude | intrinsic brightness of a star. the apparent visual magnitude the star would have if it were 10 pc away |
luminosity (L) | the total amount of energy a star radiates per second at all wavelengths |
spectral class | a star's label in the temperature classification system based on the appearance of the stars spectrum |
spectral sequence | the arrangement of spectral classes (O, B, A, F, G, K, M) ranging from hot to cool |
brown dwarf | a very cool, low-luminosity star whose mass is not sufficient to ignite nuclear fusion |
L dwarf, T dwarf | spectral classes of brown dwarf stars with lower surface temperatures and luminosities than M dwarfs |
Y dwarf | a substellar object with temperature below 500 K, having inferred properties intermediate between brown dwarfs and Jovian planets |
hertzsprung-russell (H-R) diagram | a plot of the intrinsic brightness versus the surface temperature of stars. it separates the effects of temperature and surface area on stellar luminosity |
main sequence | the region of the H-R diagram running from upper left to lower right, which includes roughly 90 percent of all stars generating energy by nuclear fusion |
giant | large, cool, highly luminous star in the upper right of the HR diagram, typically 10 to 100 times the diameter of the sun |
supergiant | exceptionally luminous star whose diameter is 100 to 1000 times that of the sun |
red dwarf | a faint, cool, low-mass, main-sequence star |
white dwarf | dying star at the lower left of the HR diagram that has collapsed to the size of Earth and is slowly cooling off |
luminosity class | a category of stars of similar luminosity, determined by the widths of lines in their spectra |
spectroscopic parallax | the method of determining a stars distance by comparing its apparent magnitude with its absolute magnitude as estimated from its spectrum |
binary star | pairs of stars that orbit around their common center of mass |
visual binary system | a binary system in which the two stars are separately visible in the telescope |
spectroscopic binary system | a star system in which the stars are too close together to be visible separately. we see a single point of light, and only by taking a spectrum can we determine that there are two stars |
eclipsing binary system | a binary star system in which the stars cross in front of each other as seen from earth |
light curve | a graph of brightness versus time commonly used in analyzing variable stars and eclipsing binaries |
nuclear forces | the two forces of nature that only affect the particles in the nuclei of atoms |
nuclear fission | reactions that break the nuclei of atoms into fragments |
nuclear fusion | reactions that join the nuclei of atoms to form more massive nuclei |
coulomb barrier | the electrostatic force of repulsion between bodies of like charge, commonly applied to atomic particles |
proton-proton chain | a series of three nuclear reactions that builds a helium atom by adding together protons. the main energy source in the sun |
deuterium | an isotope of hydrogen in which the nucleus contains a proton and a neutron |
neutrino | a neutral, nearly massless atomic particle that travels at or nearly at the speed of light |
CNO (carbon-nitrogen-oxygen) cycle | a series of nuclear reactions that use carbon as a catalyst to combine four hydrogen nuclei to make one helium nucleus plus energy, effective in stars more massive than the sun |
brown dwarf | a stellar object with such low mass that it cannot raise its central temperature high enough to sustain hydrogen fusion |
zero-age main sequence (ZAMS) | the location in the HR diagram where stars first reach stability as hydrogen-burning stars |