Some Planets May Be Warmer at Night Than During the Day

Deepayan Banik and colleagues report that a planet keeping one face permanently toward its star can end up with a night side warmer than its day side, if the starlight it receives rises and falls as it orbits. Their paper was posted Sept. 29, 2026, to the arXiv preprint server and submitted to Monthly Notices of the Royal Astronomical Society; it has not yet been peer-reviewed.

Planets on stretched orbits, planets circling two stars and planets on polar orbits around unevenly bright stars all receive starlight that rises and falls. The authors say that case had not been studied in general dynamical terms before, and they present the preprint as the first such study. They also say where to look: among planets already observed, they count 11 whose brightness over an orbit should vary by at least 10% because of the changing starlight, with HAT-P-2b the largest of them.

Their model is a single shallow layer of atmosphere, pushed toward a temperature pattern that itself changes with time. In it the atmosphere behaves like a forced damped oscillator, the physics of a swing pushed rhythmically against friction, and the paper derives expressions for the average response, its size and how far it lags behind the starlight. Held to steady starlight, the model reproduces the fixed hot spot already expected on a tidally locked planet.

The effect peaks at resonance, when the rhythm of the changing starlight matches the time atmospheric waves need to cross the planet and friction is weak. The response then forms a single standing wave across the planet, the hot spot swings right around to the point facing directly away from the star, and simulated brightness curves invert in shape as the night side warms. Nothing here was observed: the results come from analytic theory and from simulations, linear and nonlinear.

Whether the hot spot leads or trails the point directly beneath the star depends on how fast the planet spins.