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We present a model to study secularly and tidally evolving three-body systemscomposed by two low-mass planets orbiting a star, in the case where the bodiesrotation axes are always perpendicular to the orbital plane. The tidal theoryallows us to study the spin and orbit evolution of both stiff Earth-likeplanets and predominantly gaseous Neptune-like planets. The model is applied tostudy two recently-discovered exoplanetary systems containing potentiallyhabitable exoplanets (PHE): LHS-1140 b-c and K2-18 b-c. For the former system,we show that both LHS-1140 b and c must be in nearly-circular orbits. For K2-18b-c, the combined analysis of orbital evolution timescales with the currenteccentricity estimation of K2-18 b allows us to conclude that the inner planet(K2-18 c) must be a Neptune-like gaseous body. Only this would allow for theeccentricity of K2-18 b to be in the range of values estimated in recent works($e=0.20 \pm 0.08$), provided that the uniform viscosity coefficient of K2-18 bis greater than $2.4 \times 10^{19} \ \textrm{Pa s}$ (which is a valuecharacteristic of stiff bodies) and supposing that such system has an age ofsome Gyr.

Tidal evolution of exoplanetary systems hosting Potentially Habitable Exoplanets. The cases of LHS-1140 b-c and K2-18 b-c