Reassessing Exoplanet Light Curves with a Thermal Model

Abstract

We present a uniform assessment of existing near-infrared Spitzer Space Telescope observations of planet-bearing stars. Using a simple four-parameter blackbody thermal model, we analyze stars for which photometry in at least one of Spitzer's IRAC bands has been obtained over either the entirety or a significant fraction of the planetary orbit. Systems in this category comprise ten well-studied systems with Hot Jupiters on circular or near-circular orbits (HAT-P-7, HD 149026, HD 189733, HD 209458, WASP-12, WASP-14, WASP-18, WASP-19, WASP-33, and WASP-43), as well as three stars harboring planets on significantly eccentric orbits (GJ 436, HAT-P-2, and HD 80606). We find that our simple model, in almost all cases, accurately reproduces the minimum and maximum planetary emission, as well as the phase offsets of these extrema with respect to transits/secondary eclipses. For one notable exception, WASP-12 b, adding an additional parameter to account for its tidal distortion is not sufficient to reproduce its photometric features. Full-orbit photometry is available in multiple wavelengths for 10 planets. We find that the returned parameter values for independent fits to each band are largely in agreement. However, disagreements in night-side temperature suggest distinct atmospheric layers, each with their own characteristic minimum temperature. In addition, a diversity in albedos suggests variation in opacity of the photospheres. While previous works have pointed out trends in photometric features based on system properties, we cannot conclusively identify analogous trends for physical model parameters. To make the connection between full-phase data and physical models more robust, a higher signal-to-noise must come from both increased resolution and a careful treatment of instrumental systematics.

Statistical Characterization of Hot Jupiter Atmospheres Using Spitzer’s Secondary Eclipses

“…Rather than attempting to identify molecular absorptions in individual planets, we adopt a statistical approach wherein we look for trends in our total sample. Pioneering work of this type was reported by Triaud (2014a); Triaud et al (2014b); Beatty et al (2014Beatty et al ( , 2018, and also Kammer et al (2015), Adams & Laughlin (2018), and Wallack et al (2018). A statistical approach to transit (not eclipse) spectroscopy was elucidated by Sing et al (2016).…”

Discovery of Six Optical Phase Curves with K2

“…and an efficient heat transportation between day and night-side significantly weakens the phase curve signal. Another explanation could be that the planetary atmosphere at the depth the phase curve probes is rotating at a pseudo-synchronous rate (Adams & Laughlin 2018) washing out the signal as we phase fold the light curve. Note using EVEREST light curve for HATS-11b, which has comparable precision level as the K2SFF light curve, the secondary eclipse depth was detected at the level of 36±11 ppm, still a 3σ detection.…”

“…While infrared windows are often ideal for observing the thermal phase curve of planets as planetary emission often peak in such bandpass (e.g. Harrington et al 2006;Knutson et al 2007;Adams & Laughlin 2018), the limited number of space based infrared facilities has been the primary bottleneck for such studies. This is where space-based photometric missions such as Convection, Rotation, and planetary Transits (CoRoT ; Baglin et al 2002), and Kepler (Borucki et al 2010) have come to play a crucial role through optical phase curves.…”

Signatures of Obliquity in Thermal Phase Curves of Hot Jupiters

“…A tidally distorted planet can have a significantly aspherical shape. We first outlined a model of tidal asphericity in Appendix A of Adams & Laughlin (2018). Here we adopt that work's primary assumption to model the distorted shape as a prolate spheroid, with the long axis displaced clockwise in the orbital plane from the star-planet line by some lag angle λ ≡ cos −1 (r ·r ).…”

“…Bulk properties of the atmosphere such as the radiative response timescale can be directly inferred (e.g. Cowan et al 2012b;de Wit et al 2016;Adams & Laughlin 2018).…”