Most known terrestrial planets orbit small stars with radii less than 60 per cent of that of the Sun
. Theoretical models predict that these planets are more vulnerable to atmospheric loss than their ...counterparts orbiting Sun-like stars
. To determine whether a thick atmosphere has survived on a small planet, one approach is to search for signatures of atmospheric heat redistribution in its thermal phase curve
. Previous phase curve observations of the super-Earth 55 Cancri e (1.9 Earth radii) showed that its peak brightness is offset from the substellar point (latitude and longitude of 0 degrees)-possibly indicative of atmospheric circulation
. Here we report a phase curve measurement for the smaller, cooler exoplanet LHS 3844b, a 1.3-Earth-radii world in an 11-hour orbit around the small nearby star LHS 3844. The observed phase variation is symmetric and has a large amplitude, implying a dayside brightness temperature of 1,040 ± 40 kelvin and a nightside temperature consistent with zero kelvin (at one standard deviation). Thick atmospheres with surface pressures above 10 bar are ruled out by the data (at three standard deviations), and less-massive atmospheres are susceptible to erosion by stellar wind. The data are well fitted by a bare-rock model with a low Bond albedo (lower than 0.2 at two standard deviations). These results support theoretical predictions that hot terrestrial planets orbiting small stars may not retain substantial atmospheres.
Satellite measurements and radiative calculations show that Earth’s outgoing longwave radiation (OLR) is an essentially linear function of surface temperature over a wide range of temperatures (≳60 ...K). Linearity implies that radiative forcing has the same impact in warmer as in colder climates and is thus of fundamental importance for understanding past and future climate change. Although the evidence for a nearly linear relation was first pointed out more than 50 y ago, it is still unclear why this relation is valid and when it breaks down. Here we present a simple semianalytical model that explains Earth’s linear OLR as an emergent property of an atmosphere whose greenhouse effect is dominated by a condensable gas. Linearity arises from a competition between the surface’s increasing thermal emission and the narrowing of spectral window regions with warming and breaks down at high temperatures once continuum absorption cuts off spectral windows. Our model provides a way of understanding the longwave contribution to Earth’s climate sensitivity and suggests that extrasolar planets with other condensable greenhouse gases could have climate dynamics similar to Earth’s.
ABSTRACT Next-generation space telescopes will observe the atmospheres of rocky planets orbiting nearby M-dwarfs. Understanding these observations will require well-developed theory in addition to ...numerical simulations. Here we present theoretical models for the temperature structure and atmospheric circulation of dry, tidally locked rocky exoplanets with gray radiative transfer and test them using a general circulation model (GCM). First, we develop a radiative-convective (RC) model that captures surface temperatures of slowly rotating and cool atmospheres. Second, we show that the atmospheric circulation acts as a global heat engine, which places strong constraints on large-scale wind speeds. Third, we develop an RC-subsiding model which extends our RC model to hot and thin atmospheres. We find that rocky planets develop large day-night temperature gradients at a ratio of wave-to-radiative timescales up to two orders of magnitude smaller than the value suggested by work on hot Jupiters. The small ratio is due to the heat engine inefficiency and asymmetry between updrafts and subsidence in convecting atmospheres. Fourth, we show, using GCM simulations, that rotation only has a strong effect on temperature structure if the atmosphere is hot or thin. Our models let us map out atmospheric scenarios for planets such as GJ 1132b, and show how thermal phase curves could constrain them. Measuring phase curves of short-period planets will require similar amounts of time on the James Webb Space Telescope as detecting molecules via transit spectroscopy, so future observations should pursue both techniques.
Because of their intense incident stellar irradiation and likely tidally locked spin states, hot Jupiters are expected to have wind speeds that approach or exceed the speed of sound. In this work, we ...develop a theory to explain the magnitude of these winds. We model hot Jupiters as planetary heat engines and show that hot Jupiters are always less efficient than an ideal Carnot engine. Next, we demonstrate that our predicted wind speeds match those from three-dimensional numerical simulations over a broad range of parameters. Finally, we use our theory to evaluate how well different drag mechanisms can match the wind speeds observed with Doppler spectroscopy for HD 189733b and HD 209458b. We find that magnetic drag is potentially too weak to match the observations for HD 189733b, but is compatible with the observations for HD 209458b. In contrast, shear instabilities and/or shocks are compatible with both observations. Furthermore, the two mechanisms predict different wind speed trends for hotter and colder planets than currently observed. As a result, we propose that a wider range of Doppler observations could reveal multiple drag mechanisms at play across different hot Jupiters.
The effect of ocean heat uptake (OHU) on transient global warming is studied in a multimodel framework. Simple heat sinks are prescribed in shallow aquaplanet ocean mixed layers underlying ...atmospheric general circulation models independently and combined with CO2 forcing. Sinks are localized to either tropical or high latitudes, representing distinct modes of OHU found in coupled simulations. Tropical OHU produces modest cooling at all latitudes, offsetting only a fraction of CO2 warming. High‐latitude OHU produces three times more global mean cooling in a strongly polar‐amplified pattern. Global sensitivities in each scenario are set primarily by large differences in local shortwave cloud feedbacks, robust across models. Differences in atmospheric energy transport set the pattern of temperature change. Results imply that global and regional warming rates depend sensitively on regional ocean processes setting the OHU pattern, and that equilibrium climate sensitivity cannot be reliably estimated from transient observations.
Key Points
Climate response depends strongly on spatial pattern of ocean heat uptake
Different radiative feedbacks govern transient and equilibrium CO2 warming
Results are robust across an ensemble of aquaplanet climate models
Recent studies have shown that ocean dynamics can have a significant warming effect on the permanent night sides of 1:1 tidally locked terrestrial exoplanets with Earth-like atmospheres and oceans in ...the middle of the habitable zone. However, the impact of ocean dynamics on the habitable zone boundaries (inner edge and outer edge) is still unknown and represents a major gap in our understanding of this type of planet. Here, we use a coupled atmosphere-ocean global climate model to show that planetary heat transport from the day to nightside is dominated by the ocean at lower stellar fluxes and by the atmosphere near the inner edge of the habitable zone. This decrease in oceanic heat transport at high stellar fluxes is mainly due to weakening of surface wind stress and a decrease in surface shortwave energy deposition. We further show that ocean dynamics have almost no effect on the observational thermal phase curves of planets near the inner edge of the habitable zone. For planets in the habitable zone middle range, ocean dynamics move the hottest spot on the surface eastward from the substellar point. These results suggest that future studies of the inner edge may devote computational resources to atmosphere-only processes such as clouds and radiation. For studies of the middle range and outer edge of the habitable zone, however, fully coupled ocean-atmosphere modeling will be necessary. Note that due to computational resource limitations, only one rotation period (60 Earth days) has been systematically examined in this study; future work with varying rotation periods, as well as other parameters such as atmospheric mass and composition, is required.
CO2 absorbs and emits radiation, which allows it to act both as radiative forcing and feedback. Recent work has shown CO2’s feedback effect becomes dominant in hothouse climates, giving rise to a ...non‐monotonic climate sensitivity around 310 K. However, CO2’s feedback effect in colder climates is less clear. We use a line‐by‐line model to explore the CO2‐dependence of the longwave clear‐sky feedback and identify a dividing temperature. Above 290 K, feedback increases with CO2 concentration; below 290 K, feedback decreases with CO2 concentration. We explain this dependence in terms of spectral competition under CO2 increases. In hot climates, CO2’s moderate feedback replaces near‐zero feedback from the H2O bands; in cold climates, CO2’s moderate feedback replaces the large feedback from the surface. Given that global mean temperature is currently close to 290 K, our results suggest that feedback CO2‐dependence is weak at present but can be important in past and future climates.
Plain Language Summary
CO2 traps heat, causing warming. But CO2 also emits heat to space, acting as radiative feedback. Recent work has shown CO2’s feedback effect crucially helps to stabilize very hot climates, but how does it affect present‐day Earth? We show that in hot climates, more CO2 increases Earth’s feedback, while in cold climates, more CO2 decreases it. To understand why, we explain that the surface is an effective emitter, CO2 is a moderate emitter, while H2O is a poor emitter. At high temperatures, adding CO2 to the atmosphere thus replaces feedback that would have otherwise come from H2O, increasing the overall feedback; at low temperatures, adding CO2 replaces feedback that would have otherwise come from the surface, decreasing the overall feedback. Currently, Earth’s global‐mean temperature falls between these two temperature regimes, where CO2’s effect on feedback is nearly zero. Our results explain why CO2’s impact on feedback is small now but can be significant in past or future climates.
Key Points
An increase in CO2 concentration strengthens Earth’s feedback in hot climates, ∂λ/∂CO2 > 0, but weakens it in colder climates, ∂λ/∂CO2 < 0
Whether feedback CO2‐dependence, ∂λ/∂CO2, is positive or negative primarily depends on the extent of the H2O window
Feedback CO2‐dependence and forcing temperature‐dependence, ∂λ/∂CO2 = −∂F2x/∂Ts, can be important for past or future climates
Most rocky planets in the galaxy orbit a cool host star, and there is large uncertainty among theoretical models whether these planets can retain an atmosphere. The James Webb Space Telescope (JWST) ...might be able to settle this question empirically, but most proposals for doing so require large observational effort because they are based on spectroscopy. Here we show that infrared photometry of secondary eclipses could quickly identify "candidate" atmospheres, by searching for rocky planets with atmospheres thick enough that atmospheric heat transport noticeably reduces their dayside thermal emission compared to that of a bare rock. For a planet amenable to atmospheric follow-up, we find that JWST should be able to confidently detect the heat redistribution signal of an bar atmosphere with one to two eclipses. One to two eclipses is generally much less than the effort needed to infer an atmosphere via transmission or emission spectroscopy. Candidate atmospheres can be further validated via follow-up spectroscopy or phase curves. In addition, because this technique is fast it could enable a first atmospheric survey of rocky exoplanets with JWST. We estimate that the TESS mission will find ∼100 planets that are too hot to be habitable but that can be quickly probed via eclipse photometry. Knowing whether hot, rocky planets around M dwarfs have atmospheres is important not only for understanding the evolution of uninhabitable worlds: if atmospheres are common on hot planets, then cooler, potentially habitable planets around M dwarfs are also likely to have atmospheres.
Abstract
Atmospheric heat redistribution shapes the remote appearance of rocky exoplanets, but there is currently no easy way to predict a planet’s heat redistribution from its physical properties. ...This paper proposes an analytical scaling theory for the heat redistribution on tidally locked rocky exoplanets. The main parameters of the scaling are a planet’s equilibrium temperature, surface pressure, and broadband longwave optical thickness. The scaling compares favorably against idealized general circulation model simulations of TRAPPIST-1b, GJ1132b, and LHS 3844b. For these planets, heat redistribution generally becomes efficient, and a planet’s observable thermal phase curve and secondary eclipse start to deviate significantly from that of a bare rock, once surface pressure exceeds
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1
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bar. The scaling additionally points to planetary scenarios for which heat transport can be notably more or less efficient, such as H
2
and CO atmospheres or hot lava ocean worlds. The results thus bridge the gap between theory and imminent observations with the James Webb Space Telescope. They can also be used to parameterize the effect of 3D atmospheric dynamics in 1D models, thereby improving the self-consistency of such models.
The upcoming launch of the James Webb Space Telescope means that we will soon have the capability to characterize the atmospheres of rocky exoplanets. However, it is still unknown whether such ...planets orbiting close to M dwarf stars can retain their atmospheres, or whether high-energy irradiation from the star will strip the gaseous envelopes from these objects. We present a new method to detect an atmosphere on a synchronously rotating rocky exoplanet around a K/M dwarf, by using thermal emission during secondary eclipse to infer a high dayside albedo that could only be explained by bright clouds. Based on calculations for plausible surface conditions, we conclude that a high albedo could be unambiguously interpreted as a signal of an atmosphere for planets with substellar temperatures of Tsub = 410-1250 K. This range corresponds to equilibrium temperatures of Teq = 300-880 K. We compare the inferred albedos of eight possible planet surface compositions to cloud albedo calculations. We determine that a layer of clouds with optical depths greater than τ = 0.5-7, would have high enough albedos to be distinguishable from a bare rock surface. This method of detecting an atmosphere on a rocky planet is complementary to existing methods for detecting atmospheres, because it provides a way to detect atmospheres with pressures below 1 bar (e.g., Mars), which are too tenuous to transport significant heat but thick enough to host high-albedo clouds.
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