Massive Satellites of Close-In Gas Giant Exoplanets

We study the orbits, tidal heating and mass loss from satellites around close-in gas giant exoplanets. The focus is on large satellites which are potentially observable by their transit signature. We argue that even Earth-size satellites around hot Jupiters may be immune to destruction by orbital decay; detection of such a massive satellite would strongly constrain theories of tidal dissipation in gas giants, in a manner complementary to orbital circularization. The star’s gravity induces significant periodic eccentricity in the satellite’s orbit. The resulting tidal heating rates, per unit mass, are far in excess of Io’s and dominate radioactive heating out to planet orbital periods of months for reasonable satellite tidal $Q$. Inside planet orbital periods of about a week, tidal heating can completely melt the satellite. Lastly, we compute an upper limit to the satellite mass loss rate due to thermal evaporation from the surface, valid if the satellite’s atmosphere is thin and vapor pressure is negligible. Using this upper limit, we find that although rocky satellites around hot Jupiters with orbital periods less than a few days can be significantly evaporated in their lifetimes, detectable satellites suffer negligible mass loss at longer orbital periods.

💡 Research Summary

The paper presents a comprehensive theoretical investigation of large moons orbiting close‑in gas‑giant exoplanets, commonly referred to as hot Jupiters. Its primary goal is to assess whether Earth‑size or larger satellites could survive long enough to be detected via transit photometry, and what such detections would imply for tidal dissipation in the host planet.

Dynamical framework – The authors treat the star–planet–moon system as a hierarchical three‑body problem. They derive an analytic expression for the forced eccentricity imposed on the satellite by the stellar tide, e ≈ (M★/Mₚ)(aₚ/aₛ)², where M★ and Mₚ are the stellar and planetary masses, and aₚ and aₛ are the planet‑star and moon‑planet semi‑major axes, respectively. This forced eccentricity oscillates with the planet’s orbital period and is independent of any intrinsic satellite eccentricity, guaranteeing a persistent source of tidal deformation.

Tidal heating – Using the standard constant‑Q tidal model, the power per unit satellite mass is expressed as

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