932 Encyclopedia of the Solar System
Pixel A “picture element.” One element in a CCD or infrared
detector array.
Planar deformation features Planar, micrometer-sized
bands of intense deformation or glass that occur in minerals due
to the passage of a shock wave.
Planet According to the new IAU definition passed in 2006, a
planet must have three qualities: (1) it must be round, indicating
its interior is in hydrostatic equilibrium; (2) it must orbit the
Sun; and (3) it must have gravitationally cleared its zone of other
debris. According to the definition, our solar system has eight
planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus
and Neptune. Although most astronomers have accepted the
new definition, some are campaigning to have it changed. See
The Solar System and Its Place in the Galaxyand
Pluto and Charonfor more discussion.
Planetary embryo Large solid body formed by runaway and
oligarchic growth.
Planetesimal A small solid body formed in the early solar
system by accretion of dust and ice (if present) near the central
plane of the solar nebula. The terrestrial planets, asteroids,
comets, and cores of the giant planets are generally thought to
have formed through the accretion and aggregation of
planetesimals.
Plasma Ionized medium in which electrons have been
stripped from neutral matter to make a gas of charged ions and
electrons.
Plasma beta Ratio of gas pressure to magnetic field pressure
within a plasma.
Plasma tail The narrow, highly structured tail consisting of
molecular ions (and electrons) confined to magnetic field
lines wrapped around the head of the comet. The plasma
tail is normally attached to the head region. The exception is
when disconnection events occur. The orientation of the
tail, approximately anti-sunward, is produced by the solar
wind interaction. The plasma tail appears blue because of
resonance scattering of sunlight from ionized carbon monoxide
molecules.
Plasmoid A region within a magnetosphere in which plasma
is confined by a magnetic structure that is not directly linked to
the planet.
Plate tectonics The system of rigid plates, tens of kilometers
thick, that move over the surface of the Earth, causing mountain
belts to form at convergent zones.
Plume A hot blob of material that rises through the mantle,
typically causing uplift of the surface and volcanism.
Polar wandering Changes in the direction of the magnetic
pole relative to its orientation in space.
Polymict A rock unit consisting of fragments of various
pre-existing rock units.
Polymorph Crystal form of a mineral that has a different
crystal structure from that of the original mineral.
Polysaccharide Any of a group of carbohydrates consisting of
long chains of simple sugars; e.g., starch, glycogen.
Poynting–Robertson effect, P-R drag Drag on
interplanetary particles caused by their interaction with solar
radiation, which causes the particles to lose orbital angular
momentum and to spiral in towards the Sun.
Precess, Precession The slow, smooth increase or decrease
of an angle. For example, the axes of the planets’ perihelion
directions change, or precess, taking tens of thousands to
millions of years to complete an entire cycle. Pole precession is
the slow, periodic, and conical motion of the rotation axis of a
spinning body.
Precession of the equinoxes The slow rotation of the
equinoxes with respect to the stars. It has a period of about
26,000 years for the Earth.
Primary body Celestial body (usually the Sun or a planet)
around which a planet or a moon, respectively, or secondary
body, orbits.
Primitive meteorite SeeChondrite.
Prominences Cool and dense mass structures suspended
above the chromosphere, observed above the solar limb, which
are called filaments when seen on the solar disk.
Protoplanetary nebula A disk of gas and dust that surrounds
a newborn star, from which the planets, asteroids, and comets
are thought to form.
Protostar A star in the process of formation, which is
luminous due to the release of gravitational potential energy
from the infall of nebula material.
P-wave velocity Seismic body wave velocity associated with
particle motion (alternating compression and expansion) in the
direction of wave propagation.
Pyroclastic materials Fragmented materials ejected during
an explosive volcanic eruption, including ash, pumice, and rock
fragments.
Radar albedo Ratio of a target’s radar cross section in a
specified polarization to its projected area, hence a measure of
the target’s radar reflectivity.
Radar cross section Most common measure of a target’s
scattering efficiency, equal to the projected area of that perfect
metal sphere that would give the same echo power as the target
if observed at the target’s location.
Radial velocity technique Observational method used to
detect stellar reflex motions by measuring the line-of-sight
component of a star’s space velocity vector. If the radial velocity
can be measured with a precision of a few m s−^1 , then the reflex
motion due to planetary companions become detectable. Today,
this is the most successful method for finding extrasolar planets.
Radiation belts Toroidal zones containing charged particles
that are magnetically trapped in a planetary dipole field. The
Van Allen belts around the Earth include ions and electrons
with energies from hundreds of keV to tens of MeV.
Radio spectrum A graph of the brightness temperature as a
function of wavelength or frequency.
Radiolysis The dissociation of molecules by high-energy
radiation.