Step 7: Surviving the Universe

Building a planet is only half the job. Space is not a quiet place, and a habitable world has to survive billions of years of hazards. Some arrive suddenly; others creep up so slowly that no single generation would notice.

Impacts

The giant impacts of planet formation tapered off long ago, but space rocks still arrive. Tiny ones burn up as shooting stars every night. Larger ones are rare but devastating.

• 66 million years ago, an asteroid roughly 10 km wide struck what is now Mexico's Yucatán Peninsula, creating the Chicxulub crater. The impact is linked to the mass extinction that ended the age of the non-avian dinosaurs.
• In 2013, a rock about 20 metres across exploded over Chelyabinsk, Russia. Its shock wave shattered windows across the city and injured more than a thousand people, mostly from flying glass.

Earth is the first planet able to defend itself. In 2022, NASA's DART spacecraft deliberately crashed into the small asteroid moon Dimorphos and shortened its orbit around its companion by about 32 minutes — the first demonstration that humans can change an asteroid's path.

Storms from the star

Stars are not calm light bulbs. The Sun releases solar flares — sudden bursts of radiation — and coronal mass ejections, billion-tonne clouds of magnetised gas.

When one hits Earth, the magnetosphere absorbs most of the blow, and we see auroras. A strong one can still cause trouble. The Carrington Event of 1859 set telegraph systems sparking; a storm in 1989 caused a nine-hour blackout in Quebec, Canada. Today satellites, GPS and power grids are all vulnerable, which is why space weather is monitored around the clock.

Planets around small red dwarf stars may face much stronger and more frequent flares. See planets around red dwarfs.

Climate tipping points

A planet's climate can lurch from one stable state to another.

• Runaway greenhouse: heat evaporates oceans, water vapour traps more heat, until the oceans are gone. Venus may have followed this path.
• Snowball: ice reflects sunlight, the planet cools, more ice forms. Earth went through this at least twice and recovered as volcanic carbon dioxide slowly built up again.

Earth's carbon–silicate cycle (see step 5) has pulled the planet back from both extremes over geological time. But it works over hundreds of thousands of years — it cannot undo rapid changes on human timescales.

Slow leaks

Every planet loses a little atmosphere to space each day. For Earth the loss is small. For a small planet without a magnetic field, close to an active star, it can add up to the loss of most of its air, as happened on Mars.

The ultimate deadline: the star itself

Stars change as they age. The Sun has been slowly brightening since it formed and gets about 10% brighter every billion years. Climate models suggest that in roughly one to one and a half billion years, the extra sunlight will push Earth towards a runaway greenhouse and the oceans will begin to boil away. In about five billion years the Sun will swell into a red giant, and Earth's surface will be molten — if the planet is not swallowed entirely.

So even a perfectly built planet has a lifespan, set by its star. Earth's habitable period will likely total somewhere around five billion years. We are past the halfway point, but there is still a very long time left.

A planet's health bar

You can think of a planet as having health that goes up and down:

• Damage: impacts, stellar storms, atmospheric loss, climate tipping points
• Repair: volcanoes replacing lost gases, the carbon thermostat, a magnetic field, and — on Earth — a civilisation that can watch the skies

A planet lasts as long as repair keeps up with damage, and as long as its star allows.

You have built a planet

That is the whole journey: from a cloud of dust, through orbit, core, air, water and life, to a world that can survive for billions of years. Every planet you see in the night sky — and every one discovered around other stars — went through some version of these seven steps.

Now try it yourself. Design your own planet →