Astrobiology Glossary P – Space

Term Definition
Panspermia The hypothesis that life has spread from planet to planet throughout the galaxy and therefore did not originate on Earth. The term originates with Svante Arrhenius in the 1900s.
Parallax The apparent shift of an object’s position when viewed from different locations. Parallax, also called trigonometric parallax, is used to determine the distance to nearby stars. As the Earth’s position changes during its yearly orbit around the Sun, the apparent locations of nearby stars slightly shift. The stars’ distances can be calculated from those slight shifts with basic trigonometric methods.
Parsec (PC) A useful unit for measuring the distances between astronomical objects, equal to 3.26 light-years and 3.085678 * 1013 kilometers, or approximately 18 trillion miles. A parsec is also equivalent to 103,132 trips to the Sun and back.
Peptide A compound linked together by peptide bonds such as a protein.
Peptide Two or more amino acids bonded together.
Period-Luminosity Law A relationship that describes how the luminosity or absolute brightness of a Cepheid variable star depends on the period of time over which that brightness varies.
Periodic Comet A comet in a closed, elliptical orbit within our solar system. These comets typically have orbital periods of less than 200 years. Many comets have orbits that keep them in the inner solar system and allow their trajectories to be calculated with great accuracy and precision. Perhaps the best-known periodic comet is Halley’s comet, whose orbital period is 76 years.
Periodic Table (of the Elements) A chart of all the known chemical elements arranged according to the number of protons in the nucleus (also known as the atomic number). Elements with similar properties are grouped together in the same column.
pH Scale of relative acidity or alkalinity
Photoelectric Effect The release of electrons from a solid material when it is struck by radiant energy, such as visible or ultraviolet light, X-rays, or gamma rays.
Photometer An instrument that measures the intensity of light. Astronomers use photometers to measure the brightness of celestial objects.
Photometry A technique for measuring the brightness of celestial objects.
Photon A packet of electromagnetic energy, such as light. A photon is regarded as a charge-less, mass-less particle having an indefinitely long lifetime.
Photosphere The extremely thin, visible surface layer of the Sun or a star. The average temperature of the Sun’s photosphere is about 5800 Kelvin (about 10,000° F). Although the Sun is completely made up of gas, its gas is so dense that we cannot see through it. When we look at the Sun, we are seeing the photosphere.
Photosynthesis The chemical process carried out in photosynthetic bacteria and green plants in which light energy is used to synthesize carbohydrates from water and carbon dioxide.
Phylogeny The evolutionary relationships between different species of organisms as represented in a phylogenetic tree. In molecular phylogeny these relationships are determined by analysis of the differences in the sequences of genes common to the various species.
Pickoff Mirror One of four flat mirrors inside the Hubble Space Telescope. Each mirror is tilted at a 45-degree angle to the incoming light, diverting a small portion of it to the optical detectors or to one of the fine guidance sensors.
Pixel A light-sensitive picture element on a charge-coupled device (CCD) or some other kind of digital camera. A pixel is a tiny cell that, placed together with other pixels, resembles the mesh on a screen door. The Hubble Space Telescope’s Wide Field and Planetary Camera 2 has four CCDs, each containing 640,000 pixels. Each pixel collects light from a celestial object and converts it into a number. The numbers (all 2,560,000 of them) are sent to ground-based computers, which convert them into an image. The greater number of pixels, the sharper the image.
Planck Curve The graphical representation of the mathematical relationship between the frequency (or wavelength) and intensity of radiation emitted from an object by virtue of its heat energy.
Planet An object that orbits a star. Although smaller than stars, planets are relatively large and shine only by reflected light. Planets are made up mostly of rock or gas, with a small, solid core. In our solar system, the inner planets – Mercury, Venus, Earth, and Mars – are the rocky objects, and most of the outer planets – Jupiter, Saturn, Uranus, and Neptune – are the gaseous ones. Because Pluto is made largely of ice, like a comet, some astronomers do not consider it a true planet.
Planetary Nebula An expanding shell of glowing gas expelled by a star late in its life. Our Sun will create a planetary nebula at the end of its life.
Planetary Panspermia The spreading of life between planets within the solar system. In particular the hypothesis that life might have originated on Mars and been carried to Earth a suggestion which is supported by the existence of martian meteorites.
Planetesimal A small body of rock and/or ice – under 10 kilometers (6 miles) across – formed during the early stages of the solar system. Planetesimals are the building blocks of planets, but many never combined to form large bodies. Asteroids are one example of planetesimals.
Plasma A substance composed of charged particles, like ions and electrons, and possibly some neutral particles. Our Sun is made of plasma. Overall, the charge of a plasma is electrically neutral. Plasma is regarded as an additional state of matter because its properties are different from those of solids, liquids, and normal gases.
Pluto The outermost planet in our solar system. Pluto is, on average, about 40 times the Earth’s distance from the Sun. Pluto orbits the Sun in 248 Earth years.
Potential Energy The energy of an object owing to its position in a force field or its internal condition, as opposed to kinetic energy, which depends on its motion. Examples of objects with potential energy include a diver on a diving board and a coiled spring.
Pre RNA World A hypothetical early stage in the development of life which preceeded the RNA World and used some other genetic material in place of RNA or DNA.
Precambrian The period in Earth’s history from its formation up to the beginning of the Cambrian period about 540 million years ago. The Precambrian is subdivided into the Hadean, the Archean, and the Proterozoic.
Primary Mirror A large mirror in a reflecting telescope that captures light from celestial objects and focuses it toward a smaller secondary mirror. The primary mirror in the Hubble Space Telescope measures 94.5 inches (2.4 meters) in diameter.
Prime Focus The location where light reflected from the primary mirror of a reflecting telescope comes into focus. Placing a secondary mirror in the light path allows the light to be focused elsewhere, in a more convenient location for the science instruments.
Primordial Nucleosynthesis Element building that occurred in the early universe when the nuclei of primordial matter collided and fused with one another. Most of the helium in the universe was created by this process.
Prokaryote An organism belonging to the domains archaea or bacteria. Prokaryotic cells lack a nucleus and organelles and are typically much smaller than eukaryotic cells.
Prominence An eruption of gas from the chromosphere of a star. Solar prominences are visible as part of the corona during a total solar eclipse. These eruptions occur above the Sun’s surface (photosphere), where gases are suspended in a loop, apparently by magnetic forces that arch upward into the solar corona and then return to the surface.
Proper Motion The apparent motion of a star across the sky (not including a star’s parallax), arising from the star’s velocity through space with respect to the Sun.
Protein A polymer built from amino acid monomers. Living organisms use many different proteins acting as catalysts (enzymes) and for structural and other roles.
Proterozoic The period in Earth’s history from 2500 million to 540 million years ago. The last division of the Precambrian.
Protogalaxy Matter that is beginning to come together to form a galaxy. It is the precursor of a present-day galaxy and is sometimes called a ‘baby galaxy.’
Proton A positively charged elementary particle that resides in the nucleus of every atom.
Proton-Proton Chain A series of nuclear events occurring in the core of a star whereby hydrogen nuclei (protons) are converted into helium nuclei. This process releases energy.
Protoplanet A small body that attracts gas and dust as it orbits a young star. Eventually, it may form a planetary body.
Protostar A collection of interstellar gas and dust whose gravitational pull is causing it to collapse on itself and form a star.
Pulsar A neutron star that emits rapid and periodic pulses of radiation.
Punctuated Equilibrium The theory that evolutionoccurs in leaps and bounds, as a series of rapid periods of change interspersed by slow periods with no change.

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