This paper asks whether three physical processes can explain how the number of close-in sub-Neptune planets changes as stars age. Recent observations suggest that sub-Neptune occurrence rates rise from young stars (about 10–100 million years) to intermediate ages (about 100 million–1 billion years) and then drop sharply for old stars (older than about 1 billion years). The authors build a self-consistent model to see if thermal cooling, atmospheric mass loss, and one particular form of orbital migration can produce that pattern.

The team created a “coupled evolution model” that links a planet’s interior, its atmosphere, and its orbit. For the interior they used pre-computed planet structures that vary core mass, envelope (hydrogen/helium) fraction, equilibrium temperature, and internal entropy. Thermal cooling makes young, hot planets contract over time. For atmospheric loss they used a photoevaporation-style model, where stellar radiation can strip a planet’s lightweight gas envelope and shrink its radius. For orbital change they focused exclusively on high-eccentricity migration (HEM) — a process where strong gravitational kicks make an orbit very elongated and tides then shrink and circularize it close to the star. Running many synthetic planets through this framework lets them track how planets move into or out of the “close-in sub-Neptune” category as they age.

At a qualitative level the coupled treatment matters because the processes affect one another. A larger planetary radius makes tidal dissipation stronger. Moving closer to the star increases the radiation that drives atmospheric escape. Losing atmosphere changes the planet’s radius and cooling rate. In the models, HEM can act quickly and populate the close-in sub-Neptune group early. The authors find that HEM can produce a sharp rise in close-in sub-Neptune occurrence within the first ~15 million years. After that initial phase the model shows only weak age evolution.