The Astrophysics of Visible-light Orbital Phase Curves in the Space Age

Abstract

The field of visible-light continuous time series photometry is now at its golden age, manifested by the continuum of past (CoRoT, Kepler), present (K2), and future (TESS, PLATO) space-based surveys delivering high precision data with a long baseline for a large number of stars. The availability of the high quality data has enabled astrophysical studies not possible before, including for example detailed asteroseismic investigations and the study of the exoplanet census including small planets. This has also allowed to study the minute photometric variability following the orbital motion in stellar binaries and star-planet systems which is the subject of this review. We focus on systems with a main sequence primary and a low-mass secondary, from a small star to a massive planet. The orbital modulations are induced by a combination of gravitational and atmospheric processes, including the beaming effect, tidal ellipsoidal distortion, reflected light, and thermal emission. Therefore, the phase curve shape contains information about the companion's mass and atmospheric characteristics, making phase curves a useful astrophysical tool. For example, phase curves can be used to detect and measure the mass of short-period low-mass companions orbiting hot fast-rotating stars, out of reach of other detection methods. Another interesting application of phase curves is using the orbital phase modulations to look for non-transiting systems, which comprise the majority of stellar binary and star-planet systems. We discuss the science done with phase curves, the first results obtained so far, and the current difficulties and open questions related to this young and evolving subfield.

Transit Signatures of Inhomogeneous Clouds on Hot Jupiters: Insights from Microphysical Cloud Modeling

“…The presence of inhomogeneous clouds is thus likely common because all of the planets with robust two dimensional atmospheric information have signatures of inhomogeneous clouds (Shporer & Hu 2015). In observations of optical phase curves, however, there can be substantial non-atmospheric processes, such as doppler boosting, tidal ellipsoidal distortion, and planetary obliquity, that require approximations for this form of analysis (see review by Shporer 2017). This, coupled with the comparative difficulty of phase curve observations (Shporer 2017;Parmentier & Crossfield 2018) makes this method of probing inhomogeneous cloud cover difficult to generalize to the vast majority of hot Jupiters.…”

Planetary Candidates Observed by Kepler. VIII. A Fully Automated Catalog with Measured Completeness and Reliability Based on Data Release 25

“…To highlight a few examples, Barclay et al (2013) found evidence for a Moon-size terrestrial planet in a 13.3-day period orbit, Quintana et al (2014) found evidence of an Earth-size exoplanet in the habitable zone of the M dwarf Kepler-186, and Jenkins et al (2015) statistically validated a super-Earth in the habitable zone of a G-dwarf star. Additionally, for several massive planets Kepler data have enabled measurements of planetary mass and atmospheric properties by using the photometric variability along the entire orbit (Shporer et al 2011;Mazeh et al 2012;Shporer 2017). Kepler data have also revealed hundreds of compact, co-planar, multiplanet systems, e.g., the six planets around Kepler-11 (Lissauer et al 2011a), which collectively have told us a great deal about the architecture of planetary systems (Lissauer et al 2011b;Fabrycky et al 2014).…”

TESS Phase Curve of the Hot Jupiter WASP-19b

“…For the most massive planets, the periodic blue-and red-shifting of the host star's spectrum and the tidal bulge raised on the star's surface can yield additional phase curve terms that are detectable in longbaseline photometry. These contributions to the total observed photometric modulation are referred to as Doppler boosting and ellipsoidal distortion; see Shporer (2017) for a review of these processes.…”