Hundreds of Venus-like planets may soon help scientists determine why some worlds turn into scorching “greenhouse hells,” while others remain suitable for life. But before answering this question, researchers need to determine how fast these planets rotate. The PLATO mission, scheduled for launch in 2027, may help astronomers solve this problem.
Rotation Rate and a Planet’s Climate
A planet’s rotation rate helps determine how energy from its star, such as the Sun, is distributed around the planet, shapes its weather patterns, and interacts with its atmosphere and oceans, if they exist. Without knowing the planet’s true rotation rate, scientists attempting to model the climate of an exoplanet are missing a crucial piece of information.
“People tend to overlook planetary rotation, but it is absolutely a key factor in understanding a planet’s climate,” said Stephen Kane, a planetary astrophysicist at the University of California, Riverside.
In his scientific paper, Kane warns that measuring the rotation of a distant planet is not as straightforward as it might seem. Astronomers may believe they are measuring the rotation of the planet itself when, in fact, they are measuring winds racing through its atmosphere.
Venus Illustrates the Risk
Venus provides an instructive example. The planet takes 243 Earth days to complete one rotation around its axis, but its upper atmosphere circles the planet in only four days, making it appear as though Venus rotates about 60 times faster than it actually does.
When it comes to planets orbiting other stars, astronomers generally cannot see a solid surface that would allow them to determine how quickly the planet itself is rotating. Instead, measurements may be based on changes observed in the atmosphere. Kane’s paper is a warning that these atmospheric motions do not necessarily reveal the rotation of the planet beneath them.
However, this problem can be addressed. Kane proposes observing the same planet at different wavelengths, including in the infrared, allowing scientists to probe deeper layers of its atmosphere.
On Venus, wind speeds decrease closer to the surface. By comparing measurements taken at different atmospheric depths, scientists were able to build a model of how wind speed changes and use it to estimate the rotation rate of the planet itself more accurately. phys.org reports on the research.
The Importance of the PLATO Mission for Detecting Venus-Like Planets
Knowing how to do this may soon become much more important. In a separate paper published recently, Kane and his colleagues predict that a new European Space Agency mission — PLATO — will discover hundreds of Venus-like planets, providing an unprecedented opportunity to determine how common worlds resembling our neighboring planet really are.
The PLATO mission, scheduled for launch in March 2027, will survey a large portion of the sky in search of planets passing in front of their stars. Unlike previous planet-hunting missions, it will observe some regions for many years, allowing it to detect planets that take longer to complete an orbit around their stars.
Equally important, many of the planets discovered by PLATO will orbit relatively bright stars. This will make them better targets for follow-up observations with instruments such as the James Webb Space Telescope, which is capable of studying their atmospheres.
Researchers predict that the PLATO mission will discover several hundred Venus-like planets. This will give scientists something they have never had before: a large group of similar planets that can be compared with Venus.
In addition, PLATO will be able to measure the ages of host stars, allowing astronomers to determine where a particular exo-Venus lies in its evolutionary history and to investigate the boundaries of the habitable zone in greater detail.
Rotation Is Key to Understanding Conditions on a Planet
Comparing Venus-like planets in other solar systems with Venus itself may help answer one of the most important questions about our neighboring planet: why did Venus, a world similar in size to Earth, develop an atmosphere with enormous pressure and temperatures hot enough to melt lead, while Earth remained habitable?
If scientists study 100 Venus-like worlds and find that most of them also rotate extremely slowly, this could indicate that slow rotation is an important factor in transforming planets into “greenhouse worlds.” If their rotation rates differ significantly, scientists will have reason to examine other explanations more closely.
The same principle applies to the search for potentially habitable planets. Earth’s approximately 24-hour rotation helps distribute solar energy around the planet and shapes atmospheric and ocean-circulation systems that regulate the climate. Therefore, accurately measuring the rotation rates of distant Earth-like planets could improve scientists’ ability to predict whether those worlds can maintain conditions suitable for life.
“Venus is a huge mystery,” Kane said. “To understand it, we need to study Venuses in other systems and track how they have changed over time. Rotation is an important part of that puzzle, and we need to be careful to make sure that what we are measuring really is the planet’s rotation.”
