planet Any large body orbiting the Sun or another star (see planets of other stars); comets, asteroids, meteoroids (see meteor), and natural satel¬ lites are excluded. The word comes from the Greek for “wanderer,” because their positions change relative to those of the stars. The nine major planets known to revolve around the Sun, in order of increasing distance, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Nep¬ tune, and Pluto. The first four are called terrestrial planets and the next four Jovian planets. Pluto, distinct from either group, resembles a giant comet or one of the icy satellites of the Jovian planets. The terrestrial p_ planets are less than 8,000 mi (13,000 km) in diameter and rocky, with
comparatively thin or negligible atmospheres. The Sun’s heat is thought wm to have prevented the abundant gases in the original solar nebula from
condensing in them. The Jovian planets formed farther out, where the gases were cool enough to condense, so the planets grew very massive tm and accumulated huge atmospheres of light gases, mainly hydrogen and
y helium. Called gas giants, the Jovian planets appear to be similar in struc¬
ture; none has an accessible surface. Pluto is by far the smallest body tra¬ ditionally called a major planet, although many astronomers now consider it the largest known member of the Kuiper belt. Additional bodies rivaling Pluto in size may exist in this region of the solar system; one about half its size was discovered in 2002. The term minor planet is sometimes used (less now than formerly) to refer to any of the asteroids that lie between the terrestrial and Jovian planets. In astrology great importance is placed on the planets’ positions in the 12 constellations of the zodiac. See also planetesimal; solar system.
Planet X Supposed distant planet of the solar system, hypothesized on the basis of calculations of effects on the orbits of Uranus and Neptune. The term was first used by Percival Lowell c. 1905, and though his pre¬ diction eventually resulted in the discovery of Pluto (1930), Pluto’s mass was insufficient to explain the apparent perturbations of Uranus and Nep¬ tune. Proposals and searches for other planets beyond Neptune or Pluto continued sporadically during the remainder of the 20th century. How¬ ever, Voyager spacecraft data gathered about the masses of the outer plan¬ ets indicate that no additional large bodies are needed to explain the planets’ observed orbits. See also Kuiper belt.
planetarium Institution devoted to popular education in astronomy and related fields, especially space science, whose main teaching tool is a hemispheric screen onto which images of celestial objects as seen from Earth are projected from an instrument also known as a planetarium, or planetarium projector. Major planetariums have extensive exhibit space, museum collections, sizable staffs, projection domes 80 ft (25 m) or more across, and seating capacities of over 600.
planetary nebula Any of a class of bright NEBULAe that may some¬ what resemble planets when viewed through a small telescope but are, in fact, expanding shells of luminous gas around dying stars. A planetary nebula is the outer envelope shed by a red giant star not massive enough to become a supernova. Instead, the star’s intensely hot core becomes exposed (see white dwarf star) and ionizes the surrounding shell of gas, which is expanding at tens of miles per second.
planetesimal N.pla-no-'te-so-moH One of a class of hypothetical bod¬ ies that joined to form the planets after condensing from dust and gas early in the history of the solar system. According to the nebular hypothesis, a rotating cloud of interstellar dust and gas underwent gravitational con¬ traction, eventually forming a solar nebula comprising a disk of matter with a central hub. As the disk contracted further, clumps of dust left behind in its midplane coalesced successively into bodies the size of pebbles and boulders and then into planetesimals up to several hundred miles across. These then combined under the force of gravity to form pro¬ toplanets, precursors of the current planets.
planets of other stars or extrasolar planets Planets that orbit stars other than the Sun. The existence of extrasolar planets, many light- years from Earth, was confirmed in the early 1990s with the detection of three bodies circling a pulsar. The first planet revolving around a more sunlike star, 51 Pegasi, was reported in 1995. Within a decade over 100 stars with one or more planets were known. Current detection methods, based on the planets’ gravitational effects on the stars they orbit, have revealed only planets much more massive than Earth; some are several times the size of Jupiter. A number of them have highly elliptical orbits, and many are closer to their stars than Mercury is to the Sun. These find¬ ings have raised questions regarding astronomers’ ideas of how Earth’s solar system formed and how typical it is.
plankton Marine and freshwater organisms that, because they are unable to move or are too small or too weak to swim against water cur¬ rents, exist in a drifting, floating state. Plankton is the productive base of both marine and freshwater ecosystems, providing food for larger animals and indirectly for humans, whose fisheries depend on plankton. As a human resource, plankton has only begun to be developed and used. The plantlike community of plankton is called phytoplankton, and the animal¬ like community is called zooplankton, but many planktonic organisms are better described as protists. Most phytoplankton serves as food for zoo¬ plankton, but some of it is carried below the light zone. Zooplankton is used directly as food by fish (including herring) or mammals (including whales), but several links on the food chain usually have been passed before plankton is available for human consumption.
plant Any organism in the kingdom Plantae, consisting of multicellu¬ lar, eukaryotic life forms (see eukaryote) with six fundamental character¬ istics: photosynthesis as the almost exclusive mode of nutrition, essentially unlimited growth at meristems, cells that contain cellulose in their walls and are therefore somewhat rigid, the absence of organs of movement, the absence of sensory and nervous systems, and life histories that show alternation of generations. No definition of the kingdom completely excludes all nonplant organisms or even includes all plants. Many plants, for example, are not green and thus do not produce their own food by photosynthesis, being instead parasitic on other living plants (see parasit¬ ism). Others obtain their food from dead organic matter. Many animals possess plantlike characteristics, such as a lack of mobility (e.g., sponges) or the presence of a plantlike growth form (e.g., some corals and bryo- zoans), but in general such animals lack other plant characteristics. Some past classification systems (see taxonomy) placed difficult groups such as protozoans, bacteria, algae, slime molds, and fungi (see fungus) in the plant kingdom, but structural and functional differences between these organ¬ isms and plants have convinced most scientists to classify them elsewhere.
plant virus Any of various viRUSes that can cause plant disease (e.g., the tobacco mosaic virus). Plant viruses are economically important because many of them infect crop and ornamental plants. Numerous plant viruses are rodlike and can be extracted readily from plant tissue and crystallized.
© 2006 Encyclopaedia Britannica, Inc.
Plantagenet ► plastics I 1509
Most lack the fatty membrane found in many animal viruses, and all con¬ tain RNA. Plant viruses are transmitted in various ways, most importantly through insect bites, mainly by aphids and plant hoppers. Symptoms of virus infection include colour changes, dwarfing, and tissue distortion. The appearance of streaks of colour in certain tulips is caused by a virus. See also reovirus.