Earth as the Only Habitable Planet & Related Topics – Lecture 3-5 Notes

Vocabulary flashcards summarizing key terms and definitions from Lecture 3–5, covering Earth’s habitability, atmospheric layers, Earth spheres, planetary science, and recent space missions.

Habitable Planet

A planet that can sustain life by possessing liquid water, energy, and the building blocks of life.

Goldilocks Zone (Habitable Zone)

The orbital region around a star where temperatures allow liquid water to exist—neither too hot nor too cold.

Liquid Water

The essential ingredient for life that can exist only within a narrow temperature range maintained on Earth.

Greenhouse Effect

Atmospheric process in which gases trap outgoing infrared radiation, keeping Earth’s average surface temperature at about 15 °C.

Atmosphere

The thin, life-supporting gaseous envelope surrounding Earth, composed mainly of nitrogen (78 %) and oxygen (21 %).

Variable Atmospheric Components

Water vapor, aerosols, and ozone—constituents that fluctuate and influence weather, climate, and UV protection.

Magnetosphere

Earth’s magnetic field generated by its liquid iron outer core that shields the planet from solar wind and radiation.

Red Dwarf

A small, cool star whose slow hydrogen burning gives it a lifespan of trillions of years—potentially allowing long-term habitability.

Blue Giant

A massive, hot star with a short lifespan, making nearby planets unlikely to host life.

Volcanic Activity

Tectonic process that releases gases into the atmosphere; higher activity can thicken a planet’s atmosphere (e.g., Venus).

Solar Wind

A stream of charged particles emitted by the Sun that can erode planetary atmospheres without magnetic protection.

Extremophile

An organism capable of living in extreme environmental conditions, such as early Earth’s harsh settings.

Hydrosphere

All water on Earth—including oceans, glaciers, rivers, lakes, and groundwater—covering about 71 % of the surface.

Groundwater

The largest accessible reservoir of fresh water for humans, stored in underground aquifers.

Water Cycle

Continuous movement of water through evaporation, condensation, precipitation, and runoff among Earth’s spheres.

Geosphere

Earth’s solid portion—from crust to core—comprising rocks, landforms, and internal layers.

Crust

Earth’s outermost solid layer; 35 km thick on average on land and ~7 km under oceans.

Continental Crust

Thicker, less dense crust forming Earth’s continents; up to 70 km thick beneath mountains.

Oceanic Crust

Thinner, denser crust beneath oceans, averaging about 7 km in thickness.

Mantle

Layer beneath the crust extending to ~2 900 km depth; divided into upper and lower mantle.

Lithosphere

Rigid layer comprising the crust plus the uppermost solid mantle—site of tectonic plates.

Asthenosphere

Plastic, partially molten zone of the upper mantle that allows plate motion.

Outer Core

Liquid iron-nickel layer whose convection generates Earth’s magnetic field.

Inner Core

Solid, dense iron core of Earth with a radius of ~1 216 km, solidified by high pressure.

Mohorovičić Discontinuity (Moho)

Boundary between Earth’s crust and mantle characterized by a seismic velocity change.

Gutenberg Discontinuity

Seismic boundary separating the mantle from the liquid outer core.

Biosphere

All living organisms and ecosystems on Earth, interacting with air, water, and land.

Anthroposphere

Part of Earth modified or built by humans, reflecting human impact on planetary systems.

Troposphere

Lowest atmospheric layer (0–11 km) where temperature decreases with altitude and weather occurs.

Stratosphere

Atmospheric layer from ~11 km to 48 km where temperature rises due to the ozone layer.

Mesosphere

Layer above the stratosphere where temperature drops to about –90 °C and meteors burn up.

Thermosphere

Uppermost atmospheric layer receiving intense solar radiation, causing temperature to rise with altitude; includes ionosphere.

Ionosphere

Electrified region within the thermosphere where auroras occur and radio waves reflect.

Aerosol

Tiny solid or liquid particles suspended in air that serve as condensation nuclei and affect radiation.

Ozone Layer

Stratospheric zone rich in O₃ molecules that absorbs harmful ultraviolet radiation.

Rosetta Mission

ESA spacecraft (2014) first to orbit Comet 67P/Churyumov-Gerasimenko, imaging its surface and coma formation.

New Horizons

NASA probe (2015) that performed a flyby of Pluto, revealing its nitrogen atmosphere and ice-rock interior.

OSIRIS-REx

NASA mission (2016) that collected samples from asteroid Bennu and returned them to Earth in 2023.

Parker Solar Probe

NASA mission (2018) first to fly through the Sun’s corona, studying solar wind origins.

InSight Lander

NASA Mars mission (2018) designed to investigate Martian interior structure and seismic activity.

Perseverance Rover

NASA rover (2020) exploring Mars for past microbial life, collecting samples, and testing oxygen production.

James Webb Space Telescope (JWST)

Infrared space observatory launched in 2021 with a 6.5 m mirror to study early galaxies and exoplanets.

International Astronomical Union (IAU)

Organization that formalized planet definitions in 2006, introducing ‘dwarf planet’ and ‘small solar system body’.

Planet (IAU Definition)

Celestial body orbiting the Sun, nearly round from self-gravity, and having cleared its orbital neighborhood.

Dwarf Planet

Solar-orbiting body that is nearly round but has not cleared its orbital path (e.g., Pluto).

Small Solar System Body

All other objects orbiting the Sun that are neither planets nor dwarf planets, such as asteroids and comets.

Solar Nebula Theory

Model stating the Solar System formed 4.6 billion years ago from a rotating cloud of gas and dust.

Accretion

Process by which dust and particles clump together under gravity to form larger bodies like planets.

Terrestrial Planet

Rocky planet formed near the Sun (Mercury, Venus, Earth, Mars) where high temperatures favored metal-silicate condensation.

Gas Giant (Jovian Planet)

Large, mostly gaseous planet (Jupiter, Saturn, Uranus, Neptune) formed in cooler outer regions allowing ices and gases to accumulate.

Extremophile

Microorganism that thrives in extreme environments such as high temperature, acidity, or salinity.

Cyanobacteria

Photosynthetic bacteria credited with producing Earth’s early oxygen atmosphere.

Aerospace Engineer

Professional who designs, constructs, and tests aircraft, spacecraft, and satellites; includes aeronautical and astronautical specializations.

Solar Wind

Continuous flow of charged particles from the Sun that can strip atmospheres lacking magnetic protection.

Kuiper Belt

Region beyond Neptune containing icy bodies and dwarf planets like Pluto and Haumea.

Comet Tail

Dust and gas stream created when solar radiation and wind act on a comet’s nucleus as it nears the Sun.

Asteroid Bennu

Near-Earth asteroid sampled by OSIRIS-REx to study Solar System formation and impact risk.

Aurora

Natural light display in polar skies caused by solar particles interacting with Earth’s magnetic field and atmosphere.

Greenhouse Gas

Atmospheric component (e.g., CO₂, H₂O vapor) that absorbs infrared radiation, contributing to warming.

Solar Corona

Sun’s outer plasma atmosphere explored by the Parker Solar Probe.

Habitat

Environment where an organism lives, influenced by temperature, water availability, and atmospheric conditions.