Venus should be “locked” with one side facing the sun. Here’s why it’s not

When two bodies in space are close enough to each other, their gravity acts as a brake, slowing them down until one’s rotation “locks” to match its orbit.

This tidal lock means that one side of the smaller body always faces the larger one – it’s the reason we only see one side of the Moon from Earth.

Earth’s evil twin, Venus, sits near the tidal lock. So close it could have been tidally locked to the Sun…but it’s not.

It takes Venus 225 days to orbit the Sun – and 243 days to complete one rotation. Moreover, other planets, it rotates in the opposite direction to its orbit around the Sun.

It’s a small difference, but significant. The only thing keeping Venus from fully locking is the planet’s thick, stormy atmosphere, which orbits Venus in just four days: 60 times faster than the planet itself.

This, according to astrophysicist Stephen Kane of the University of California, Riverside, means we need to pay much more attention to planetary atmospheres when studying not just Venus, but other planets orbiting distant stars.

“We think of the atmosphere as a thin, almost separate layer above a planet that has minimal interaction with the solid planet,” Kane explains.

“Venus’ powerful atmosphere teaches us that it’s a much more integrated part of the planet that affects absolutely everything, even the rotational speed of the planet.”

Venus’ atmosphere is quite wild, considered one of the key features that led Earth and Venus – so similar in so many other ways – to become such different worlds.

It’s a riddle planetary scientists would love to solve, because it means the difference between a lush, habitable world (Earth) and a toxic, acidic hellscape (Venus).

On Venus, the incredibly fast atmosphere – due to the phenomenon of superrotation – drives winds in excess of 400 kilometers per hour (about 250 miles per hour).

This superrotation causes the atmosphere to slide across the planetary surface – the result of which appears to be slowing the planet’s rotation, as well as fighting the Sun’s gravity grip and preventing tidal locking.

The slow, retrograde rotation means that a single day on Venus, from sunrise to sunset, lasts about 117 days. The thick, toxic atmosphere traps most of the Sun’s heat: only 3% of incident sunlight reaches the surface, meaning the rest is absorbed by the atmosphere.

The result is that Venus has the hottest surface of any body in the solar system except the Sun, averaging around 482 degrees Celsius (900 Fahrenheit).

“It’s an incredibly alien experience, very different from being on Earth,” Kane says. “Standing on the surface of Venus would be like standing on the bottom of a very hot ocean. You couldn’t breathe on it.”

Because the atmosphere traps solar energy, Venus only traps heat: an uncontrollable greenhouse effect. We don’t know what role tidal locking might play in contributing to this runaway greenhouse effect, but the study of Venus might provide some clues.

Most of the exoplanets we find are very close to their host stars; the tools we use to search for them are much better at finding nearby worlds. Many of these worlds would therefore be locked.

Since a planet with a runaway greenhouse effect is inhospitable to life as we know it, understanding how tidal locking affects habitability could help us identify habitable worlds orbiting other stars.

When looking for habitable exoplanets, one of the things astronomers look for are objects roughly the size of Earth. But just being Earth-sized probably won’t be enough.

Venus is more or less the size of Earth, and yet any terrestrial organism trying to live there could not survive. Simply using Earth as a model for exoplanets, even tidally locked ones, might not yield accurate results.

“Venus is our opportunity to get these models correct, so we can properly understand the surface environments of planets around other stars,” Kane said.

“We’re not doing a good job of considering that right now. We’re mostly using Earth-like models to interpret the properties of exoplanets. Venus is waving both arms saying, ‘Look over here!

Venus, he says, is a tool right here in our own solar system that we can use to try to understand the climates of extraterrestrial worlds.

The research has been published in natural astronomy.