Exploring Exoplanet Dynamics with JWST: Tides, Rotation, Rings, and Moons

Abstract

Although nearly 6,000 exoplanets are currently known, in most cases our knowledge is limited to a handful of the planet's orbital characteristics and bulk properties such as radius and mass. The James Webb Space Telescope (JWST) can expand our knowledge not only by probing exoplanet atmospheres, but also by measuring additional orbital and physical properties of exoplanets, thanks to its superior light-gathering power and measurement precision. Here, we describe the potential of JWST to unveil dynamical phenomena that were previously beyond our reach, such as tidal distortion and inflation, rotational flattening, planetary rings, and moons.

Figures (3)

• Figure 1: The brightest stars with transiting planets. The histograms are cumulative distributions of the apparent Gaia magnitudes of stars with known transiting planets, for the Kepler survey (dark blue) and over the entire sky (light blue). One would expect the distribution for a complete all-sky catalog to resemble that of the Kepler survey shifted leftward by 4.3 mag (dotted blue line). For hot Jupiters (top panel), this is nearly the case, suggesting that many of the brightest hosts are already known. For planets with periods of months or longer, such as cold Jupiters (bottom panel), many brighter hosts surely exist but remain undiscovered. Because it is based on the Kepler survey, this comparison is restricted to Sun-like stars with effective temperatures 4000--6000 K and does not address the completeness of surveys of lower-mass stars that emit most of their radiation at infrared wavelengths.
• Figure 2: Detectability of various phenomena as a function of orbital period. We consider a fiducial system consisting of a Jupiter-like planet with $Q'=10^6$ orbiting a Sun-like star with $Q_{\star}' = 10^7$. Tidal orbital decay is fast ($\tau_{\mathrm{decay}} < 10$ Myr, equation \ref{['eq: tau_a']}) for very short-period planets and is observable as a gradual reduction in the time between transits. For planets with longer periods, the changing transit period is not detectable on human timescales, but is still astrophysically relevant if $\tau_{\mathrm{decay}} <$ Gyr. Tidal inflation requires the planet's orbit to be eccentric and close enough to the star for a significant tidal luminosity $L_{\mathrm{tide}}$ relative to the incident stellar power $L_{\mathrm{irr}}$. We used expressions from 2010AA...516A..64L for $L_{\mathrm{tide}}$ and the tidal circularization timescale $\tau_{\mathrm{circ}}$. In the eccentric regime, we assumed a fiducial value, $e = 0.2$. Oblateness and rings are only expected to be detectable for planets in wide enough orbits to avoid tidal spin-down. We used the expression for the tidal synchronization timescale $\tau_{\mathrm{sync}}$ from 1996Icar..122..166G, assuming an initial rotation rate equal to Jupiter's current rate. Moons are dynamically unstable for planets with very short orbital periods due to the planet's small Hill radius and the effects of tidal evolution. The approximate moon instability cutoff of $P\approx10$ days is based on 2021PASP..133i4401D.
• Figure 3: Illustrations of some phenomena observable in transit light curves. The host star is assumed to be identical to the Sun; the other relevant parameters are indicated in each panel. The limb darkening parameters are calculated for a wavelength of $3 \mathrm{\mu m}$ using the Exoplanet Characterization Toolkit Limb Darkening Calculator 2021zndo...4556063B. The illustrations are to scale unless otherwise noted. (a) A tidally-distorted short-period planet. Shown are the deviations between the light curve of the distorted planet (calculated with the ellc code 2016AA...591A.111M) and the best-fit light curve of a spherical planet. The planet's size is to scale, including at mid-transit and quadrature, but the orbit is scaled down by a factor of two. (b) An oblate planet with a tilted spin axis (blue). The red dotted circle has the same projected area as the oblate planet, for comparison. Shown are the deviations between the light curve of the oblate planet (calculated with the JoJo code 2025AJ....169...79L) and the best-fit light curve of a spherical planet. The planet's size is $2\times$ larger and the oblateness is $3\times$ larger than the values used to calculate the light curve. (c) A planet with a tilted ring system. Shown are the deviations between the light curve of the ringed planet (calculated with the pyPplusS code 2019MNRAS.490.1111R) and the best-fit light curve of a spherical ringless planet. (d) A transit of a Jupiter-sized planet and its super-Earth-sized moon.