Venus

Venus is almost the same size as Earth but has a crushing carbon dioxide atmosphere and a surface hot enough to melt lead. A case study in how a planet build can go wrong.

Diameter: 12,104 km (95% of Earth)
Mass: 0.815 × Earth
Surface gravity: 0.91 × Earth
Distance from Sun: 0.72 AU (about 108 million km)
Year: 225 Earth days
Rotation: 243 Earth days, backwards
Moons: 0
Surface temperature: about 465 °C
Surface pressure: about 92 × Earth

The build: same blueprint, different result

On paper, Venus and Earth look like the same project. They are nearly identical in size, mass and density, and they formed in neighbouring parts of the solar system from similar materials. If you were building a habitable planet, Venus would seem an excellent start.

Instead, Venus is the hottest planet in the solar system — hotter than Mercury, which is closer to the Sun. The difference is all in the atmosphere.

What went wrong: a runaway greenhouse

Venus's air is about 96.5% carbon dioxide, so thick that standing on the surface would feel like being nearly a kilometre underwater. Its clouds are made of sulphuric acid droplets.

The likely story:

Venus receives almost twice as much sunlight as Earth.
Any early oceans warmed and evaporated faster. Water vapour traps heat, which caused more evaporation — a runaway greenhouse.
High in the atmosphere, sunlight split water molecules apart and the light hydrogen escaped into space. The water was lost for good.
Without liquid water, there was no way to pull carbon dioxide out of the air and lock it into rock. Volcanic carbon dioxide piled up.

Venus has much more deuterium (heavy hydrogen) relative to normal hydrogen than Earth does, which fits the idea that it once had far more water and lost it. Whether Venus ever had true oceans, and for how long, is still debated.

Spinning backwards

Venus rotates very slowly and in the opposite direction to most planets — the Sun would rise in the west. One rotation takes 243 Earth days, longer than its 225-day year. Because the rotation and the orbit go opposite ways, a full day–night cycle lasts about 117 Earth days. The cause may be a giant impact or the slow pull of the Sun's tides on its massive atmosphere.

A hidden, young surface

Thick clouds hide the ground from ordinary cameras. NASA's Magellan spacecraft mapped it with radar in the early 1990s, revealing volcanoes, lava plains and relatively few craters. That suggests much of the surface was resurfaced by lava within the last several hundred million years. Soviet Venera landers sent back the only photographs from the surface; none survived more than about two hours.

New missions — NASA's VERITAS and DAVINCI and ESA's EnVision — are planned to study Venus in the coming years.

Lessons for planet builders

Distance from the star matters: Venus sits just inside the inner edge of the Sun's habitable zone.
Water is what keeps carbon dioxide in check. Lose it and the climate spirals.
A planet's size can be perfect and the result still uninhabitable.