Linking the interior structure of terrestrial exoplanets to their (tidal) observables

Tidal effects probe the interior of celestial bodies and are an important source of information on their thermal state and the prevailing deformation mechanisms. In the case of exoplanets, estimation of quantities related to tides might add important constraints on the interior structure of those worlds that would complement the existing measurements of masses and radii [e.g., 1]. In recent years, the fluid Love number h_f, which characterises the deformed figure of a rotating celestial body, has been measured for the gaseous exoplanet WASP-103b [2], and the rate of tidal dissipation has also been estimated for several extrasolar gas giants [e.g., 3]. Although not directly detectable today, future measurements might also assess the deformation of Earth-sized exoplanets. Moreover, the measurements of thermal emission light curves, accessible to JWST [4], might shed light on the actual spin states of low-mass exoplanets, which is another parameter affecting the long-term evolution and the habitability prospects of the extrasolar worlds. Detailed analysis of the light curves can also unveil global-scale volcanism that would be indicative of the magnitude of tidal dissipation [5].

Here, we discuss and illustrate the link between various aspects of the planet’s interior structure and a set of potential observables related to tides on close-in rocky exoplanets without atmosphere. Specifically, we focus on the role of different rheological models and their parameters and on the major features of the interior structure, such as liquid layers or low-viscosity zones. We address the stability of various spin-orbit resonances, surface tidal heat flux, the magnitude of the tidal Love numbers h₂ and k₂, and the present-day effect of tides on the orbital elements. Since the tidal deformation and the rate of energy dissipation in close-in rocky exoplanets also govern the secular orbital evolution, we further discuss the effect of changes in the interior structure, induced by variations in the thermal state, on the long-term orbital dynamics of tidally loaded exoplanets or moons [6].

 

Acknowledgement:

The work presented in this contribution has been supported by the Czech Science Foundation grant nr. 23-06513I.

 

References:

[1] Baumeister & Tosi (2023), doi:10.1051/0004-6361/202346216.

[2] Barros et al. (2022), doi:10.1051/0004-6361/202142196.

[3] Barker et al. (2024), doi:10.1093/mnras/stad3530.

[4] Zieba et al. (2023), doi:10.1038/s41586-023-06232-z.

[5] Selsis et al. (2013), doi: 10.1051/0004-6361/201321661.

[6] Walterová & Běhounková (2020), doi:10.3847/1538-4357/aba8a5.