The discovery of Venus-like conditions outside our solar system could reveal why some rocky worlds remain havens for life while others turn into scorching hothouse infernos. But before this mystery can be solved, astronomers must determine how fast these distant planets really rotate.
Slow rotations could cause the hellish conditions astronomers see on Venus, but new research suggests that the planet’s atmosphere may make the world appear to be rotating much faster than it actually is, complicating efforts to understand the climates of planets outside our solar system. Research by Stephen Kanea planetary astrophysicist at the University of California, Riverside, explores how high-speed atmospheric winds can be confused with the rotation of the rocky planet’s solid surface. This issue could become even more important when future observatories begin studying Earth- and Venus-sized worlds.
“People tend to overlook planetary rotation, but it is fundamental to understanding the planet’s climate,” Keane said.
Venus’ solid surface takes approximately 243 Earth days to complete one orbit, yet its atmosphere orbits the planet at cloud level in about four to five days. An observer just tracking those clouds can conclude that Venus is rotating about 60 times faster than it does. Scientists call this air movement Super rotationThe winds move around the planet much faster than the surface beneath them.
Kane’s study, accepted for publication in Astronomical magazineA simulation model is used to see if scientists can distinguish between surface rotation and atmospheric winds by studying light reflected by an exoplanet.
Reality may not be as it seems at first glance
Astronomers usually cannot see the surfaces of rocky exoplanets because the planets are small, distant, and difficult to separate from their host stars. Future direct imaging observatories may examine small changes in reflected light. As the planet rotates, one side moves toward the observer while the other side moves away, resulting in shifts in the wavelengths of light across Doppler effect. Researchers can use the resulting spectral pattern to estimate how fast the planet is rotating.
Kane’s model shows that atmospheric winds can produce a similar signal. Under certain conditions, scientists may think they are measuring the planet’s rotation when they are actually tracking clouds and gases. However, different spectral features can sample gases at different atmospheric pressures. Comparing these measurements can reveal how wind speed changes with height.
A rapidly rotating solid planet should produce a relatively consistent apparent speed across atmospheric levels. A slowly rotating planet with a super-rotating atmosphere would show faster motion near the cloud tops and slower motion near the surface. Venus follows this second pattern. Future instruments, including NASA’s proposed Habitable Worlds Observatory and high-dispersion spectrometers on large ground-based telescopes, could eventually use this approach to study rocky exoplanets.
Precisely measuring rotation may help explain why some rocky worlds remain benign while others turn into hell. The rotation does not add heat to the planet, but it affects the circulation of the atmosphere, weather systems and cloud formation. These changes determine how incoming stellar energy moves around the planet and how much it is reflected back into space.
On a slowly rotating water-rich planet, prolonged heating on the side facing its star can pull moist air upward and create a broad layer of bright clouds. These clouds may reflect enough starlight to protect the planet from severe global warming. Some climate models suggest that this effect could allow the slowly rotating planet to remain habitable while receiving much more energy from its star than Earth does.
This protection may have limits. If the planet receives too much stellar energy or loses enough water that it no longer maintains its reflective cloud cover, the balance may shift. Long periods of daylight can expose parts of the surface to extended heating, while variable circulation can affect how efficiently that heat is transferred.
These higher temperatures would, in turn, cause more surface water to evaporate. Since water vapor is a potent greenhouse gas, the added moisture will trap more heat, leading to more evaporation and warming. The planet could eventually reach a point where it cannot release enough heat to balance the energy it is absorbing from its star. A runaway greenhouse effect could vaporize the remaining oceans and rid the planet of all water.
However, slow rotation alone will not cause this transformation. Its importance comes from how it changes clouds and heat transfer, which could affect when or whether the planet crosses the runaway greenhouse threshold.
The flower shows the possible end result. Although similar in size and mass to Earth, it has a carbon dioxide-dominated atmosphere that produces a surface pressure about 93 times greater than Earth’s at sea level. Its average surface temperature is about 870 degrees Fahrenheit, which is hot enough to melt lead. In fact, scientists are still debating whether Venus once had oceans, and what caused its climate to change so dramatically.
