G
Glossary
Ab initio: Latin term meaning “from the beginning.”
Abiotic: Of or relating to nonliving things; independent of life or living organisms.
Adaptive optics: A set of techniques to adjust the shape of mirrors on time scales of a fraction of a second to correct for rapid fluctuations in image quality. In the astronomical community, this term is used for systems that correct for distortions in images of ground-based telescopes due to atmospheric turbulence. The essential feature of the implementation is use of real-time sensing of the wave front from a distant source to provide the signals to the actuators that control the shape of the mirror.
Aerosol: A suspension of fine solid particles or liquid droplets in a gas.
Aerosol scattering: The scattering of light produced by small solid particles or liquids that are suspended in a gas. Albedo: The fraction of light that is reflected from the surface of a planetary body.
Amorphous metals: Metals that have a noncrystalline atomic structure and are instead arranged in a glass-like atomic structure. However, they are highly conductive.
Aperture: The diameter of the primary lens or mirror of a telescope; hence, the simplest single measure of the light-gathering power of a telescope.
Asteroseismology: The study of the interior structure of stars based on observed stellar oscillations.
Astrometry: The precise measurement of stellar positions. In exoplanet hunting, the astrometric method consists of making precise stellar location measurements and searching for motion of a star around a common center of mass with a planet.
Biogenic: A substance produced by living organisms; produced or brought about by living organisms.
Biosignature: A global impact of life on its planetary environment that may be detectable at interstellar distances. Examples include atmospheric gases produced by life, characteristic surface reflectivity due to vegetation pigments or internal leaf structure, and seasonal changes in either of these due to life.
Bolometric: Measured over all wavelengths.
Chord: A “line,” or the path of an object, as it transits in front of another body.
Circumstellar disk: A broad ring of material orbiting around a star.
Coronagraph: A telescopic piece employed to block out the direct light from a star, to enable the viewing of nearby objects and coronal activity.
Debris disk: A disk created by ongoing collisions or evaporation of planetesimals that can exist for long times after the primordial disk (which was largely molecular hydrogen) has disappeared.
Diffraction limit: The finest detail that can be discerned with a telescope. The physical principle of diffraction limits this to a value proportional to the wavelength of the light observed divided by the diameter of the telescope.
Doppler shift: The change in frequency or wavelength of a wave, due to movement relative to the source. When movement is toward the observer, waves shift to shorter wavelength. When the movement is away from the observer, waves shift to longer wavelength.
Emission spectrum: A spectrum composed solely or predominantly of emission, including lines indicating the presence of a hot gas or thermal radiation as a result of a body’s heat.
Ephemeris: The precise current, and sometimes future, positions of celestial objects.
eta-Earth (η⊕): The mean number per star of rocky planets with between 1 and 1.5-2 Earth-radii that reside in the habitable zone of their host star.
Exozodiacal dust: 1-100 micrometer grains of warm dust in extrasolar planetary systems; the analogue to zodiacal dust in the Solar System.
Fabry-Pérot etalon: An interferometer that uses interference conditions of a wave reflected between two surfaces to maximize wavelength resolution.
Free-floating planet: A planetary-mass object that has been ejected from its home system, and orbits the galactic center directly, not gravitationally bound to any star or brown dwarf.
Habitable zone: The region around a star where a terrestrial planet with an Earth-like atmosphere could support surface liquid water. The habitable zone is calculated using stellar irradiation, orbital parameters and a 1D or 3D climate model. The inner edge occurs when an Earth-like planet experiences a runaway greenhouse. The outer edge calculation assumes that atmospheric CO2 increases with orbital distance (and decreasing irradiation) until a maximum greenhouse limit is reached.
Inner working angle: For a coronagraph, the inner working angle is the smallest angular separation from the host star beyond which a planet of a given flux ratio can be imaged.
Insolation: The amount of stellar radiation reaching a given surface area; power per unit area.
Interferometer: An instrument that utilizes interference between beams of light to extract information about the source based upon the interference pattern generated.
Magnetohydrodynamics: The branch of physics that studies the motion of electrically conductive fluids in electric and magnetic fields.
Magnetorotational instability (MRI): A theory for how turbulence in an ionized gas subjected to an external magnetic field is generated. Turbulence is required to remove angular momentum from disk gas and allow accretion of the disk onto the star.
Metallicity: The sum total of elements within an object other than hydrogen and helium. Astronomers often lump these disparate elements together because they follow a common abundance trend in many astronomical objects. That is, an object that has low or high metallicity will have similarly low and high abundances of individual heavy elements like carbon, oxygen, silicon, and iron.
Microlensing: A technique used in the search for extrasolar planets that measures the changes in brightness of a background star as it is gravitationally lensed by a foreground star and its planet.
Occulter: An object or instrument that is used to block the light of an intended target.
Ohmic heating: Heating that occurs due to electric current flowing through a resistant medium.
Photochemical haze: A haze created from the interaction of atmosphere and stellar radiation (most commonly ultraviolet radiation).
Photochemistry: Chemical reactions caused by interaction with light. Most commonly this is used to describe interactions from the absorption of ultraviolet, visible, and infrared radiation.
Photoevaporation: The interaction of light with the molecules in a planet’s atmosphere, which results in the loss of atmosphere. High-energy photons can accelerate or heat atmospheric molecules, enabling them to reach escape speed and be stripped from the atmosphere.
Photoionization: The process in which a photon interacts with an atom or molecule and energizes the atom to the point that it ejects an electron.
Photolyze: To break down molecules with light.
Photometry: The precise measurement of an object’s electromagnetic radiation, usually summed over a small range of wavelengths at once.
Photosystem: The biochemical mechanism in planets in which chlorophyll absorbs light energy to use for photosynthesis.
Planetesimal: A rocky and/or icy body, of size from about 100 m to tens of kilometers, which was formed in the protoplanetary disk.
Polarimetry: The measurement and interpretation of the polarization of waves. This is an exoplanet hunting technique as well. Light reflected by a planet is polarized due to a number of phenomena, such as gaseous scattering, aerosol scatter, and reflection from the surface, and this light can be used as a direct-detection method for exoplanets.
Protoplanetary disk: A circumstellar disk of matter, including gas and dust, from which planets may eventually form or be in the process of forming.
Reducing atmosphere: An atmospheric condition in which free oxygen or other oxidizing gases are limited, preventing oxidation.
Rovibrational transitions: Molecular transitions involving both vibrational and rotational states, used in rotational-vibrational spectroscopy.
Semimajor axis: One-half of the major axis (which runs through both foci) of an ellipse. For most planetary orbits, this is comparable to the separation between the planet and its star.
Serpentization: A metamorphic process in which ultrabasic rocks react with water to create a variety of hydrous, magnesium-iron phyllosilicate minerals known collectively as serpentine. The process is endothermic and results in the liberation of hydrogen, methane, and hydrogen sulfide.
Snow line: The region of a disk that separates where a volatile, such as water or CO, is in the gas or solid phase. This location depends on temperature and pressure and is thus a line that changes as a function of radial and vertical location in the disk as well as over time. Snow lines are thought to be important for changing the composition of solid material and for changing the surface density of the disk.
Speckle noise: A form of granular “noise” caused by aberrations in an optical system that can look like a point source or planet.
Spectroscopy: The process of dissecting electromagnetic radiation from an object into its component wavelengths.
Starshade: A design for an external occulter for space-based telescopes. The shade is positioned between a telescope and a target star system. The shade is used to block the light coming from the star in order to enable the detection of light from an exoplanet.
Sun-like stars: Main sequence stars with internal structures similar to the Sun and with spectral types ranging from mid-to-late F through K. These stars are considered distinct from M dwarfs for the purposes of this report.
Telluric: Of or relating to Earth; telluric “lines” refers to features from elements and molecules in Earth’s atmosphere, and these may interfere with ground-based spectra of celestial objects.
Throughput: The ratio of incoming photons or flux from a point source, to those collected by the detector. Throughput can be degraded by elements such as a telescope central obscuration (support struts) and absorption by optical elements in the system.
Tidal locking: During long-term orbital interaction, when two co-orbiting bodies interact to drive the rotation rate of at least one of the bodies into a state where the orbital period is equivalent to, or an integer multiple of, the rotation rate.
Transit: The passage of an astronomical object across the face of one of larger angular diameter.
Volatile: A substance that vaporizes at a relatively low temperature.
Wavefront aberrations: The deviation or deformation of a wavefront from the desired formation, resulting in degradation of image quality. Aberrations disturb the convergence of energy to a point image, which is the ideal outcome.
Weathering: The breaking down of materials, through either physical or chemical forces. Physical processes include destruction through direct contact with abrasive forces, heat, or pressure. Chemical weathering is the breaking down of materials through chemical reactions, and examples include hydrolysis and oxidation.
