← Back to Cookie Club
🌍 Science & Nature

Earth Is Where We Live

June 14, 2026

Planet Earth

🌍💙

Earth is our home! We all live here. 🏠

Earth has big blue water and green land. 🌊🌳

Animals, people, and plants all share Earth together. 🐶🌸

What Is Earth?

Earth is a planet. It is a big, round ball that floats in space! If you could look at Earth from far away, it would look like a blue and green marble. The blue parts are water, and the green and brown parts are land. 🌍

Why Is Earth Special?

Earth is the only planet we know of where things can live. It has air we can breathe, water we can drink, and soil where food can grow. The Sun keeps us warm, but not too hot. It is just right! ☀️

What Lives on Earth?

So many things live here! Dogs, cats, fish, birds, bugs, trees, flowers, and YOU. Scientists think there are about 8 million different kinds of living things on Earth. That is a LOT of neighbors! 🐝🦜🐠

Land and Water

Most of Earth is covered in water. If you filled a big bucket with 10 cups of water to show Earth, about 7 cups would be ocean! Only 3 cups would be land. That is why Earth looks so blue from space. 💧

Our Planet in Space

Earth is the third planet from the Sun, sitting between Venus and Mars. It takes 365 and a quarter days for Earth to travel all the way around the Sun. That extra quarter-day is why we add February 29th every four years (called a leap year). Earth also spins like a top, making one full turn every 24 hours, which gives us day and night.

The Blue Planet

About 71% of Earth's surface is covered in water. Most of that water is in the oceans, which are salty. Only about 3% of all the water on Earth is fresh water, the kind we can drink. And most of that fresh water is locked up in ice at the North and South Poles!

If you could shrink Earth down to the size of a basketball, the layer of air we breathe (the atmosphere) would be thinner than a sheet of plastic wrap. We live in a very thin shell of air wrapped around a very big rock!

Inside the Earth

Earth is not solid all the way through. It has layers, like an onion. The outer layer we walk on is called the crust, and it is actually pretty thin compared to the rest. Underneath is the mantle, which is made of very hot rock that flows slowly like thick honey. At the very center is the core, where temperatures reach about 5,400°C. That is nearly as hot as the surface of the Sun!

Life Everywhere

Living things have been found in almost every corner of Earth. There are tiny creatures called tardigrades that survive in boiling hot springs, frozen ice, and even outer space. Fish live in the deepest ocean trenches, 11 kilometers below the surface. Bacteria grow inside rocks deep underground. Earth is bursting with life in places you would never expect.

One Planet, Many Places

Earth has deserts where it almost never rains, rainforests where it pours every day, frozen tundras near the poles, and coral reefs under the ocean that look like underwater cities. Each of these places is called a biome, and each one has its own special set of plants and animals that have adapted to live there.

Earth by the Numbers

Earth has a diameter of about 12,742 kilometers and a mass of roughly 5.97 × 10²⁴ kilograms. It orbits the Sun at an average distance of 149.6 million kilometers (1 astronomical unit), traveling at approximately 107,000 km/h. Despite this speed, we do not feel the motion because everything around us, the air, the ground, and all the objects, moves at the same velocity.

Earth's axial tilt of 23.5° is what produces seasons. When the Northern Hemisphere tilts toward the Sun, it receives more direct sunlight and experiences summer. Six months later, it tilts away and experiences winter. Without this tilt, every day would have roughly equal amounts of daylight everywhere on the planet.

Plate Tectonics: A Planet in Motion

Earth's crust is not one solid shell. It is broken into about 15 major tectonic plates that float on the semi-fluid mantle beneath them. These plates move between 1 and 15 centimeters per year. Where they collide, mountains form (the Himalayas grow about 5 mm taller each year as the Indian Plate pushes into the Eurasian Plate). Where they pull apart, magma rises to create new crust (the Mid-Atlantic Ridge adds about 2.5 cm of new ocean floor annually).

The Himalayas are growing at about 5 mm/year. Mount Everest is currently 8,849 meters tall. At this rate, in 1,000 years it would gain only 5 meters, or about the height of a one-story building. Erosion actually removes material at a similar rate, so the mountain stays roughly the same height over human timescales.

The Atmosphere: More Than Just Air

Earth's atmosphere is a layered blanket of gases: 78% nitrogen, 21% oxygen, and 1% other gases including argon, carbon dioxide, and water vapor. The atmosphere extends about 480 km above the surface, but 99% of its mass is concentrated in the lowest 30 km. The troposphere (0 to 12 km) is where all weather occurs. The stratosphere (12 to 50 km) contains the ozone layer, which absorbs 97 to 99% of the Sun's ultraviolet radiation.

Carbon dioxide makes up only about 0.04% of the atmosphere, but it plays an outsized role in regulating temperature through the greenhouse effect. Without any greenhouse gases, Earth's average surface temperature would be about -18°C instead of the current +15°C. The concern is that human activities have increased CO₂ concentrations from about 280 parts per million (pre-industrial) to over 420 ppm today, trapping additional heat.

The Water Cycle

Earth's water constantly moves between the oceans, atmosphere, land, and ice in a process called the hydrological cycle. Every day, the Sun evaporates about 1.2 trillion liters of water from the oceans. This water rises, cools, forms clouds, and falls as rain or snow. Rivers carry it back to the sea, and the cycle repeats. The water molecule you drink today may have fallen as rain during the age of dinosaurs, 66 million years ago. Water is endlessly recycled.

Earth's Magnetic Shield

Deep inside Earth, the liquid iron outer core flows and churns, generating a magnetic field that extends tens of thousands of kilometers into space. This magnetosphere acts as a shield, deflecting the solar wind (a stream of charged particles from the Sun) that would otherwise strip away the atmosphere. Mars likely lost most of its atmosphere because its core cooled and its magnetic field died billions of years ago. Earth's magnetic field is one reason our planet kept its air and water while Mars became a cold, dry desert.

Earth in the Habitable Zone

Earth orbits within the Sun's "habitable zone," the range of distances where liquid water can exist on a planet's surface. But location alone is insufficient. Venus orbits at the inner edge of this zone and suffered a runaway greenhouse effect, with surface temperatures now reaching 465°C. Mars sits near the outer edge but lacks the mass to retain a thick atmosphere. Earth's habitability results from a convergence of factors: sufficient mass (5.97 × 10²⁴ kg) to retain an atmosphere, a magnetic field to protect that atmosphere, plate tectonics to recycle carbon, and a large stabilizing moon that prevents chaotic axial wobble.

The Moon stabilizes Earth's axial tilt to within about 1° of 23.5°. Without the Moon, simulations suggest Earth's tilt could vary chaotically between 0° and 85° over millions of years, producing extreme climate swings that might make complex life impossible. Mars, which has no large moon, experiences axial tilt variations of 15° to 35° over timescales of 100,000 years.

The Carbon-Silicate Cycle: Earth's Thermostat

Over geological timescales (millions of years), Earth regulates its own temperature through the carbon-silicate cycle, sometimes called the "geological thermostat." When temperatures rise, chemical weathering of silicate rocks accelerates, drawing CO₂ out of the atmosphere and converting it to carbonate minerals (CaCO₃) that wash into the ocean and eventually become limestone. When temperatures drop, weathering slows, CO₂ accumulates from volcanic outgassing, and the greenhouse effect strengthens.

This negative feedback loop has kept Earth's surface temperature within a range compatible with liquid water for roughly 4 billion years, despite the Sun's luminosity increasing by about 30% over that period (the "faint young Sun paradox"). The early Sun was dimmer, yet geological evidence shows liquid water existed on Earth's surface as far back as 4.4 billion years ago, implying a stronger greenhouse effect in the past.

Biodiversity and the Sixth Extinction

Earth hosts an estimated 8.7 million eukaryotic species, of which roughly 1.2 million have been formally described. The fossil record preserves evidence of five mass extinction events:

Current extinction rates are estimated at 100 to 1,000 times the background rate observed in the fossil record. Whether this constitutes a "sixth mass extinction" is debated in terms of magnitude (we have not yet reached the 75% threshold that defines the Big Five), but the trajectory, the rate of loss, is unprecedented outside of asteroid impacts.

The Pale Blue Dot

On February 14, 1990, the Voyager 1 spacecraft, then 6 billion kilometers from Earth, turned its camera back and photographed our planet. Earth appeared as a pale blue dot, less than a single pixel in size, suspended in a sunbeam. Carl Sagan's reflection on that image remains one of the most quoted passages in science writing: "Everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives" on that tiny speck.

The scientific point behind the poetry is real. Among the thousands of exoplanets discovered since the 1990s, we have yet to find one that matches Earth's combination of liquid surface water, oxygen-rich atmosphere, plate tectonics, magnetic field, and large stabilizing moon. Earth may not be unique in the universe, but it is the only confirmed example of its kind. That distinction carries weight for how we think about stewardship.

The Unlikely Planet

Earth's habitability is the product of a series of contingencies, each necessary and none individually sufficient. The right distance from a G2V star. A mass large enough to retain an atmosphere but small enough to maintain a solid surface. Plate tectonics to regulate the carbon cycle. A magnetic dynamo powered by a convecting liquid iron core. A disproportionately large moon formed by a Mars-sized impactor (Theia) roughly 4.5 billion years ago, stabilizing axial obliquity and driving tidal rhythms. Remove any single element and the cascade toward habitability breaks.

The Faint Young Sun and the Thermostat

The Sun was approximately 30% less luminous 4 billion years ago, yet geological evidence (pillow basalts, sedimentary structures, zircon oxygen isotopes) indicates liquid water on Earth's surface by 4.4 Ga. The resolution of this paradox remains one of planetary science's foundational questions. The standard answer, a stronger CO₂ greenhouse, is necessary but probably insufficient alone. Methane (CH₄), produced abiotically and later by methanogenic archaea, likely contributed a significant greenhouse supplement before the Great Oxidation Event (~2.4 Ga) introduced free oxygen that would have photolyzed methane.

The carbon-silicate weathering feedback provides a plausible long-term thermostat, but its response time is on the order of 100,000 to 500,000 years. Shorter-term climate excursions, including "Snowball Earth" events (~717 to 635 Ma, when ice sheets may have reached equatorial latitudes), demonstrate that the thermostat can fail temporarily. Recovery from Snowball Earth likely required millions of years of volcanic CO₂ accumulation under an ice-sealed surface, followed by a rapid greenhouse rebound that produced some of the most extreme climate swings in the geological record.

Plate Tectonics as Biological Engine

Plate tectonics is not merely a geological curiosity. It is arguably the most important single process maintaining Earth's habitability. Beyond carbon cycling, it drives:

No other rocky planet in our solar system has active plate tectonics. Venus has a stagnant lid regime with episodic resurfacing. Mars's plates froze billions of years ago. Whether plate tectonics is common among rocky exoplanets remains unknown, but its apparent rarity in our solar system (1 of 4 rocky planets) suggests it may be a significant filter in the emergence of complex biospheres.

The Anthropocene Question

The proposal to designate a new geological epoch, the Anthropocene, reflecting the scale of human impact on Earth systems, was formally rejected by the International Union of Geological Sciences' Subcommission on Quaternary Stratigraphy in March 2024. The rejection was procedural (the proposed Global Boundary Stratotype Section and Point at Crawford Lake, Ontario, did not meet stratigraphic standards) rather than substantive. The evidence that human activity has altered Earth's major biogeochemical cycles is not in dispute.

Atmospheric CO₂ has risen from ~280 ppm (pre-industrial) to over 420 ppm, a level not seen in at least 3 million years (the Piacenzian stage of the Pliocene, when sea levels were 15 to 25 meters higher). Nitrogen fixation through the Haber-Bosch process now exceeds all natural terrestrial nitrogen fixation combined. Humans move more sediment annually (through construction, mining, agriculture) than all the world's rivers. The biomass of domesticated mammals (cattle, pigs, sheep) now exceeds wild mammal biomass by approximately 15:1.

Whether one calls this epoch the Anthropocene or not, Earth in the mid-21st century is, in measurable geochemical terms, a different planet from the one that existed in 1800. The question is not whether the changes are significant but whether the planetary systems that have maintained habitability for 4 billion years can absorb the rate of change we are imposing.

Sources

  1. Sagan, C. Pale Blue Dot: A Vision of the Human Future in Space. Random House (1994).
  2. Kasting, J. F. "Earth's Early Atmosphere." Science 259.5097 (1993): 920-926.
  3. Canup, R. M., and E. Asphaug. "Origin of the Moon in a Giant Impact near the End of the Earth's Formation." Nature 412 (2001): 708-712.
  4. Hoffman, P. F., et al. "Snowball Earth Climate Dynamics and Cryogenian Geology-Geobiology." Science Advances 3.11 (2017): e1600983.
  5. Bar-On, Y. M., R. Phillips, and R. Milo. "The Biomass Distribution on Earth." Proceedings of the National Academy of Sciences 115.25 (2018): 6506-6511.
  6. Lenton, T. M. Earth System Science: A Very Short Introduction. Oxford University Press (2016).
  7. Laskar, J., et al. "Stabilization of the Earth's Obliquity by the Moon." Nature 361 (1993): 615-617.
  8. Ceballos, G., et al. "Accelerated Modern Human-Induced Species Losses: Entering the Sixth Mass Extinction." Science Advances 1.5 (2015): e1400253.
  9. Zalasiewicz, J., et al. "The Anthropocene: A New Epoch of Geological Time?" Philosophical Transactions of the Royal Society A 369 (2011): 835-841.
  10. IPCC Sixth Assessment Report, Working Group I. "The Physical Science Basis" (2021).