Big History Project
Glossary: Our Solar System & Earth
All of the following terms appear in this unit. The terms are arranged here in alphabetical order.
accretion — The process by which an object collects matter. For example, planet formation takes place as material orbiting a star gathers together through gravitational or electrostatic attraction, forming larger and larger bodies over time.
Archaean eon — The second eon in Earth’s history, a time from 3.8 to 2.5 billion years ago, during which the first living organisms appeared.
asteroid — Small rocky, icy, and metallic celestial bodies left over from the formation of the Solar System which can range from a few meters to several hundred kilometers in width.
atmosphere — The mixture of gases surrounding a planet. The composition of the Earth’s atmosphere has played a critical role in the development of life on Earth.
circadian rhythm — The “master clock” that controls the body’s coordinated timing system, telling it when to work, eat, and sleep. Based on the Sun’s 24-hour cycle, circadian rhythms speed up the body’s metabolism before sunrise to provide energy, and prepare it for sleep at night by lowering blood pressure and slowing the activity of the brain.
continental drift — The idea that the Earth’s continents move in relation to each other, so that continents currently separated by oceans were joined together in the past. The theory of plate tectonics explains why continental drift occurs.
convergent plate boundaries — Found where two plates move toward each other and collide. Depending on what types of plates are colliding, one may dive beneath the other to be recycled back into the mantle while the other rises up, or both may rise to form a new system of mountains.
core (of the Earth) — The dense center of the Earth, made mostly of iron, and some nickel. The movement of molten iron and nickel in the outer core generates the Earth’s magnetic field.
crust (of the Earth) — The solid outer layer of the Earth, consisting of moving plates both of the continental (lighter, made of granite) and oceanic (heavier, made of basalts) varieties.
differentiation (chemical) — A process early in the Earth’s history that produced different layers within the Earth’s interior, with denser metals sinking to form the Earth’s core, while progressively lighter materials formed the upper layers.
divergent plate boundaries — Found where two plates move away from each other. When both of the separating plates are oceanic plates, material from the mantle rises up and creates new seafloor.
Earth — The third planet from the Sun in our Solar System, home to many complex life forms and modern human society.
exoplanet — A planet outside of our Solar System.
gas giant — A type of planet that is composed primarily of gases rather than rock or other solid material. Examples from our own Solar System include Jupiter, Saturn, Uranus, and Neptune.
geology — The scientific study of the Earth, including its composition and history.
greenhouse effect — A process by which certain trace gases in the Earth’s atmosphere trap heat near the Earth’s surface and so keep the Earth’s climate warmer than it would be otherwise. The Earth emits some of the radiation it receives from the Sun back into space, but greenhouse gases trap some of this radiation before it can escape, thus warming up the climate on Earth, as in a greenhouse.
Hadean eon — The earliest period in Earth’s history (4.5 to 4.0 billion years ago), the “hellish era,” when the planet’s formation was still ongoing and was unsuited to life.
light spectrum — Electromagnetic radiation arranged in the order of its wavelength; a rainbow is a natural spectrum of visible light from the Sun. Human eyes can only perceive light within the range of the visible light spectrum. We cannot perceive infrared (slightly less energetic) or ultra- violet (slightly more energetic) light.
mantle (of the Earth) — The layer of the Earth between the core and the crust; it is mostly solid, but over long periods of time can flow like a thick syrup. Convection currents in the mantle drive plate tectonics.
orbit — The path of a body’s motion through space, often dictated by the gravitational pull of one or more larger bodies.
ozone — A molecule consisting of three oxygen atoms, in contrast to the more common form consisting of just two oxygen atoms. A thin layer of ozone high in the atmosphere shields the Earth’s surface from harmful forms of ultraviolet radiation.
Pangaea — The vast supercontinent formed more than 200 million years ago as plate movements joined the major continental plates together. It is probable that such supercontinents have formed periodically throughout Earth’s history. The existence of a single huge landmass probably reduced biodiversity.
planet —A spherical ball of rock, gas, or both, that’s in orbit around a star. Unlike a dwarf planet, a planet clears the area around its orbit of smaller objects through accretion.
planetesimal — An object, at least a kilometer or so across but much smaller than a planet, that forms through accretion during the early stages of planet formation. Planetesimals may combine with one another to form protoplanets and eventually planets.
plate tectonics — The idea that the Earth’s crust (together with the upper mantle) is broken up into separate plates that are in constant motion, explaining continental drift as well as the distribution of earthquakes, volcanoes, mountain ranges, and other rock structures, and many other features of the planet. Plate tectonics has been a central unifying theory in modern Earth sciences (geology) since the 1960s.
protoplanetary disk — A rotating disk of gas and dust grains surrounding a newly formed star or protostar. Over time, accretion within the disk tends to produce planets.
rocky (or terrestrial) planets — A type of planet that is composed primarily of rock and other solid material. Examples from our own Solar System include Mercury, Venus, Earth, and Mars.
seafloor spreading — A process in which new ocean floor is created as molten material from the Earth’s mantle rises and spreads out at the boundary between two plates.
Solar System — The Sun and the objects that orbit it; the area in space in which the Sun’s gravitational pull is the dominant force.
spectrum (light) — Electromagnetic radiation (light) arranged in the order of its wavelength; a rainbow is a natural spectrum of visible light from the Sun.
subduction zones — An area of convergence (collision) between two tectonic plates where the heavier plate sinks downward beneath the lighter one, which rises up. As the lighter plate rises, it forms volcanic mountains (if the rising plate has continental crust) or volcanic islands (if the plates are converging on the seafloor).
Sun — The star at the center of our Solar System.
tectonic plates — The huge rigid slabs of rock that the Earth’s crust (together with the upper mantle) is broken up into, which are in constant motion. Some plates carry continents, producing continental drift that dramatically changes the relative locations of continents over millions of years.
transform plate boundaries — Found where two plates grind past each other without either producing or destroying crust.