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HarvardX: SPU30x Super-Earths and Life

Glossary

This page provides definitions of scientific terms from our course. If you want more detail, we encourage you to also look up these topics on sites such as Wikipedia, NASA.gov, NIH.gov, and other fine websites.

  • Abiogenesis - The process by which life, specifically the earliest living cells, arose from non-living materials such as amino acids. Also known as "abiotic synthesis." Contrast with spontaneous generation, an older theory that has been disproven.
  • Absorption Spectrum - The colors of light absorbed by an object when it is illuminated. The exact colors depend on the composition of the object, and are identical to those in the Emission Spectrum.
  • Accretion - The process by which planets and some moons or other astronomical objects form via accumulating matter.
  • Acidic - Having an excess of hydrogen ions. Acids are known for their sour taste and caustic nature, but most are very mild. Acids are often found in digestive processes, such as in the human stomach. Opposite of Alkaline. See also pH.
  • Adiabatic - Describes a process during which heat transfer does not occur.
  • Aerobic Metabolism - The use of oxygen by living things to obtain energy from chemical sources such as sugars, fats, and proteins.
  • Albedo - The proportion of light that shines onto a surface that is reflected.
  • Alkaline - Having the ability to remove hydrogen ions from other chemicals. Alkaline substances (or "bases") are known for their bitter taste and caustic nature, but most are very mild. Bases are often found in cleaning compounds. Opposite of Acidic. See also pH.
  • Amino Acid - An organic molecule from which larger, more complicated proteins are created. A particular subset of amino acids are found in all living creatures on Earth, and they are considered one of the fundamental building blocks for life.
  • Angular Momentum - It measures an object's tendency to continue to spin/rotate.
  • Angular Size - How large an object appears to be from a certain viewpoint. While an object's actual size (as measured by a ruler) does not change, a particular object might have an angular size of 70 degrees when viewed up close, or just a few degrees when viewed from far away.
  • Aphelion - The point in the orbit of an astronomical object when it is farthest from the Sun.
  • Archaea - Single-celled microorganisms which lack membrane-bound organelles. They reproduce asexually. Many thrive in extreme environments, thanks in part to the structure of their cell membrane. One of the three domains of the Darwinian Tree of Life with eukaryotes and prokaryotes.
  • Asteroid - A minor planet orbiting the Sun, especially one in the inner Solar System (closer to the Sun than Jupiter). They are solid objects, stony or metallic in composition.
  • Astronomical Unit - The distance from Earth to the Sun: 150 million kilometers, or 93 million miles.
  • Atmosphere - The layers of gases surrounding a planet. Planets with stronger gravity have thicker atmospheres. Atmospheric pressure and temperature determine the gaseous or liquid state of the atmosphere.
  • ATP Molecule - Adenosine triphosphate is a highly efficient energy-storage molecule within a cell. It is used to transfer energy during cellular metabolism.
  • Autocatalyses - An enzyme or other catalyst that is capable of facilitating its own formation.
  • Bacteria - Single-celled microorganisms which lack membrane-bound organelles. Most have the shape of spheres, rods or spirals. Bacteria are prokaryotes.
  • Banded Iron Formation - Distinctive units of sedimentary rock, predominantly from Proterozoic rocks, that contain alternating layers of iron-rich and silica materials.
  • Batholith - A large mass of intrusive igneous rock that forms when magma cools deep within the Earth's crust.
  • Big Bang - The current hypothesis for the origin of the universe via rapid expansion of matter from a state of extremely high density and temperature.
  • Binary Star System - A star system that consists of two stars orbiting around their common center of mass. These systems are very important in astrophysics because calculations of their orbits allow the masses of the two stars to be directly determined, in which in turn allows the estimation of other stellar parameters, such as radius and density. More Info in Course
  • Bio-molecule - Any molecule that is present in a living organism; especially used to describe larger molecules that are produced by living organisms.
  • Brown Dwarf - Substellar objects which do not have enough mass to maintain hydrogen fusion. They have masses between those of the largest planets and the smallest stars.
  • Buoyant Force - The upward force exerted by a fluid on a body placed in that fluid.
  • Cambrian Explosion - The rapid diversification of animals that occurred during the Cambrian period, approimately 541 million years ago; resulted in the appearance of most major animal phyla in the fossil record.
  • Captured Asteroid - An asteroid which has been captured by a planet's gravity but neither collides with the surface nor moves back into space. Instead, it orbits the planet. Many of our our own solar system's outer planets have moons that are probably captured asteroids.
  • Captured Comet - A comet which has been drawn by a planet's gravitational pull into orbit. See also Captured Asteroid.
  • Carbohydrates - Biological molecules that contains carbon, hydrogen, oxygen, and sometimes other elements; they often have the general formula (CH2O), explaining their name carbo (carbon) hydrate (water).
  • Carbon Star - A luminous red giant, whose atmosphere contains more carbon than oxygen.
  • Catalyst - A substance which speeds up or controls the rate of a chemical reaction, but survives the reaction unchanged.
  • Cell Division - This process occurs as part of the cell cycle whereby a parent cell is divided into two or more daughter cells with identical genetic material.
  • Cell Membrane - A protective layer shielding the inside of the cell from the outside world, but allows nutrients to enter and waste byproducts to leave.
  • Cell Replication - The process of cell division that creates two cells from one parent. The process assures that each new cell produced inherits all of the genetic material essential to survival and reproduction.
  • Cenozoic Era - It is an interval of geological time from roughly (66 Ma to present day).
  • Chemical Bond - An interaction between two or more atoms where electrons are shared and exchanged, resulting in the formation of a molecule.
  • Chemical Reaction - An interaction between two or more chemical species that results in their conversion into new chemical species.
  • Chondrite - Meteorites that contain small mineral granules because they have not been subject to melting or differentiation.
  • Coma - The cloud of dust and gas that surrounds a comet.
  • Comet - Small bodies composed of rock, ice, frozen gases and dust. Comets often have very large and highly elliptical orbits. When they pass into our solar system, the sun heats the frozen gases and releases some materials, creating the visible coma and tail.
  • Companion Star - The smaller of two stars in a binary star system.
  • Conduction - The transfer of thermal energy from a warmer substance to a colder one via contact.
  • Continental Plate - A geologic plate that makes up continental landmass; tend to be less dense than oceanic plates.
  • Continent Shelf - The region of underwater continental crust that surrounds a large landmass where the sea is relatively shallow (compared with the open ocean).
  • Convection - The transfer of heat due to the bulk movement of molecules within a fluid from warmer regions to colder regions and vice versa.
  • Convective Current - A current in a fluid that results from convective heat transfer.
  • Convergent Boundary - A boundary between two tectonic plates that are moving towards each other.
  • Convergent Evolution - The independent evolution of similar traits in species that are under a similar selective pressure.
  • Core - The central part of an object. Terrestrial planets, ice giants, gas giants, and stars all have cores that are different from their outer layers. For instance, terrestrial planets often have molten metallic cores, while their surfaces are colder and most often solid.
  • Cosmic Abundance - a measurement of how common a particular element is, and/or how much of the element is present in a given environment. Abundance is typically measured by comparison to all other elements. More Info in Course
  • Cosmos - A complex and orderly system, such as our Universe; an alternate name for the universe.
  • Covalent Bond - A bond that forms between atoms that share electrons.
  • Crater - a large bowl-shaped cavity left in the ground or on the surface of a planet or moon by the impact of a meteorite or other celestial body or by an explosion.
  • Crust - The solid, low-density surface layer of a planet that has undergone differentiation. More Info in Course
  • Cryovolcanism - An activity associated with volcanoes that are composed of ice.
  • Cyanobacteria - Photosynthetic prokaryotes.
  • Cytoplasm - Part of a cell which includes ribosomes, organelles (if present), and other inclusions, suspended in a combination of water, salt, and other organic molecules.
  • Differentiation - The process by which different constituents of a planetary body are separated into layers due to their physical or chemical behaviors.
  • Density - A physical property of matter defined as mass per unit volume.
  • Dipeptide - Two amino acids covalently attached to each other.
  • Direct Imaging - A method for detecting planets which involves taking an image of a star, and then filtering or blocking out the star from the light itself, enabling us to see its planets.
  • Divergent Boundary - A boundary between two tectonic plates that are moving away from each other.
  • DNA - Deoxyribonucleic acid, a long-chain molecule that contains the genetic instructions used for all living organisms and viruses. DNA consists of two polymer strands formed into a double helix. Between the strands are nucleotides composed of either guanine (G), adenine (A), thymine (T), or cytosine (C).
  • Doppler Effect - When an object moves toward or away from an observer, that observer will detect waves from that object as being higher frequency (toward) or lower frequency (away). The faster the motion, the stronger the effect will be. This applies to sound, light, and any other waves. The effect also applies if the observer is moving.
  • Doppler Spectroscopy - A method for detecting extrasolar planets. Doppler Spectroscopy looks for changes in the spectrum of a star, which indicate that it is moving back and forth. The more the star is moving, the more massive the planet must be that is orbiting it. Also known as the Wobble Method.
  • Dust - Tiny solid flecks of material. In astronomy, we often discuss interplanetary dust (found within a star system) or interstellar dust (found between the stars in a galaxy).
  • Dwarf Planet - Planet-like objects which are in orbit of a star and assume hydrostatic equilibrium, but have not cleared their orbits of other objects.
  • Element - A pure chemical substance. Atoms of different elements can be identified by the number of protons in their atomic nucleus. Molecules are made up of elements; elements are made up of protons, neutrons, and electrons.
  • Emission Spectroscopy - The measurement of light as it is emitted from a hot object. In the visible range, this means light coming from stars, but planets emit enough infrared light for this technique to be useful in that spectrum.
  • Emission Spectrum - The colors of light emitted by an object when it is heated or subjected to a very strong electrical current. The exact colors depend on the composition of the object, and are identical to those absorbed in the Absorption Spectrum.
  • Endosymbiosis - The process by which mitochondria and chloroplasts are thought to have arisen in eukaryotic cells; the mitochondria or chloroplast was engulfed by an ancestral eukaryote and began to "live" inside the eukaryotic cell, eventually becoming dependent on it for survival.
  • Endosymbiotic Theory - An evolutionary theory which seeks to explain the origin of eukaryotic cells from prokaryotes. According to this theory, several key organelles of eukaryotes originated as symbiosis between separate single-celled organisms. Also known as symbiogenesis.
  • Enzyme - A protein that can act as a catalyst and speed up a chemical reaction.
  • Erosion - The gradual destruction of an object due to wind, water, or other natural agents flowing over its surface.
  • Escape Velocity - The speed at which an object must move in order to escape from a planet forever, without any rockets or other propulsion at work.
  • Eukaryotes - A eukaryotic organism is made of complex cells that contain a nucleus (stores the DNA), cytoskeleton, and organelles (membrane bound structures that do work within the cell). One of the three domains of the Darwinian Tree of Life with archaea and prokaryotes.
  • Eukaryotic Cell - A cell from organisms that possess a nucleus, which contains their genetic material, and other intracellular compartments.
  • Evaporation - The physical transformation that occurs when a liquid becomes a gas.
  • Evolution - The gradual change in populations of living organisms. Evolution is responsible for transforming life on Earth from its primitive origins to the great diversity of life seen today.
  • Exoplanet - Short for "extrasolar planet," a planet that does not orbit the Sun and instead orbits a different star, stellar remnant, or brown dwarf. More than 1,800 exoplanets have been discovered. There are also rouge planets, which do not orbit any star and which tend to be considered separately, especially if they are gas giants. More Info in Course
  • Extrasolar Planet - see Exoplanet
  • Extremophile - A living organisms that is adapted to conditions that are"extreme" by human standards, such as very high or low temperature or a high level of salinity or radiation.
  • Fatty Acid - A hydrocarbon chain with a carboxyl (-COOH) group at one end.
  • Flood Basalt - Large formation of igneous rock that is created when basalt lava erupts from a giant volcano or a series of eruptsions covers a large area with basalt lava.
  • Foraminifera - Tiny organisms that produce external shells, often containing calcium carbonate (CaCO3).
  • Fossil Fuel - A fuel that was formed long ago from the remains of living organisms, for example coal or gas.
  • Frequency - The more often a wave oscillates, the higher its frequency is. Measured in Hertz. A wave that has an oscillation of, for example, 47 times per second, would have a frequency of 47 Hertz (Hz). Frequency is inversely related to wavelength, so higher frequencies mean shorter wavelengths. Frequency in light is directly related to energy: higher frequency means higher energy.
  • Galaxy - A large collection of stars that is gravitationally bound together, and is distant from other such collections. Galaxies are typically home to billions of stars, though they vary widely. Our home galaxy is called the Milky Way.
  • Gamma Ray - A very-high-energy, very-high-frequency form of light. Gamma rays are very damaging to living creatures.
  • Gas Giant - A giant planet composed mainly of hydrogen and helium. In our solar system Jupiter and Saturn are considered gas giants. The term gas giant was initially synonymous with the term "giant planet," but in the 1990s, Uranus and Neptune were reclassified as ice giants due to their composition of heavier volatile substances.
  • Gene - The DNA sequence that directs the production of a particular RNA or protein in a cell.
  • Genome - The genetic material for a cell or organism.
  • General Relativity - Our best current theory of gravity and acceleration, which describes everything from gravity on Earth and planets in orbit to the effects of black holes. General Relativity is Einstein's generalization of his Special Theory of Relativity, best-known for its description of time dilation at near-light-speed.
  • Genera - The biological classification grouping that comes above species; most genera contain multiple species.
  • Genetic Code - The 3-letter code by which DNA and RNA sequences can be translated into amino acid sequences.
  • Genetic Material - The information encoded in the DNA or RNA, found in either the nucleus or cytoplasm, which determines the inherited characteristics of an organism.
  • Glycolysis - The process by which cells break down the sugar glucose to obtain energy.
  • Gradient - A change in a quantity from one point to another. Some examples: A hillside has a height gradient. The end of a freshwater river as it runs into the salty ocean has a salt gradient. There is a gravity gradient that exists between the surface of our planet and deep space. This is a more general term than the mathematical "gradient" operation.
  • Gravitational Attraction - The attractive force between two objects due to their gravities.
  • Gravitational Force - The force that attracts one mass to another; the larger an object, the stronger the force "pulling" other objects toward it.
  • Gravitational Energy - A form of potential energy that can be released if an object moves from a higher position to a lower position.
  • Gravitational Lensing - The magnification or distortion (into arcs, rings, or multiple images) of an image caused by light bending through a gravitational field. Gravitational lensing was predicted by Einstein's general theory of relativity, and was one of the early pieces of evidence for the accuracy of that theory.
  • Gravity - A force exerted by all objects that have mass on all other objects that have mass. The larger the masses involved, the greater the force. Gravity obeys the Inverse Square Law and is described by General Relativity.
  • Great Oxidation Event - The sudden and rapid appearance of oxygen (O2) in Earth's atmosphere as a result of oxidative photosynthesis performed by cyanobacteria.
  • Greenhouse Effect - The trapping of energy from the sun in a planet's lower atmosphere due to differences in the permeability of the atmosphere to visible and infrared radiation; visible radiation can get in, but infrared radiation can't get out.
  • Habitable Planet - A planet that, by virtue of its distance from its star and other features, is thought to be capable of potentially supporting life.
  • Habitable Zone - The region around a star in which planets could potentially have surface temperatures at which liquid water could exist.
  • Hertzsprung-Russell Diagram - A two-dimensional plot of stallar temperature (color) versus brightness.
  • Heteroatoms - Any atom in a hydrocarbon that is not carbon or hydrogen; also used to indicated that non-carbon atoms have replaced carbon in the backbone of the molecular structure.
  • Hot Spot Volcanism - The formation of volcanoes at sites where hot plumes rise through the mantle and melt the rock near the surface.
  • Hydrocarbon - An organic compound made up of hydrogen and carbon, possibly with other elements as well.
  • Hydrogen Bond - A bond formed between a hydrogen atom with an electronegative atom, such as fluorine, oxygen or nitrogen. Hydrogen bonds are weaker than most other chemical bonds.
  • Hydrogen Fusion - The process by which two hydrogen nuclei fuse together to make a single, more massive nucleus, and release energy in the process. This process is also known as atomic fusion or nuclear fusion; hydrogen fusion is only the most common type.
  • Hydrophilic - Molecules that are attracted to and tend to be dissolved by water are called hydrophilic.
  • Hydrophobic - Molecules that are not attracted to a mass of water and tend not to interact with water are called hydrophobic.
  • Hydrostatic Equilibrium - A balanced state in which forces pulling in one direction are countered by forces pushing in the other direction. This equilibrium is what makes stars and planets approximately round.
  • Hydrothermal Vent - An opening in the sea floor through which hot, mineral-rich water emerges.
  • Hypothesis - One of multiple possible explanations of some aspect of the natural world. Hypotheses often have evidence to support them, but there may also be evidence or arguments against them.
  • Ice Giant - A giant planet whose composition includes substances heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, sulfur, methane, and ammonia. In our solar system there are two ice giants, Uranus and Neptune. Those two have become known as ice giants because their constituent compounds were ices when they were incorporated into the planets during their formation, either directly in the form of ices or trapped in water ice. See gas giants.
  • Igneous Rock - Rock that is formed through the solidification of magma or lava.
  • Image Processing - The analysis and alteration of images by computer. Direct Imaging techniques use image processing to remove the light of a star while leaving the light from its planets. More Info in Course
  • Infrared Light - Light with a wavelength that falls in the portion of the electromagnetic spectrum between radio waves and visible light. Infrared light is fairly low-energy and low-frequency. It is best-known for use in "heat-vision" or "thermal imaging" cameras that see the infrared light emitted by warm objects.
  • Insolation - The amount of solar radiation that reaches a given area.
  • Interstellar Cloud - An accumulation of gas, plasma, and dust that forms between the star systems in a galaxy.
  • Interstellar Medium - The gas and dust that fills the space between stars in a galaxy.
  • Invertebrate - Animals that lack a backbone such as insects, mollusks, etc.
  • Inverse Square Law - When a force obeys the Inverse Square Law, its strength drops off by the square of the distance involved. For instance, if two objects were moved from a certain separation distance to 5 times that distance, the force between them would decrease by a factor of 25.
  • Ion - An atom with extra or missing electrons, giving it a negative or positive electrical charge.
  • Isotope - A form of an element that has the same number of protons, but different numbers of neutrons.
  • Kinetic Energy - The energy of a body in motion.
  • Kuiper Belt - The comet-rich region of our solar system that spans the distance of about 30-100 astronomical units from the Sun. Kuiper belt comets have orbits that lie fairly close to the plane of planetary orbits and travel around the Sun in the same direction as the planets.
  • Light - A form of energy; specifically, light is electromagnetic radiation. Light includes a wide variety of phenomena, including radio waves, microwave radiation, infrared light, visible light, ultraviolet light, x-rays and gamma rays.
  • Light Curve - A graph that shows the intensity of light coming from an object versus time. Light curves are used in the Transit Method for detecting exoplanets. More Info in Course
  • Light Year - A distance; specifically, the distance that light travels in one year.
  • Limestone - A sedimentary rock composed largely of calcium carbonate.
  • Lipid - Molecules that contain hydrocarbons, which make up the building blocks of the structure and function of living cells. Examples of lipids include fats, oils, waxes, certain vitamins, hormones, and most of the non-protein membrane of cells.
  • Lithosphere - The outermost layer of Earth which includes the crust and the solid uppermost portion of the mantle.
  • Luminosity - It is a measure of the brightness and is defined as the total energy emitted per unit second by a stellar object.
  • Macroevolution - Large-scale evolutionary changes that lead to speciation.
  • Magma - Hot material below or within the Earth's crust from which lava and other igneous rock forms; may be fluid or semifluid.
  • Magnetic Field - A vector field that describes the magnetic influence of magnetized materials and electrical currents.
  • Main-Sequence stars - The continuous and distinctive band of stars that appear when stellar temperature (color) is plotted against stellar brightness, for example on a Hertzsprung-Russel Diagram.<\li>
  • Mantle - The semi-molten layer of rock in a terrestrial planet, between the solid crust and the molten core.
  • Mass - A measure of the amount of matter in an object. More massive objects require more force to accelerate them.
  • Mesozoic Era - It is an interval of geological time from roughly (252-72 Ma), and contains the Triassic (252-209 Ma), Jurassic (209-152 Ma), and Cretaceous periods (145-72 Ma).
  • Metabolism - All of chemical reactions that occur within a living system. The term "metabolize" is typically used to mean "chemically break down food so that its energy and nutrients can be used by an organism."
  • Meteorite - A solid piece fo debris from an extraterrestrial body.
  • Methanogenic Bacteria - Bacteria that produce methane as a byproduct of metabolism in the absence of oxygen.
  • Microevolution - Small-scale evolutionary changes, generally occurring within a population over relatively short time scales.
  • Microbe - A single-cell organism, so tiny that millions can fit into the eye of a needle. Microbes are the oldest form of life on Earth; their fossils date back more than 3.5 billion years. More info at Microbeworld
  • Microlensing - One of the methods of discovering exoplanets. Microlensing is a method which uses a special property whereby light from a distant object is bent by gravity as it travels; our telescopes therefore see the distant object at a different location from where it actually is. It may also appear brighter or dimmer, depending on whether we receive more or less of that light than normal, and often appears distorted. More Info in Course
  • Micrometeorite - An extraterrestrial particle that is collected on the Earth's surface. It used to be a part of micrometeoroid, which has survived entry through the Earth's atmosphere.
  • Microwave Radiation - Light with wavelengths in the range of micrometers to millimeters. Microwaves are generally considered to be a subset of the radio wave portion of the electromagnetic spectrum.
  • Mid-ocean Ridge - An underwater mountain range, generally found at divergent plate boundaries.
  • Milankovitch Cycle - Cyclical changes in Earth's climate due to changes in Earth's movements.
  • Minor Planet - An astronomical object that orbits around the Sun but is not considered a planet nor a comet. Some examples of a minor planet include dwarf planets, asteroids, trojans, centaurs, Kuiper belt objects, and other trans-Neptunian objects.
  • Molecule - A collection of any number of atoms that are chemically bonded to one another.
  • Momentum - The product of an object's mass and velocity.
  • Monomer - A single molecule that has the ability to chemically connect or bind to other molecules, forming a polymer.
  • Moon - A natural object that orbits a planet.
  • Morphology - The physical form of an organism.
  • Mutation - A change in a DNA sequence.
  • Natural Selection - The mechanism, first proposed by Charles Darwin, to explain the evolution of living things; in a population, individuals that are better adapted to their environment are more likely to survive and produce more offspring, so that over time the population comes to resemble its best-adapted members.
  • Noble Gas - The group of elements on the far right-hand side of the periodic table. These elements all have similar chemical properties - they are the most stable elements in the periodic table, and therefore the least reactive.
  • Node - A branch point on a phylogenetic tree that represents the last common ancestor of the branches that descend from it.
  • Nucleic Acid - Very large molecules that are essential to life. The backbone of a nucleic acids is made of alternating sugar and phosphate molecules bonded together in a long chain. Nucleic acids store information which details the specific structure of proteins that determine daily functions of our bodily systems. See also DNA and RNA.
  • Nuclear Fission Reaction - A reaction that releases energy when the nucleus of a heavy element splits, forming two lighter elements.
  • Nuclear Fusion Reaction - A reaction that releases energy when two nuclei of low atomic number fuse to form a new heavier element.
  • Nucleus - The core of an atom, containing positively charged protons and neutral neutrons. The same word is also used for the central part of a living cell, which contains the cell's DNA.
  • Nucleotide - The bulding blocks of DNA or RNA, consisting of a sugar, phosphate group(s), and nitrogenous base.
  • Observable Universe - The portion of the entire universe that, at least in principle, can be seen from Earth at present time. Light and other signals from these objects have had time to reach the Earth since the beginning of the cosmological expansion.
  • Ocean Crust - The portion of the crust that is formed of oceanic plates.
  • Ocean Plate - A geologic plate at the bottom of the ocean; tend to be denser than continental plates.
  • Orbit - The path followed by a celestial body because of gravity. An orbit may be bound (elliptical), in which case the celestial body moves around a central point over and over, or unbound (parabolic or hyperbolic), in which case it is deflected in its path but never returns.
  • Orbital Eccentricity - The degree to which an orbit deviates from a perfect circle.
  • Orbital Migration - A shift in a planet or satellite's orbital parameters.
  • Orbital Period - The length of time that it takes one astronomical object to complete an orbit around another astronomical object.
  • Organelle - A specialized subunit within a cell that has a specific function. Eukaryotic cells (like human cells or plant cells) have many organelles suspended in their cytoplasm; prokaryotic cells (like bacteria) have none.
  • Organic Chemistry - Chemistry that involves molecules containing carbon and hydrogen.
  • Organic Matter - Material that comes from living things.
  • Organic Molecule - Molecular structures that have carbon atoms; also known as organic compounds. Their importance is pronounced when it comes to forming life: each carbon atom can form strong, stable bonds with four other atoms at a time. The other elements commonly found in organic molecules are oxygen, hydrogen, nitrogen, sulfur and phosphorus. Carbon can also bond to other carbon atoms to form chains that are almost unlimited in length.
  • Outgassing - The release of a substance as a gas or vapor.
  • Oxidation - The loss of electrons by a molecule, atom, or ion.
  • Parallax - The apparent shifting of an object against the background that is caused by viewing it from different positions.
  • Perihelion - The point in the orbit of an astronomical object when it is closest to the Sun.
  • pH - A measure of the H+ ion concentration in solution; low pH (less than 7) means that there is a high concentration of H+ (the solution is acidic); high pH (greater than 7) means that there is a low concentration of H+, pH of 7 indicates a neutral solution (neither acidic nor basic)
  • Photometrics - The science of the measurement of light, often phrased in terms of its perceived brightness to the human eye. Used in measuring transits of a planet against its star.
  • Photon - A particle that represents a quantum of light or other form of electromagnetic radiation.
  • Photosynthesis - The process by which organisms, predominantly algae and green plants, capture light energy from the sun and convert it into chemical energy.
  • Phyla - The biological classification grouping that comes below kingdom.
  • Phylogenetic - The study of the evolutionary relationships between groups of organisms.
  • Pixel - An individual "picture element" on a screen, camera, or other device.
  • Planet - A moderately large object that orbits a star and shines primarily by reflecting light from its star. More precisely, according to a definition approved in 2006, a planet is an object that 1) orbits a star (but is itself neither a star nor a moon), 2) is massive enough for its own gravity to give it a nearly round shape, and 3) has cleared the neighborhood around it orbit. Objects that meet the first two criteria but not the third, including Ceres, Pluto, and Eris, are designated dwarf planets.
  • Planetesimals - Small bodies that exist during the process of planet formation; they can be found in protoplanetary discs and can aggregate to form planets.
  • Planetoid - See minor planet
  • Plate Tectonics - A theory that explains the structure and movement of the Earth's crust as resulting from an interaction between the rigid plates on the surface and the flowing mantle below.
  • Plutoid - A body that orbits beyond Neptune that is large enough to be round in shape. Also known as an"ice dwarf" or "Trans-Neptunian dwarf planet."
  • Polar Molecule - A molecule in which chemical bonding creates an uneven distribution of electrons, resulting in an electrical field in that area. Many molecules are polar; water is an example of a polar molecule.
  • Polymer - Multiple molecules all strung together to form really long chains, such as DNA or proteins, which are fundamental to biological structure and function. The individual unit of a polymer is a monomer.
  • Polypeptide - A string of several (often ~3-50) amino acids that form a polymer.
  • Precipitation - Removal of a substance from a solution via formation of a solid; rain, snow, sleet, or hail that falls to the ground.
  • Primary Transit - When a planet passes in front of its star as seen from Earth. See also transit, secondary transit.
  • Prokaryotes - A single-celled organism that lacks a membrane-bound nucleus, mitochondria, or other membrane-bound organelles. One of the three domains of the Darwinian Tree of Life with eukaryotes and archaea.
  • Proteins - Large and complex molecules that play many critical roles in living organisms. Proteins do most of the work in cells and are required for the structure, function, and regulation of the body's tissues and organs. They are made up of hundreds or thousands of smaller units called amino acids.
  • Protocell - A model primitive cell that could be similar to those that existed when life began.
  • Protoplanetary disk - A rotating disk of dense gas and dust that surrounds a newly formed star; as the materials collect into larger clumps they can form planets, planetesimals, and other astronomical objects.
  • Protostar - The contracting mass of gas that occurs early during star formation (before the nucleus of the star starts to form).
  • Quasar - The nucleus of an active galaxy, which produces exceptional amounts of energy. Short for"quasi-stellar object".
  • Radial Velocity Method - Another name for the Wobble Method and Doppler Spectroscopy.
  • Radiation - A catch-all term for any fast-moving particles that are emitted from an object. Most radiation is in the form of light (electromagnetic radiation) or subatomic particles (nuclear radiation). Some radiation is damaging to living cells, some is not.
  • Radio Wave - Light with very low energy and low frequency (and hence long wavelength). Radio waves pass through many materials.
  • Radioactive Decay - The emission of energy from an unstable atomic nucleus in the form of a particle (such as an alpha or beta particle) or ray (such as a gamma ray).
  • Radioactive Dating - The process by which specimens can be dated using the radioactive half-life of different elements that they contain.
  • Radiocarbon Dating - A technique that uses the ratio of isotopes present in a sample to determine its age.
  • Radioactive Element - An element that emits ionizing radiation or particles.
  • Radius - The distance from the center of a circle (or sphere) to its edge.
  • Red Dwarf - A small and relatively cool star, with a mass range between .075 to .5 solar masses, and a surface temperature of less than 4,000 K.
  • Ribonucleotides - The building blocks of RNA, including a ribose sugar, phosphate group(s), and nitrogenous base.
  • Ribosome - A large and complex molecular machine, found within all living cells, that serves as the primary site of biological protein synthesis (called translation). The ribosome links amino acids together in the order specified by Messenger RNA molecules.
  • Ribozyme - Ribonucleic acid enzyme (or RNA enzyme) catalyzes chemical reactions. They play a role in reactions such as RNA splicing, transfer RNA biosynthesis, and viral replication.
  • RNA - Ribonucleic acid, one of the three major biological macromolecules that are essential for all known forms of life (the others are DNA and proteins). Genetic information in a cell is passed on from DNA through RNA to proteins. RNA is like the DNA's"photocopy" of the cell - RNA carries the viral genetic information and plays an important role in regulating cellular processes from cell division, differentiation and growth to cell aging and death. More Info at the RNA Society site
  • RNA World Hypothesis - A theory which posits that RNA was the first major component of life that appeared on Earth, providing genetic material and acting as a catalyst for other reactions.
  • Satellite - An object in orbit around a larger object.
  • Secondary Transit - When a planet completely disappears behind its star as seen from Earth. See also transit, primary transit.
  • Sedimentary Rock - Rock that is formed by the deposition of mineral or organic particals into layers on the floor of oceans or other bodies of water.
  • Seismic Wave - A wave of energy that travels through the layers of the Earth.
  • Seismometer - A piece of equipment that is used to measure the motion of the ground caused by earthquakes, volcanic eruptions, explosions, etc.
  • Selective Pressure - A factor that influences an organism's survival and reproduction and thus shapes the evolution of a population or species.
  • Small Solar System Bodies (SSSB) - An object in the Solar System that is neither a planet, nor a dwarf planet, nor a satellite. Examples of SSSB include comets, minor planes, asteroids, trojans, centaurs, and trans-Neptunian objects.
  • Solar Eclipse - The most dramatic example of a transit, when the moon passes between the Sun and Earth. The sunlight we receive dims dramatically because the moon blocks our view of the Sun.
  • Solar System - A star (sometimes more than one star) and all the objects that orbit it. Also known as star system.
  • Solar Wind - A stream of plasma released from the upper atmosphere of the Sun; it consists of electrons and protons. The stream of particles varies in density, temperature and speed.
  • Solvent - A substance that dissolves a chemically different substance. In a salt water solution, for example, water is the solvent.
  • Space Probe - An unmanned spacecraft that travels through space to collect science information. Probes send data back to Earth for scientists to study. More Info at NASA.gov
  • Specific Heat - The amount of heat required to raise the temperature of some unit of a substance by some amount.
  • Spectrometry - A technique for measuring the distribution of light across the whole electromagnetic spectrum. Spectrometry usually focuses on a smaller range, such as the range from ultraviolet to infrared.
  • Speed of Light - 300,000,000 meters per second, which is 186,282 miles per second. This is a universal constant, a value that is the same everywhere across our universe.
  • Star - A collection of hydrogen (with traces of other elements) that is so large that is has collapsed under its own weight, causing hydrogen fusion in the core and making the whole object glow.
  • Star Cluster - A group of stars that is gravitationally bound together.
  • Stellar parallax - The apparent shift in the position of a nearby star (relative to distant objects) that occurs as we view the star from different positions in Earth's orbit of the Sun each year. A specialized case of parallax.
  • Stratosphere - The second major layer of the Earth's atmosphere, just above the trophosphere.
  • Stromatolite - Layers of chalky material that are produced by lime-secreting cyanobacteria mixed with trapped sediment to form a mound; often refers to fossils of such mounds from the Precambrian era.
  • Sugar - A small hydrocarbon molecule that living cells use to produce or store energy.
  • Subduction - A geological process that takes place at the convergent boundaries of tectonic plates where one plate moves under another; in some cases, one plate is forced into the mantle by the other, or it may sink into the mantle due to gravity.
  • Subduction Zone - The region where two plates come together and one sinks under the other.
  • Sun - When we say "sun" in this course, we typically mean our sun, the star closest to Earth. However, one could also look at the star closest to another planet and call that star the planet's "sun."
  • Super-Earth - A terrestrial planet that is larger than Earth, typically 2-10 times more massive.
  • Supercontinent - A large landmass that includes most or all of Earth's continents (such as Pangea).
  • Surface Tension - An attraction between the molecules in a liquid causes the molecules at the surface to exhibit tension, since they are "held" on the surface by the molecules in the bulk of the liquid.
  • Telescope - An instrument that magnifies distant objects using lenses and/or curved mirrors, or a device that can be used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation.
  • Terrestrial Planet - Rocky planets similar in overall composition to Earth. Terrestrial planets typically have a crust and mantle with large amounts of silicon and oxygen, and a molten iron core.
  • Theory - A widely accepted explanation of some aspect of the natural world, with substantial evidence to support it.
  • Thermodynamic Equilibrium - A state where all the components of a system have the same temperature and there are no flows of material.
  • Tidal Force - An apparent force that stretches a body towards the center of mass of another body due to a gradient in the gravitational field from the other body.
  • Tidal Lock - Synchronization of the rotational period and orbital period of one astronomical object around another such that the same face of the orbiting object is always towards the object that it is orbiting.
  • Torque - A twisting force that causes rotation.
  • Transform Boundary - A boundary between two tectonic plates that are sliding past each other.
  • Transit - When one astronomical object moves across another from our point of view, that is called a transit. Solar eclipses are good examples of transits.
  • Transmission Spectroscopy - The measurement of light after it passes through a gas, such as an interstellar cloud or the atmosphere of a planet.
  • Transit Method - A method for detecting extrasolar planets that looks for brief decreases in the light given off by the planet's home star. These changes may be indicative of a planet moving across the face of the star in a transit.
  • Triple Point - The temperature and pressure at which the solid, liquid, and vapor phases of a pure substance can coexist in equilibrium.
  • Ultraviolet Light - Light with a wavelength of 10-400 nm. Ultraviolet light can cause surface damage to living creatures and to individual cells.
  • Universe - All of the objects and space that are connected in some way to our own world. Another term for the cosmos.
  • Velocity - The combination of speed and direction of motion. Velocity can be stated as a speed in particular direction, such as 100 kilometers per hour due north or 40 meters per second upward.
  • Vesicle - A small compartment with a membrane composed of fatty acids or other lipids that can contain water and other substances.
  • Virus - A small infectious agent that replicates only inside the living cells of other organisms.
  • Visible Light - The portion of the electromagnetic spectrum that is visible to human beings. This light has moderate energies and frequencies, and is typically not damaging to living beings, though there are exceptions.
  • Volatile - A chemical or compound that is readily converted into its gaseous state (e.g., water, nitrogen).
  • Wavelength - The length of one full oscillation of a wave, from peak to peak. The larger a wave's wavelength is, the lower its frequency and energy.
  • Weathering - The process by which rocks, soil, and minerals are broken down through contact with air, water, etc.
  • White Dwarf - A very dense stellar core remnant with a mass comparable to that of the Sun and a volume comparable to that of the Earth.
  • Wobble Method - A method for detecting extrasolar planets. The Wobble Method, also known as Doppler Spectroscopy, looks for changes in the spectrum of a star which indicate that it is moving back and forth. The more the star is moving, the larger the planet must be that is orbiting that star.
  • X-ray - A high-energy, high-frequency form of light. X-rays can travel through some materials but not others. Living beings who are exposed to x-rays for long periods of time have an increased risk of DNA damage.