Tidal Evolution of the Planetary System around HD 83443

Abstract

Two planets with an orbital period ratio of approximately 10:1 have been discovered around the star HD 83443. The inner and more massive planet, HD 83443b, has the smallest semimajor axis among all currently known exoplanets. Unlike other short-period exoplanets, it maintains a substantial orbital eccentricity, e_1 = 0.079 ± 0.008, in spite of efficient tidal damping. This is a consequence of its secular interactions with HD 83443_c, whose orbital eccentricity e_2 = 0.42 ± 0.06. Dissipation, associated with tides the star raises in the inner planet, removes energy but not angular momentum from its orbit, while secular interactions transfer angular momentum but not energy from the inner to the outer planet's orbit. The outward transfer of angular momentum decreases the tidal decay rate of the inner planet's orbital eccentricity while increasing that of the outer planet. The alignment of the apsides of the planets' orbits is another consequence of tidal and secular interactions. In this state the ratio of their orbital eccentricities, e_1/e_2, depends on the secular perturbations the planets exert on each other and on additional perturbations that enhance the inner planet's precession rate. Tidal and rotational distortions of the inner planet along with general relativity provide the most important of these extra precessional perturbations, each of which acts to reduce e_1/e_2. Provided the planets' orbits are coplanar, the observed eccentricity ratio uniquely relates sin i and C ≡ (k_2/k_(2J))(R_1/R_J)^5, where the tidal Love number, k_2, and radius, R_1, of the inner planet are scaled by their Jovian equivalents.