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The transmission spectrum of the Neptune-mass exoplanet GJ 436b is shown to be featureless, implying that the planet has either a hydrogen-poor atmosphere or a high cloud layer. A tale of two planets Two papers in this issue of Nature report Hubble Space Telescope observations of two separate sub-Jupiter-sized extrasolar planets. Heather Knutson et al . observed four transits of the Neptune-mass planet GJ 436b and Laura Kreidberg et al . observed 15 transits of the smaller 'super-Earth' GJ 1214b. The transmission spectra of starlight passing through the atmospheres of these planets should give a good indication of the nature of their respective atmospheres, and for both planets the spectra obtained from Hubble's Wide Field Camera 3 are virtually featureless. Knutson et al . argue that their data are consistent with either a high cloud deck at pressures of 0.1–10 mbar or a hydrogen-poor atmosphere on GJ 436b. Kreidberg et al . conclude that their near-infrared spectra are consistent with the presence of high-altitude clouds that obscure the lower layers of GJ 1214b. GJ 436b is a warm—approximately 800 kelvin—exoplanet that periodically eclipses its low-mass (half the mass of the Sun) host star, and is one of the few Neptune-mass planets that is amenable to detailed characterization. Previous observations 1 , 2 , 3 have indicated that its atmosphere has a ratio of methane to carbon monoxide that is 10 5 times smaller than predicted by models for hydrogen-dominated atmospheres at these temperatures 4 , 5 . A recent study proposed that this unusual chemistry could be explained if the planet’s atmosphere is significantly enhanced in elements heavier than hydrogen and helium 6 . Here we report observations of GJ 436b’s atmosphere obtained during transit. The data indicate that the planet’s transmission spectrum is featureless, ruling out cloud-free, hydrogen-dominated atmosphere models with an extremely high significance of 48 σ . The measured spectrum is consistent with either a layer of high cloud located at a pressure level of approximately one millibar or with a relatively hydrogen-poor (three per cent hydrogen and helium mass fraction) atmospheric composition 7 , 8 , 9 .

The transmission spectrum of the super-Earth exoplanet GJ 1214b is observed to be featureless at near-infrared wavelengths and its atmosphere must contain clouds to be consistent with the data. A tale of two planets Two papers in this issue of Nature report Hubble Space Telescope observations of two separate sub-Jupiter-sized extrasolar planets. Heather Knutson et al . observed four transits of the Neptune-mass planet GJ 436b and Laura Kreidberg et al . observed 15 transits of the smaller 'super-Earth' GJ 1214b. The transmission spectra of starlight passing through the atmospheres of these planets should give a good indication of the nature of their respective atmospheres, and for both planets the spectra obtained from Hubble's Wide Field Camera 3 are virtually featureless. Knutson et al . argue that their data are consistent with either a high cloud deck at pressures of 0.1–10 mbar or a hydrogen-poor atmosphere on GJ 436b. Kreidberg et al . conclude that their near-infrared spectra are consistent with the presence of high-altitude clouds that obscure the lower layers of GJ 1214b. Recent surveys have revealed that planets intermediate in size between Earth and Neptune (‘super-Earths’) are among the most common planets in the Galaxy 1 , 2 , 3 . Atmospheric studies are the next step towards developing a comprehensive understanding of this new class of object 4 , 5 , 6 . Much effort has been focused on using transmission spectroscopy to characterize the atmosphere of the super-Earth archetype GJ 1214b (refs 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 ), but previous observations did not have sufficient precision to distinguish between two interpretations for the atmosphere. The planet’s atmosphere could be dominated by relatively heavy molecules, such as water (for example, a 100 per cent water vapour composition), or it could contain high-altitude clouds that obscure its lower layers. Here we report a measurement of the transmission spectrum of GJ 1214b at near-infrared wavelengths that definitively resolves this ambiguity. The data, obtained with the Hubble Space Telescope, are sufficiently precise to detect absorption features from a high mean-molecular-mass atmosphere. The observed spectrum, however, is featureless. We rule out cloud-free atmospheric models with compositions dominated by water, methane, carbon monoxide, nitrogen or carbon dioxide at greater than 5 σ confidence. The planet’s atmosphere must contain clouds to be consistent with the data.

Most known terrestrial planets orbit small stars with radii less than 60 per cent of that of the Sun 1 , 2 . Theoretical models predict that these planets are more vulnerable to atmospheric loss than their counterparts orbiting Sun-like stars 3 – 6 . 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 7 – 10 . 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 11 . 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. Phase curve measurements for the small (1.3 Earth radii) terrestrial exoplanet LHS 3844b show absence of a thick atmosphere, in agreement with theoretical predictions.

A correlation between giant-planet mass and atmospheric heavy elemental abundance was first noted in the past century from observations of planets in our own Solar System and has served as a cornerstone of planet-formation theory. Using data from the Hubble and Spitzer Space Telescopes from 0.5 to 5 micrometers, we conducted a detailed atmospheric study of the transiting Neptune-mass exoplanet HAT-P-26b. We detected prominent H₂O absorption bands with a maximum base-to-peak amplitude of 525 parts per million in the transmission spectrum. Using the water abundance as a proxy for metallicity, we measured HAT-P-26b’s atmospheric heavy element content ( 4.8 − 4.0 + 21.5 times solar). This likely indicates that HAT-P-26b’s atmosphere is primordial and obtained its gaseous envelope late in its disk lifetime, with little contamination from metal-rich planetesimals.

Observations of extrasolar planets were not projected to be a substantial part of the Spitzer Space Telescope’s mission when it was conceived and designed. Nevertheless, Spitzer was the first facility to detect thermal emission from a hot Jupiter-sized planet, and the range of its exoplanetary investigations grew to encompass transiting planets, microlensing, brown dwarfs, and direct imaging searches and astrometry. Spitzer used phase curves to measure the longitudinal distribution of heat as well as time-dependent heating on hot Jupiters. Its secondary eclipse observations strongly constrained the dayside thermal emission spectra and corresponding atmospheric compositions of hot Jupiters, and the timings of eclipses were used for studies of orbital dynamics. Spitzer’s sensitivity to carbon-based molecules such as methane and carbon monoxide was key to atmospheric composition studies of transiting exoplanets as well as imaging spectroscopy of brown dwarfs, and complemented Hubble Space Telescope spectroscopy at shorter wavelengths. Its capability for long continuous observing sequences enabled searches for new transiting planets around cool stars and helped to define the architectures of planetary systems such as TRAPPIST-1. Spitzer measured masses for small planets at large orbital distances using microlensing parallax. Spitzer observations of brown dwarfs probed their temperatures, masses and weather patterns. Imaging and astrometry from Spitzer was used to discover new planetary-mass brown dwarfs and to measure distances and space densities of many others. The Spitzer Space Telescope launched when the study of exoplanets was in its infancy, and yet it was remarkably successful in characterizing both exoplanet and brown dwarf systems through their mid-infrared emissions. This Review collates the highlights of Spitzer-based research in these fields.

A spectroscopic comparison of ten hot-Jupiter exoplanets reveals that the difference between the planetary radius measured at optical and infrared wavelengths allows atmosphere types ranging from clear to cloudy to be distinguished; the difference in radius at a given wavelength correlates with the spectral strength of water at that wavelength, suggesting that haze obscures the signal from water. Diversity in the air on hot Jupiters David Sing et al . present a set of ten broadband exoplanet spectra from Hubble Space Telescope and Spitzer observations that resolve both the optical scattering and infrared molecular absorption spectroscopically. They find that the difference between the planetary radius measured at optical and infrared wavelengths provides a metric that can distinguish between different atmospheric types. Significantly, strong water absorption lines are seen in clear-atmosphere planets, while the weakest features are associated with clouds and hazes, strongly arguing against primordial water depletion during formation, and indicating that clouds and hazes are the cause of weaker spectral signatures. These results clarify the diversity seen in hot Jupiters and illustrate the interplay of clouds, hazes and metallicity in exoplanet atmospheres. Thousands of transiting exoplanets have been discovered, but spectral analysis of their atmospheres has so far been dominated by a small number of exoplanets and data spanning relatively narrow wavelength ranges (such as 1.1–1.7 micrometres). Recent studies show that some hot-Jupiter exoplanets have much weaker water absorption features in their near-infrared spectra than predicted 1 , 2 , 3 , 4 , 5 . The low amplitude of water signatures could be explained by very low water abundances 6 , 7 , 8 , which may be a sign that water was depleted in the protoplanetary disk at the planet’s formation location 9 , but it is unclear whether this level of depletion can actually occur. Alternatively, these weak signals could be the result of obscuration by clouds or hazes 1 , 2 , 3 , 4 , as found in some optical spectra 3 , 4 , 10 , 11 . Here we report results from a comparative study of ten hot Jupiters covering the wavelength range 0.3–5 micrometres, which allows us to resolve both the optical scattering and infrared molecular absorption spectroscopically. Our results reveal a diverse group of hot Jupiters that exhibit a continuum from clear to cloudy atmospheres. We find that the difference between the planetary radius measured at optical and infrared wavelengths is an effective metric for distinguishing different atmosphere types. The difference correlates with the spectral strength of water, so that strong water absorption lines are seen in clear-atmosphere planets and the weakest features are associated with clouds and hazes. This result strongly suggests that primordial water depletion during formation is unlikely and that clouds and hazes are the cause of weaker spectral signatures.

A key legacy of the recently launched the Transiting Exoplanet Survey Satellite (TESS) mission will be to provide the astronomical community with many of the best transiting exoplanet targets for atmospheric characterization. However, time is of the essence to take full advantage of this opportunity. The James Webb Space Telescope (JWST), although delayed, will still complete its nominal five year mission on a timeline that motivates rapid identification, confirmation, and mass measurement of the top atmospheric characterization targets from TESS. Beyond JWST, future dedicated missions for atmospheric studies such as the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) require the discovery and confirmation of several hundred additional sub-Jovian size planets (Rp < 10 R⊕) orbiting bright stars, beyond those known today, to ensure a successful statistical census of exoplanet atmospheres. Ground-based extremely large telescopes (ELTs) will also contribute to surveying the atmospheres of the transiting planets discovered by TESS. Here we present a set of two straightforward analytic metrics, quantifying the expected signal-to-noise in transmission and thermal emission spectroscopy for a given planet, that will allow the top atmospheric characterization targets to be readily identified among the TESS planet candidates. Targets that meet our proposed threshold values for these metrics would be encouraged for rapid follow-up and confirmation via radial velocity mass measurements. Based on the catalog of simulated TESS detections by Sullivan et al., we determine appropriate cutoff values of the metrics, such that the TESS mission will ultimately yield a sample of ∼300 high-quality atmospheric characterization targets across a range of planet size bins, extending down to Earth-size, potentially habitable worlds.

This tutorial is an introduction to techniques used to characterize the atmospheres of transiting exoplanets. We intend it to be a useful guide for the undergraduate, graduate student, or postdoctoral scholar who wants to begin research in this field, but who has no prior experience with transiting exoplanets. We begin with a discussion of the properties of exoplanetary systems that allow us to measure exoplanetary spectra, and the principles that underlie transit techniques. Subsequently, we discuss the most favorable wavelengths for observing, and explain the specific techniques of secondary eclipses and eclipse mapping, phase curves, transit spectroscopy, and convolution with spectral templates. Our discussion includes factors that affect the data acquisition, and also a separate discussion of how the results are interpreted. Other important topics that we cover include statistical methods to characterize atmospheres such as stacking, and the effects of stellar activity. We conclude by projecting the future utility of large-aperture observatories such as the James Webb Space Telescope and the forthcoming generation of extremely large ground-based telescopes.

Short-period planets exhibit day–night temperature contrasts of hundreds to thousands of kelvin. They also exhibit eastward hotspot offsets whereby the hottest region on the planet is east of the substellar point 1 ; this has been widely interpreted as advection of heat due to eastward winds 2 . We present thermal phase observations of the hot Jupiter CoRoT-2b obtained with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. These measurements show the most robust detection to date of a westward hotspot offset of 23 ± 4°, in contrast with the nine other planets with equivalent measurements 3 – 10 . The peculiar infrared flux map of CoRoT-2b may result from westward winds due to non-synchronous rotation 11 or magnetic effects 12 , 13 , or partial cloud coverage, that obscure the emergent flux from the planet’s eastern hemisphere 14 – 17 . Non-synchronous rotation and magnetic effects may also explain the planet’s anomalously large radius 12 , 18 . On the other hand, partial cloud coverage could explain the featureless dayside emission spectrum of the planet 19 , 20 . If CoRoT-2b is not tidally locked, then it means that our understanding of star–planet tidal interaction is incomplete. If the westward offset is due to magnetic effects, our result represents an opportunity to study an exoplanet’s magnetic field. If it has eastern clouds, then it means that a greater understanding of large-scale circulation on tidally locked planets is required. Global circulation theory predicts strong equatorial jets at the equators of hot gas giant exoplanets that blow hot gas to the east, resulting in an eastward hotspot. Here, Dang et al. present a detection of a hotspot significantly offset to the west.

With no analogues in the Solar System, the discovery of thousands of exoplanets with masses and radii intermediate between Earth and Neptune was one of the big surprises of exoplanet science. These super-Earths and sub-Neptunes probably represent the most common outcome of planet formation1,2. Mass and radius measurements indicate a diversity in bulk composition much wider than for gas giants3; however, direct spectroscopic detections of molecular absorption and constraints on the gas mixing ratios have largely remained limited to planets more massive than Neptune4–6. Here we analyse a combined Hubble/Spitzer Space Telescope dataset of 12 transits and 20 eclipses of the sub-Neptune exoplanet GJ 3470 b, whose mass of 12.6 M⊕ places it near the halfway point between previously studied Neptune-like exoplanets (22–23 M⊕)5–7 and exoplanets known to have rocky densities (7 M⊕)8. Obtained over many years, our dataset provides a robust detection of water absorption (>5σ) and a thermal emission detection from the lowest irradiated planet to date. We reveal a low-metallicity, hydrogen-dominated atmosphere similar to that of a gas giant, but strongly depleted in methane gas. The low metallicity (O/H = 0.2–18.0) sets important constraints on the potential planet formation processes at low masses as well as the subsequent accretion of solids. The low methane abundance indicates that methane is destroyed much more efficiently than previously predicted, suggesting that the CH4/CO transition curve has to be revisited for close-in planets. Finally, we also find a sharp drop in the cloud opacity at 2–3 µm, characteristic of Mie scattering, which enables narrow constraints on the cloud particle size and makes GJ 3470 b a key target for mid-infrared characterization with the James Webb Space Telescope.A comprehensive set of Hubble and Spitzer observations reveal a hydrogen-rich, low-metallicity atmosphere on the sub-Neptune exoplanet GJ 3470 b. Water vapour is detected, but the planet is surprisingly depleted in methane, possibly because of photochemical or thermal processes. Sub-millimetre-sized Mie-scattering cloud particles partially attenuate the molecular signatures at short wavelength, but are largely transparent beyond 3 µm.