Earth

The only planet known to host life. What makes Earth work — oceans, plate tectonics, a magnetic field, a large moon and a stable climate — and how they fit together.

Diameter: 12,756 km
Mass: 5.97 × 10²⁴ kg
Surface gravity: 9.8 m/s²
Distance from Sun: 1 AU (about 149.6 million km)
Year: 365.25 days
Day: 24 hours
Moons: 1
Average temperature: about 15 °C
Age: about 4.54 billion years

The build that worked

Earth is the planet every other world gets compared with, because it is the only one where we know life exists. That is not down to one lucky feature. It is the result of several systems that support each other — the same systems laid out in the seven build steps.

The features that matter

A good orbit

Earth sits in the middle of the Sun's habitable zone on a nearly circular orbit. Its distance from the Sun changes by only about 3% over the year.

The right size

Earth is the largest rocky planet in the solar system. It is big enough to hold a substantial atmosphere and to stay hot inside for billions of years, which keeps volcanoes, plate tectonics and its magnetic field going.

Oceans and continents

About 71% of the surface is ocean. Water moves heat around the planet, drives weather and, through rain and rock weathering, controls carbon dioxide over long timescales.

Moving plates

Earth's crust is broken into plates that slide, collide and sink back into the mantle. This recycling keeps the carbon thermostat working and renews the surface. Earth is the only planet known to have plate tectonics today.

A magnetic field

Churning liquid iron in the outer core generates a magnetic field that deflects much of the solar wind. The visible sign of this shield at work is the aurora near the poles.

A big moon

The Moon, probably formed in a giant impact about 4.5 billion years ago, helps keep Earth's 23.4° axial tilt stable and drives the tides.

Life itself

Life has reshaped the planet. Photosynthesis filled the atmosphere with oxygen, which allowed the ozone layer to form and block harmful ultraviolet light. Earth's air, soils and even many of its rocks carry the marks of living things.

A planet that changed a lot

Earth has not always looked like this. It started as a molten ball, had an atmosphere with no free oxygen for about half its history, froze nearly solid at least twice and has been hit by large asteroids. Its continents have joined and split several times. What has stayed constant for around four billion years is liquid water at the surface.

Lessons for planet builders

Habitability comes from systems working together, not one ingredient.
An active interior is as important as a good orbit.
Life, once established, becomes part of how the planet works.