Geometric model explains the brightness temperature of planet 55 Cancri e

Star system: 55 Cancri A | Distance from the Sun: 41.1 light years

The thermal characteristics of 55 Cancri e, an ultra-short period rocky planet, are better understood by accounting for the planet’s three-dimensional geometry and its rotation. By calculating how sunlight hits the planet at various angles throughout its day, it is possible to model the temperature distribution across its surface more accurately. This approach clarifies why the planet exhibits a specific thermal imprint observed by telescope measurements.

The research utilizes an upgraded computer model, InstellCa-2.0, to determine the average amount of starlight received across different latitudes on the planet. This model factors in the varying zenith angle—the angle between the star and the point directly above the observer—throughout the planet's daily cycle. By treating the planet as a rotating body, the model estimates the surface heat distribution for worlds that may not be tidally locked.

Applying this geometric approach to 55 Cancri e reveals that a bare rocky surface, combined with an assumed Bond albedo—the fraction of light reflected by the planet—of 0.3, perfectly aligns with previously measured brightness temperatures of 1796 Kelvin. This agreement suggests that the thermal signatures previously debated in scientific literature can be explained without needing to assume a complex atmosphere.

Key findings
- New geometric model accounts for planetary rotation and 3D surface shape to calculate temperatures.
- The calculated brightness temperature of 1796 Kelvin aligns with precise JWST observations.
- Results support the conclusion that 55 Cancri e functions as a bare rocky world.
- Geometric irradiation effects reconcile long-standing debates regarding the planet's thermal profile.

Scientific study

The Role of Extended-Source Geometry and Diurnal Cycles on Exoplanetary Thermal Baselines: Reconciling the Brightness Temperature of 55 Cancri e (external link)