Spin‐Orbit Evolution of Short‐Period Planets

Abstract

The negligible eccentricity of all extrasolar planets with periods less than 6 days can be accounted for by dissipation of tidal disturbances within their envelopes that are induced by their host stars. In the period range of 7-21 days, planets with circular orbits coexist with planets with eccentric orbits. These are referred to as the borderline planets. We propose that this discrepancy can be attributed to the variation in spin-down rates of young stars. In particular, prior to spin-down, dissipation of a planet's tidal disturbance within the envelope of a sufficiently rapidly spinning star can excite eccentricity growth and, for a more slowly spinning star, at least reduce the eccentricity-damping rate. In contrast, tidal dissipation within the envelope of a slowly spinning lowmass mature star can enhance the eccentricity-damping process. On the basis of these results, we suggest that short-period planets around relatively young stars may have a much larger dispersion in eccentricity than those around mature stars. We also suggest that because the rate of angular momentum loss from G and K dwarfs via stellar winds is much faster than the tidal transfer of angular momentum between themselves and their very short (3-4 days) period planets, they cannot establish a dynamical configuration in which the stellar and planetary spins are approximately parallel and synchronous with the orbital frequency. In principle, however, such configurations may be established for planets (around G and K dwarfs) with orbital periods of up to several weeks. In contrast to G and K dwarfs, the angular momentum loss due to stellar winds is much weaker in F dwarfs. It is therefore possible for synchronized short-period planets to exist around such stars. The planet around Tau Boo is one such example.

KOI-200 b and KOI-889 b: Two transiting exoplanets detected and characterized withKepler, SOPHIE, and HARPS-N

“…Borderline planets are in the period range 6-30 days that lie in the crossover between a regime where the tides are sufficient to explain circularization and a regime where tidal effects are negligible. The dispersion in eccentricity of these systems can be seen as the result of different spin-down rates of young stars, as argued by Dobbs-Dixon et al (2004). The case of KOI-889 b is particularly interesting since it is close to the envelope of the maximum observed eccentricity for systems in this period range.…”

Discovery and characterization of WASP-6b, an inflated sub-Jupiter mass planet transiting a solar-type star

“…Neglecting the possible decrease due stellar wind (Dobbs-Dixon et al 2004), L tot should be a conserved quantity during the whole tidal evolution of the system. Figure 6 shows the obtained evolution for a, e, H, L tot and the orbital and stellar rotation period from 2 Gyr ago to 5 Gyr in the future.…”

Evolution of angular-momentum-losing exoplanetary systems

“…Lanza et al 2011). In a more general approach, Dobbs-Dixon et al (2004) have considered how the evolution of the spin of the host star can affect the eccentricity of a planetary orbit, and propose that all main-sequence dwarfs attain a quasi-steady equilibrium state in which the host star's AML through the stellar wind is balanced by the tidal transfer of angular momentum from their planets. Due to insufficient data at the time, their theory remained conjectural.…”