A planet’s rotation controls how heat from its star spreads across its surface.
Kane offers a warning: Astronomers observing a distant planet’s atmosphere can easily mistake the speed of its winds for the speed of the planet’s own rotation.
Anyone observing only the clouds would calculate a rotation rate nearly 60 times faster than the planet’s actual rate.
“This is what people originally thought about Venus,” Kane said.
They must infer a planet’s rotation from patterns moving through its atmosphere.
Venus spins 60 times slower than its clouds suggest. A new study warns hundreds of other planets could fool us the same way.
Astronomers are preparing to study hundreds of planets that resemble Venus. Before they can determine what turned Venus into a furnace while Earth remained habitable, they need one basic measurement for each planet: how fast it rotates.
That measurement is easy to get wrong, and Venus itself proves it.
A planet’s rotation controls how heat from its star spreads across its surface. It shapes wind patterns and, on a planet with oceans, influences how the water and atmosphere interact.
Astronomers who start with the wrong rotation rate could build an inaccurate climate model around it.
“People tend to overlook planetary rotation, but it is absolutely key to understanding a planet’s climate,” said Stephen Kane, a planetary astrophysicist at the University of California, Riverside.
Kane offers a warning: Astronomers observing a distant planet’s atmosphere can easily mistake the speed of its winds for the speed of the planet’s own rotation.
Venus spins slowly, but its clouds don’t
Venus takes 243 Earth days to rotate once on its axis. Its clouds, however, race around the planet in about four days.
Anyone observing only the clouds would calculate a rotation rate nearly 60 times faster than the planet’s actual rate.
“This is what people originally thought about Venus,” Kane said. “If you just look at the atmosphere of a planet like Venus, you’d think it rotates once every four days, and you’d be wrong by almost two orders of magnitude.”
Venus’s solid surface reveals that mistake because radar can penetrate the clouds and measure the planet’s rotation directly.
Planets around other stars offer no such shortcut. Astronomers cannot see their solid surfaces.
They must infer a planet’s rotation from patterns moving through its atmosphere. They then have to determine whether those patterns move with the surface beneath them.
One cloud layer can fake a spin
Kane built a model showing what a telescope would detect if astronomers mistook fast winds for a rapidly rotating planet.
A single layer of moving clouds can mimic a rotating planet closely enough that the two appear identical from one viewing angle.
The solution is to observe a planet at multiple wavelengths, including infrared light, which penetrates deeper into its atmosphere than visible light. On Venus, wind speeds decrease closer to the surface.
Astronomers could compare atmospheric movement at several depths and map that decrease in speed. They could then use the map to determine the rotation of the solid planet beneath the clouds.
PLATO will supply the targets
PLATO should soon give Kane plenty of targets on which to test his method. The European Space Agency mission will search for tiny dips in a star’s light that occur when a planet passes in front of it. The agency plans to launch PLATO in March 2027.
Unlike earlier planet-hunting missions that conducted quick sweeps, PLATO will observe some regions of the sky for years. This approach will allow it to detect planets with longer orbits that shorter observations might miss.
Many of its targets will also orbit bright, nearby stars. Instruments such as the James Webb Space Telescope could study these planets in detail after PLATO identifies them.
In a separate paper, Kane and three colleagues used PLATO’s expected findings to estimate how many Venus-like planets the mission will discover.
PLATO should find between 170 and 280 planets ranging from the size of Earth to twice its diameter. These planets will occupy a temperature zone where they could develop conditions resembling those on Venus. Of those planets, 40 to 80 should be close to Earth’s size.
“We’ve learned a lot about Venus itself, but there is still so much we don’t understand about its history,” said Emma Miles, a doctoral student at UC Riverside and co-author of the PLATO paper.
“Exo-Venus candidates, which are Venus-like planets in other solar systems, are going to play a huge role in filling in that knowledge gap,” she added.
PLATO adds crucial context
Miles said PLATO’s ability to provide context could prove as valuable as the number of planets it discovers.
“For example, PLATO will be able to measure the ages of the host stars, which will allow us to determine where a given exo-Venus sits in its own evolutionary history, and to further explore the boundaries of habitability,” she said.
That context could turn a collection of individual planets into something resembling an experiment. If most Venus-like planets rotate slowly like Venus, the pattern would suggest that rotation helps drive runaway greenhouse conditions.
If their rotation rates vary widely, however, the explanation for Venus’s fate probably lies elsewhere.
Rotation could reveal habitability
The same logic applies to planets that might support life rather than bake beneath thick clouds. Earth’s roughly 24-hour rotation helps distribute the sun’s energy around the globe. It also drives ocean and air currents that help stabilize the climate.
A planet rotating much more slowly or quickly could struggle to do the same. Measuring its rotation is therefore an important part of determining whether a distant world could support life.
Astronomers have not yet measured the true rotation rate of an exo-Venus. Kane’s method offers a way to make that measurement once PLATO identifies suitable targets and more powerful telescopes become available.
“Venus is a giant mystery,” Kane said. “To understand it, we need to see Venuses in other systems and see how they changed through time.”
“Rotation is a huge piece of that puzzle, and we need to be careful that what we think we’re measuring is really the rotation of the planet.”
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