THE STATISTICS OF ALBEDO AND HEAT RECIRCULATION ON HOT EXOPLANETS
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If both the day-side and night-side effective temperatures of a planet can be measured, it is possible to estimate its Bond albedo, 0 < AB < 1, as well as its day–night heat redistribution efficiency, 0 < ε < 1. We attempt a statistical analysis of the albedo and redistribution efficiency for 24 transiting exoplanets that have at least one published secondary eclipse. For each planet, we show how to calculate a sub-stellar equilibrium temperature, T0, and associated uncertainty. We then use a simple model-independent technique to estimate a planet's effective temperature from planet/star flux ratios. We use thermal secondary eclipse measurements—those obtained at λ>0.8 μm—to estimate day-side effective temperatures, Td, and thermal phase variations—when available—to estimate night-side effective temperature. We strongly rule out the “null hypothesis” of a single AB and ε for all 24 planets. If we allow each planet to have different parameters, we find that low Bond albedos are favored (AB < 0.35 at 1σ confidence), which is an independent confirmation of the low albedos inferred from non-detections of reflected light. Our sample exhibits a wide variety of redistribution efficiencies. When normalized by T0, the day-side effective temperatures of the 24 planets describe a uni-modal distribution. The two biggest outliers are GJ 436b (abnormally hot) and HD 80606b (abnormally cool), and these are the only eccentric planets in our sample. The dimensionless quantity Td/T0 exhibits no trend with the presence or absence of stratospheric inversions. There is also no clear trend between Td/T0 and T0. That said, the six planets with the greatest sub-stellar equilibrium temperatures (T > 2400 K) have low ε, as opposed to the 18 cooler planets, which show a variety of recirculation efficiencies. This hints that the very hottest transiting giant planets are qualitatively different from the merely hot Jupiters. We propose an explanation of this trend based on how a planet's radiative and advective times scale with temperature: both timescales are expected to be shorter for hotter planets, but the temperature dependence of the radiative timescale is stronger, leading to decreased heat recirculation efficiency.
Subfields: Astronomy and Astrophysics, Instrumentation
View Paper: https://www.semanticscholar.org/paper/c7e146a8afa9dc513dbde4c711cf59f1df7be731
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Citations in the corpus
- HST hot-Jupiter transmission spectral survey: evidence for aerosols and lack of TiO in the atmosphere of WASP-12b (2013, background): “As many hot Jupiters have very low albedos (Cowan & Agol 2011), the modest albedo of WASP-12b prompted Cowan et al. (2012) to mention Rayleigh scattering and/or reflective clouds as a possibility.”
- ROTATION AND WINDS OF EXOPLANET HD 189733 b MEASURED WITH HIGH-DISPERSION TRANSMISSION SPECTROSCOPY (2015, background): “Observational constraints on the energy recirculation of hot-Jupiter atmospheres have been placed by broadband observations of secondary eclipses and thermal phase curves (see Cowan & Agol 2011, for a review).”
- SWEET-Cat: A catalogue of parameters for Stars With ExoplanETs - I. New atmospheric parameters and masses for 48 stars with planets (2013, methodology): “In addition, a combination of different techniques and methods is also giving the possibility to explorethe planetary properties, including the study of their atmospheres and internal structure (e.g. Valencia et al. 2010; Cowan & Agol 2011; Demory et al. 2012).”
- Carbon monoxide and water vapor in the atmosphere of the non-transiting exoplanet HD 179949 b (2014, background): “4 of Cowan & Agol (2011) in order to get an estimate of the dayside temperature of HD 179949 b, which was found to be Teq ' 1950 K.”
- 3.6 AND 4.5 μm PHASE CURVES AND EVIDENCE FOR NON-EQUILIBRIUM CHEMISTRY IN THE ATMOSPHERE OF EXTRASOLAR PLANET HD 189733b (2012, methodology): “…surveys seeking to estimate the relative amount of day-night circulation based on fits to the dayside emission alone, such as those presented in Cowan & Agol (2011), should obtain reasonable estimates for this quantity despite the limited data available and the relative simplicity of the…”
- ATMOSPHERIC HEAT REDISTRIBUTION ON HOT JUPITERS (2013, unknown): “This demonstrates that HAT- 2 Perez-Becker & Showman ison of secondary eclipse depths with predicted dayside equilibrium temperatures in a broad sample of∼24 systems performed by Cowan & Agol (2011).”
- THERMAL PHASE VARIATIONS OF WASP-12b: DEFYING PREDICTIONS (2011, unknown): “On the other hand, we find that the 4.5 μ m phase amplitude and offset, 16(4) ◦ E, are consistent with a Cowan & Agol (2011a) model with heat transport efficiency of (cid:10) ≡ τ rad /τ adv ≈ 0 .”
- WASP-121 b: a hot Jupiter close to tidal disruption transiting an active F star (2015, result): “This would agree with the trend noted by Cowan & Agol (2011) that hottest planets are less efficient at redistributing the incident stellar energy than colder planets, the explanation proposed for this trend being that the radiative timescale is shorter than the advective timescale for hotter…”