Why did Earth become a comfortable home for life while its similar-sized neighbor Venus turned into a scorching world with a crushing atmosphere?
Scientists think part of the answer could involve something surprisingly simple: how fast a planet spins.
Planetary astrophysicist Stephen Kane of the University of California, Riverside says rotation is a crucial but sometimes overlooked part of understanding a planet’s climate.
A planet’s rotation helps determine how heat from its star is distributed across its surface. It also influences weather, atmospheric circulation and interactions between the atmosphere and oceans.
Earth, for example, rotates roughly once every 24 hours, helping shape the circulation systems that regulate our climate.
Venus is dramatically different. It takes about 243 Earth days to complete a single rotation, making it one of the slowest-spinning planets in the Solar System.
However, measuring rotation on distant planets could be surprisingly difficult, Kane explains in a new paper published in The Astronomical Journal.
The problem is that astronomers may accidentally measure the movement of a planet’s atmosphere rather than the planet itself.
Venus provides a striking example. Although the planet rotates once every 243 Earth days, its upper atmosphere races around the planet in only about four days. If astronomers could see only Venus’s atmosphere, they might conclude that the planet spins around 60 times faster than it actually does.
This creates a challenge when studying planets orbiting other stars. Astronomers generally cannot see their solid surfaces, so they may have to rely on changes in their atmospheres to estimate rotation.
Kane suggests that scientists could solve the problem by observing planets at several different wavelengths of light. Infrared observations, for example, can reveal deeper layers of an atmosphere. Comparing wind speeds at different depths could help researchers separate atmospheric movement from the rotation of the planet underneath.
This technique could soon become especially useful. The European Space Agency’s PLATO mission, scheduled to launch in March 2027, is expected to discover hundreds of planets resembling Venus.
PLATO will search for planets as they pass in front of their stars. Because the mission will observe some areas of the sky for years, it should be able to detect planets with longer orbits that previous surveys may have missed.
Many of these planets are also expected to orbit relatively bright stars, making them good candidates for further study with powerful observatories such as the James Webb Space Telescope.
Having hundreds of “exo-Venuses” to study could help scientists understand why Venus became so hostile.
If most Venus-like planets turn out to rotate extremely slowly, it could suggest that slow rotation plays an important role in creating extreme greenhouse conditions. If their rotation speeds vary widely, scientists may need to look for other explanations.
The same knowledge could improve the search for life elsewhere. By accurately measuring how Earth-like planets rotate, researchers could build better climate models and determine which distant worlds might have temperatures and conditions suitable for life.
Venus remains one of our Solar System’s biggest mysteries. Studying similar worlds around other stars may finally reveal why one Earth-sized planet became a furnace while another became our home.
