Glossaire beginning with P


For an object of the Solar system, the parallax is the angle subtended by the radius of the Earth as seen from the object. It can be measured, for example, by photographing the same object from the background of stars from various points of the Earth. Knowing the radius of the Earth, one can then calculate the distance of the object.

Parent molecule
A molecule directly ejected from the nucleus of a comet under the effect of solar radiation. It is generally not observable in the visible range, but only in the infrared and in radio. The parent molecules are dissociated by the solar ultraviolet radiation into atoms and daughter molecules which are observable by spectroscopy in the visible.
The point of the orbit of a planet or comet where it is nearest to the Sun.
Petite planète
Synonyme d’astéroïde.
Polarization of light
Property acquired by the light reflected or transmitted, or when passing to a crystal called polarizer, whereby the vibrations of the electric field are entirely or partially in a given plane.
Prebiotic molecule
Organic molecule essentially formed of the atoms H, C, N and O, which is one of the building blocks of proteins and DNA. Amino acids are such molecules.
Protosolar nebula
Mass of gas and interstellar dust from which the solar system formed.
Protosolar, protoplanetary
Adjectives denoting what existed in the future Solar system before the formation of the Sun and planets.