The Tidal Threshold

Some exoplanets are heated from the inside more than from the outside. The boundary is a single dimensionless number.

Every planet receives stellar irradiation. The absorbed flux sets the equilibrium temperature. But planets on eccentric orbits also experience tidal heating — gravitational flexing from the varying tidal force dissipates energy inside the planet, warming it from within. Earth's tidal heating is negligible compared to solar flux. Io's is not — Jupiter's tidal forces melt its interior.

The framework (arXiv:2603.23557) classifies ~2,000 exoplanets by the ratio Λ = F_absorbed / F_tidal. When Λ >> 1, the star dominates — familiar territory. When Λ << 1, tides dominate — the planet's thermal state is set by its orbit, not its star. At Λ ≈ 1, both contribute comparably, and neither can be neglected.

The dominant controls are semi-major axis and eccentricity. Close-in planets on eccentric orbits are tidally dominated: high tidal flux (from proximity and eccentricity) and high stellar flux, but the tidal scaling with orbital parameters is steeper. Far-out planets on circular orbits are irradiation-dominated: low tidal flux (from distance and low eccentricity), moderate stellar flux.

The finding: a significant fraction of the known exoplanet population falls in or near the Λ ≈ 1 regime. These planets cannot be characterized by stellar irradiation alone. Their surface temperatures, atmospheric dynamics, and habitability assessments require accounting for tidal heating — a thermal source that depends on orbital mechanics, not stellar properties.

For habitability, this matters. A planet too far from its star for liquid water might still have it if tidal heating makes up the deficit. The habitable zone broadens when you include the planet's own interior heat.

The heat comes from the star and the orbit. The boundary between them is Λ = 1.