Step 6: Making It Habitable

So far you have a planet in a good orbit, with a core, a crust, an atmosphere and oceans. That is a remarkable achievement — but is it habitable? Astrobiologists use a short list of requirements for life as we know it, and then a longer list of features that help life last.

The basic requirements

• Liquid water. A solvent where chemistry can happen. Covered in step 5.
• Energy. Sunlight at the surface, or chemical energy from the planet's interior. On Earth's sea floor, whole ecosystems live around hydrothermal vents without any sunlight.
• The right elements. Life on Earth is built mainly from carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur — often abbreviated CHNOPS. These are common in the universe, but they need to be available at the surface.

Features that keep a planet habitable

Many planets may be habitable briefly. Earth has been habitable for around four billion years. That long run seems to depend on several features working together.

A magnetic shield

Earth's liquid iron core generates a magnetic field that deflects much of the solar wind and helps protect the atmosphere and surface. See magnetosphere. Mars lost its field early, and much of its atmosphere followed.

Plate tectonics

Moving plates recycle carbon, renew the surface and bring fresh nutrients up from the interior. They are a key part of the long-term climate thermostat described in step 5.

A large moon

Earth's Moon is unusually big compared with its planet. Its gravity helps keep Earth's axial tilt within a narrow range, which keeps seasons from swinging to extremes. Calculations suggest Mars, with only two tiny moons, has seen its tilt vary far more over millions of years. The Moon also drives ocean tides. Read more in What if the Moon disappeared?

A calm, long-lived star

Life needs time. Earth's first life appeared within the first billion years or so — some evidence points to around 3.5 to 3.7 billion years ago — but complex animals took about three billion years more. A star that burns out in a few hundred million years would not allow this. Stars like the Sun last about 10 billion years. See star types explained.

Good neighbours

Giant planets shape the delivery of water and the rate of impacts on inner planets. It is often said that Jupiter shields Earth from comets, but simulations show the picture is mixed: Jupiter deflects some objects away and throws others inward. Its exact role is still debated.

Is Earth special?

Some scientists argue that the combination of features that made Earth habitable is rare (the “Rare Earth” view). Others point out that we have only one example and the universe is huge. Space telescopes have now found thousands of planets around other stars, including many rocky planets in their stars' habitable zones. Whether any of them have the full package is one of the biggest open questions in science.

How astronomers check a planet from far away

We cannot visit exoplanets, but we can study their light. When a planet passes in front of its star, some starlight filters through its atmosphere, and the gases there leave fingerprints in the spectrum. The James Webb Space Telescope is now doing this for small planets. Signs that would make astronomers look closer include water vapour, carbon dioxide, methane and oxygen in combinations that are hard to explain without life.

Builder's checklist for step 6

• Liquid water, energy and the CHNOPS elements
• A magnetic field from a hot, liquid core
• Plate tectonics to recycle carbon and nutrients
• A stabilising moon (helpful, perhaps not essential)
• A long-lived, steady star
• Patience: billions of years

Your planet can now support life. The last challenge is keeping it alive in a dangerous universe: Step 7: Surviving the universe.