Advancing Physics and Astronomy in Uganda: Max Planck Collaboration Delivers Landmark PhD at Kyambogo University

Juma Kamulali is the first astrophysics PhD graduate from Kyambogo University in Uganda. In collaboration with the partner group of the Max Planck Institute for Astrophysics (MPA), he conducted ground-breaking research that introduces a powerful new machine-learning tool, MAISTEP, which enables the precise determination of stellar properties from basic observational data. Applying this method to thousands of exoplanet-host stars, Kamulali discovered a clearer, deeper 'radius valley' between super-Earths and sub-Neptunes, shedding new light on planetary evolution and atmospheric loss.

Space missions such as Kepler and TESS have increased the number of known exoplanet-host stars, allowing more detailed studies of planet formation and evolution. However, inferring stellar parameters without asteroseismic data (i.e. information embedded in stellar oscillations) often introduces uncertainties. In his PhD thesis, Kamulali addressed this issue by developing MAISTEP (Machine-learning Algorithms for Inferring STEllar Parameters). This tool is based on four tree-based machine learning models that can determine the core parameters of exoplanet host stars with unprecedented precision using just three basic parameters: effective temperature, metallicity and luminosity. When benchmarked against Kepler asteroseismic data, MAISTEP achieved median uncertainties of just 2% in radius, 3% in mass and 18% in age.

When applied to 1,255 main-sequence exoplanet hosts from the SWEET-Cat catalogue, MAISTEP revealed a sharper and deeper 'radius valley' — a striking deficit of planets between 1.9 and 2 Earth radii — than had been recorded previously in the NASA Exoplanet Archive. The location of the valley correlates with orbital period, stellar mass and incident stellar flux. Crucially, it also evolves over time: in older systems, the valley becomes shallower and shifts towards larger radii while the super-Earth-to-sub-Neptune ratio increases. Juma Kamulali also used MAISTEP’s age precision to study giant planets, finding that hot Jupiters orbit stars that are, on average, younger than their cold-Jupiter counterparts, which suggests that tidal interactions may drive orbital decay over time.

Juma Kamulali is the first PhD graduate in Physics from Kyambogo University in Uganda since its foundation in 2003, and the first to specialise in astrophysics. He also contributes to outreach initiatives designed to encourage secondary school students to pursue STEM subjects (Science, Technology, Engineering and Mathematics).