Earth

Background

The 4.5 billion-year-old Earth is the only known astronomical object to harbor life, giving rise to billions of species of stunning diversity, including ours, Homo sapiens. It has formed the backdrop of an estimated 110 billion human lives.

At 13.1 septillion pounds and 25,000 miles in circumference, the third planet from the sun long formed the horizon of all human experience and knowledge (watch overview).

Recent discoveries have revealed our home planet’s relative size and location in the universe: a pale blue dot within the Orion Spur, located 26,000 light-years from the center of the Milky Way Galaxy, one of 100,000 galaxies within the Laniakea Supercluster.

Formation

Early Earth is theorized to have formed alongside the other planets within a solar nebula, where a massive cloud of spinning, interstellar gas and dust contracted under its own gravity and flattened into a hot disk (watch visualization).

The core of the disk became dense with lighter elements like hydrogen, eventually heating up and triggering nuclear fusion, forming the sun. Solar wind pushed lighter elements farther out into the system, while heavier metals like iron gathered into increasingly larger masses known as planetismals in a process called accretion to form the Earth and other inner rocky planets.

As the protoplanet grew, heat from the colliding material and radioactive decay differentiated Earth’s heavier iron-rich core from its lighter rocky mantle, giving rise to Earth’s magnetic field and long-term stability. Various models suggest Earth’s formation took tens of millions of years.

Two billion years later, Earth changed dramatically when cyanobacteria, a microbe, evolved to generate energy from sunlight (i.e., photosynthesis) and release oxygen as a byproduct into the atmosphere during the Great Oxidation Event.

Structure and Composition

Earth is the densest planet in the solar system and the most massive of the four rocky terrestrials. Shaped into a sphere by gravity, Earth is flattened at its poles and bulges at its equator due to its roughly 1,000-mile-per-hour eastward spin (Jupiter spins 28 times faster).

By analyzing seismic waves, researchers theorize that a solid, 9,800-degree Fahrenheit inner core is surrounded by an outer core of liquid iron and nickel—common elements that consolidate into solids at high pressures.

Above the core, a slow-moving rocky mantle moves the crust's tectonic plates, causing volcanoes and earthquakes (see overview).

Earth’s spin combines with the core’s electrical conductivity and extreme heat to produce a magnetic field that protects its surface from damaging solar winds, cosmic rays, and deep space radiation. This so-called geodynamo process is expected to last for billions of years.

Surface and Climate

Situated within the solar system’s “Goldilocks zone,” Earth is the only planet with conditions able to sustain liquid surface water, key to the formation of life. Roughly 71% of its surface is water; the rest is land. An estimated 300 million planets in our galaxy are located in similar zones.

The Earth’s five-layer atmosphere traps solar energy and maintains an average global surface temperature of 59 degrees Fahrenheit. Roughly 21% is oxygen, crucial for respiration but highly flammable. Nitrogen (78%) dilutes the oxygen and prevents rapid combustion.

Seasons result from the Earth’s 23.4-degree tilt in relation to the orbital plane. Ice ages last millions of years and result from shifting climatic conditions—like ocean currents and the position of tectonic plates—that drop average temperatures by double digits.

We live amid the fifth major ice age, though we are in the middle of a warmer interglacial period that began 11,000 years ago.