Venus Highlights a Big Problem With How We Measure Alien Planets

A slowly rotating planet can become a hellscape, but initial looks can often be deciving.

Discovering Venus-like conditions beyond our Solar System could reveal why some rocky worlds remain havens for life while others become scorching greenhouse infernos. Before solving that mystery, however, astronomers must determine how quickly those distant planets are truly spinning.

Slower rotations can cause hellish conditions astronomers see on Venus, but new research suggests a planet’s atmosphere may make the world appear to rotate much faster than it really does, complicating efforts to understand the climates of planets beyond our solar system. Research by Stephen Kane, a planetary astrophysicist at the University of California, Riverside, explores how high-speed atmospheric winds could be mistaken for the rotation of a rocky planet’s solid surface. The issue could become more important as future observatories begin studying Earth- and Venus-sized worlds.

“People tend to overlook planetary rotation, but it is absolutely key to understanding a planet’s climate,” Kane said.

Venus’ solid surface takes approximately 243 Earth days to complete one rotation, yet its atmosphere circles the planet at cloud level in roughly four to five days. An observer tracking only those clouds could conclude that Venus rotates about 60 times faster than it does. Scientists call this atmospheric movement superrotation, in which winds travel around a planet much faster than the surface beneath them.

Kane’s study, accepted for publication in The Astronomical Journal, uses a simulation model to investigate whether scientists could distinguish surface rotation from atmospheric winds by studying light reflected by an exoplanet.

Reality might not be what it first seems

Astronomers cannot usually 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 instead examine small changes in reflected light. As a planet rotates, one side moves toward the observer while the other moves away, producing shifts in the light’s wavelengths through the Doppler effect. Researchers can use the resulting spectral pattern to estimate how quickly the planet appears to turn.

Kane’s model demonstrates that atmospheric winds can produce a similar signal. Under certain conditions, scientists could believe they are measuring the planet’s rotation when they are, in actuality, tracking clouds and gases. Different spectral features, however, can sample gases at different atmospheric pressures. Comparing those measurements could reveal how wind speed changes with altitude.

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A rapidly rotating solid planet should produce a relatively consistent apparent speed across atmospheric levels. A slowly rotating planet with a superrotating atmosphere would show faster movement near its cloud tops and slower movement closer to the surface. Venus follows this second pattern. Future instruments, including NASA’s proposed Habitable Worlds Observatory and high-dispersion spectrographs on large ground-based telescopes, could eventually use this approach to study rocky exoplanets.

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Measuring rotation accurately may help explain why some rocky worlds remain temperate while others become infernos. Rotation does not add heat to a planet, but it affects atmospheric circulation, weather systems and cloud formation. Those changes determine how incoming stellar energy moves around the planet and how much is reflected into space.

On a slowly rotating, water-rich planet, prolonged heating on the side facing its star can draw moist air upward and create a broad layer of bright clouds. These clouds may reflect enough starlight to protect the planet from extreme warming. Some climate models suggest that this effect could allow a slowly rotating planet to remain habitable while receiving considerably more energy from its star than Earth does.

That protection may have limits. If a planet receives too much stellar energy or loses enough water where it no longer maintains its reflective cloud cover, the balance could change. Long periods of daylight would expose parts of the surface to extended heating, while altered circulation could affect how efficiently that heat is transported.

These higher temperatures, in turn, would cause more surface water to evaporate. Because water vapor is a powerful greenhouse gas, the added moisture would trap more heat, leading to further evaporation and warming. The planet could eventually reach a point where it can no longer release enough heat to balance the energy it absorbs from its star. This runaway greenhouse effect could evaporate the remaining oceans and shed the planet of all water.

However, slow rotation alone would not cause this transformation. Its importance comes from how it changes clouds and heat transport, potentially influencing when—or whether—a planet crosses the runaway greenhouse threshold.

Venus shows the possible end result. Although similar to Earth in size and mass, it has a carbon dioxide-dominated atmosphere that produces surface pressure about 93 times greater than Earth’s at sea level. Its average surface temperature is approximately 870 degrees Fahrenheit, 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.