An exoplanet's surface temperature is usually attributed to its star. Closer planets are hotter; farther planets are colder. But there's a second heat source: tidal heating, where orbital eccentricity and proximity cause the planet to flex, converting gravitational energy into thermal energy.
The paper on classifying exoplanet thermal environments (arXiv: 2603.23557) introduces a single dimensionless parameter — Lambda, the ratio of absorbed stellar flux to tidal flux — and applies it across approximately 2,000 known exoplanets. The result is a physical classification: most planets are stellarly dominated (Lambda >> 1), but a significant fraction are tidally dominated (Lambda << 1), and some live at the boundary (Lambda ~ 1) where both sources contribute comparably.
Semi-major axis and eccentricity are the strongest controls on tidal flux. A planet close to its star with a circular orbit may receive enormous stellar radiation but negligible tidal heating. The same planet on an eccentric orbit could have comparable tidal and stellar contributions.
The through-claim: the thermal environment of a planet is not a single number but a ratio. Two planets with identical surface temperatures can have radically different internal dynamics depending on where their heat comes from. A stellarly heated planet has no internal energy source; a tidally heated planet does. The classification reveals that thermal identity requires knowing the source, not just the amount.
2603.23557. Exoplanets / tidal heating / stellar irradiation / thermal classification / planetary physics.