Watching a distant planet spin could conceal a trap: an apparent rotation may come from its atmosphere, while the surface spins much more slowly. Stephen R. Kane of the University of California, Riverside, shows that this problem could complicate the study of rocky worlds resembling Venus.
Venus provides the clearest example. Its surface takes about 243 Earth days to complete one rotation. Yet its upper atmosphere circles the planet in just a few days. The clouds are therefore carried much faster than the surface, a phenomenon known as superrotation.
View of Venus by Mariner 10
For an exoplanet, astronomers generally cannot see the surface directly. Instead, they analyze the light reflected by the planet. The lines present in this light shift slightly depending on the speed of the material reflecting them. This method can be used to estimate a rotation, but it actually measures the atmospheric layer from which the photons originate.
This is where the ambiguity appears.
The researcher built a model combining the rotation of the surface, winds circulating from east to west, and the illuminated portion of the planet. The result: with a single spectral line formed within a narrow pressure range, certain winds can exactly reproduce the signal expected from a rapidly rotating planet.
In other words, a planet comparable to Venus could provide different answers depending on the altitude observed. In the model using Venus-like winds, the apparent period ranges from several hundred days in the lower layers to about four or five days at cloud level.
One solution is to examine multiple wavelengths. They do not all pass through the atmosphere in the same way and can probe layers located at different pressures. If the apparent speed changes with altitude, astronomers then have an indication that winds dominate the signal. A single speed measurement would therefore not always be enough to determine the length of the day on the planet.
This distinction matters for understanding hot rocky planets. Rotation speed influences air circulation, heat distribution, and clouds. Confusing the rapid movement of the atmosphere with that of the surface could therefore lead to reconstructing a climate based on the wrong day-night rhythm.
The method will now need to be tested against the capabilities of future instruments. The study estimates in particular the spectral precision and signal quality needed to distinguish speeds between several layers. NASA’s future Habitable Worlds Observatory and very high-resolution spectrographs could search for this “false rotation” signature on worlds close to Venus.