Step 4: Adding an Atmosphere

A bare rocky planet is a harsh place: no weather, no protection from radiation, and temperature swings of hundreds of degrees between day and night. An atmosphere changes all of that. It spreads heat around, shields the surface and, with the right gases, keeps it warm enough for liquid water.

The first atmosphere: borrowed from the disk

While the protoplanetary disk still has gas, a growing planet can pull some of it in. This primary atmosphere is mostly hydrogen and helium — the same mix as the star.

Giant planets keep theirs: Jupiter and Saturn are essentially huge primary atmospheres wrapped around heavy cores. Small rocky planets usually cannot. Hydrogen and helium are very light, and a young star's intense ultraviolet light heats them until they escape. Earth's primary atmosphere, if it had a significant one, was lost early.

The second atmosphere: breathed out by the planet

The air we have today came mostly from inside the planet and from material that fell onto it. When rocks melt, gases trapped in them are released — this is outgassing, and volcanoes still do it today. Volcanic gases are mostly water vapour, carbon dioxide and nitrogen, with smaller amounts of sulphur gases.

Impacts from asteroids and comets added more volatile material. Together, these built a secondary atmosphere. On the young Earth it was probably rich in carbon dioxide and nitrogen, with no free oxygen at all.

Three planets, three outcomes

Venus, Earth and Mars probably started with broadly similar ingredients. Look at how differently they turned out:

	Venus	Earth	Mars
Main gases	96.5% carbon dioxide, 3.5% nitrogen	78% nitrogen, 21% oxygen, ~0.9% argon	~95% carbon dioxide
Surface pressure	about 92 times Earth's	1 bar	less than 1% of Earth's
Average surface temperature	about 465 °C	about 15 °C	about −65 °C

Venus has too much atmosphere, Mars too little. Earth's is the exception — and understanding why is the heart of this step.

Keeping the air: atmospheric escape

Gas molecules are always moving. If they move fast enough, they escape into space. Whether a planet keeps its atmosphere depends on a few factors:

• Gravity. A more massive planet has a higher escape velocity. Earth's is about 11.2 km/s; Mars's is only about 5 km/s, which makes losing gas much easier.
• Temperature. Hotter gas moves faster. Planets close to their stars lose light gases more easily.
• Gas weight. Hydrogen escapes far more easily than heavier gases like nitrogen or carbon dioxide.
• The stellar wind. Charged particles streaming from the star can strip gas from the upper atmosphere. NASA's MAVEN spacecraft has measured this happening at Mars today and concluded that it removed much of Mars's early atmosphere over billions of years.

A magnetic field is often described as a shield against the stellar wind. It certainly helps, but scientists still debate how important it is overall: Venus has no global magnetic field yet keeps a massive atmosphere, mainly because it is large and its air is made of heavy carbon dioxide.

Where did the oxygen come from?

Almost none of the oxygen in Earth's air came from volcanoes. It was produced by life. Microbes called cyanobacteria began releasing oxygen through photosynthesis, and around 2.4 billion years ago oxygen started building up in the atmosphere — an event known as the Great Oxidation Event. It took most of Earth's history for oxygen to reach today's level of about 21%.

That is why astronomers searching for life on other planets get excited about oxygen in an atmosphere: on a planet like Earth, it is hard to keep that much around without something constantly producing it.

Builder's checklist for step 4

• Expect to lose any light hydrogen–helium atmosphere if your planet is small
• Build a secondary atmosphere from volcanic gases and impacts
• Make the planet massive enough — and far enough from its star — to hold on to it
• Don't let carbon dioxide pile up without a way to remove it (see Venus)
• For oxygen, you need life

The air is in. Next comes the ingredient every habitable planet needs: Step 5: Bringing water.