Exoplanet transit observations are now sensitive enough to detect when a planet is flattened slightly by its rotation. Using a new code tailor-made for the purpose, researchers have placed constraints on the rotation period of a cold, puffy exoplanet.
(Don’t) Assume a Spherical Planet
Though countless astronomy problem sets have asked students to consider planets to be spheres, not all planets are so perfect. Rapid rotation turns gaseous planets into slightly squished, or oblate, spheroids. Measurements of this subtle departure from a perfect sphere have the potential to reveal the rotation rates of distant exoplanets and the amount of angular momentum they carry, helping to disentangle how these planetary systems formed.
However, measuring this subtle signal is hard, and so far oblateness has been measured for only a handful of planets. Transiting planets can reveal themselves to be slightly squished only when crossing in front of the edge of their host star, requiring sensitive, high-time-cadence observations — and the right software to analyze the signal.
greenlantern Lights the Way
Examples of how the flux during a planetary transit differs between an oblate planet model and a spherical planet model. For all of the examples plotted, the oblate planet and its comparison spherical planet cover the same area of their host star mid-transit. The line color signifies the impact parameter. [Price et al. 2025]
After walking through the technical aspects of the model, Price’s team introduced their test subject, the sub-Neptune exoplanet HIP 41378f. HIP 41378f is notable for being the outermost of five known planets in its star system, for having an extremely low bulk density of just 0.09 gram per cubic centimeter, and for having a long, chilly, 542-day orbit around its host star.
Price’s team used their code to fit Kepler space telescope data of HIP 41378f and constrain the planet’s oblateness and, by extension, its rotation rate. They determined that the planet’s rotation period is at minimum 15.3 hours, making it a slower rotator than either Jupiter (10 hours) or Saturn (10.5 hours).
Joint probability distribution for the flattening and projected obliquity (axial tilt) of HIP 41378f. [Price et al. 2025]
Rotating Through the Possibilities
In addition to constraining the rotation rate, this work also showed that current observations of HIP 41378f are consistent with a broad range of axial tilts. What does this mean for how HIP 41378f formed? Given the planet’s mass — 12 Earth masses, give or take a few — a large axial tilt could arise in a variety of ways, including gravitational interactions between the planets in the system. However, with HIP 41378f orbiting quite far from its siblings, it’s not clear why it would be so tilted, and more data are needed to pin down HIP 41378f’s axial tilt.
Finally, Price’s team interpreted the results in the context of HIP 41378f’s possible ring system. Given the planet’s extremely low bulk density, it’s possible that a system of rings has artificially inflated the planet’s apparent radius. In this case, the measured oblateness would reflect the influence of the ring system and could not be used to constrain the planet’s rotation rate.
Citation
“A Long Spin Period for a Sub-Neptune-Mass Exoplanet,” Ellen M. Price et al 2025 ApJL 981 L7. doi:10.3847/2041-8213/adb42b