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Register a new userThe Featureless Transmission Spectra of Two Super-Puff Planets
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Added by
Jessica Libby-Roberts
Publication date
2019
fields
Physics
and research's language is
English
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The Kepler mission revealed a class of planets known as super-puffs, with masses only a few times larger than Earths but radii larger than Neptune, giving them very low mean densities. All three of the known planets orbiting the young solar-type star Kepler 51 are super-puffs. The Kepler 51 system thereby provides an opportunity for a comparative study of the structures and atmospheres of this mysterious class of planets, which may provide clues about their formation and evolution. We observed two transits each of Kepler 51b and 51d with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope. Combining new WFC3 transit times with re-analyzed Kepler data and updated stellar parameters, we confirmed that all three planets have densities lower than 0.1 g/cm$^{3}$. We measured the WFC3 transmission spectra to be featureless between 1.15 and 1.63 $mu$m, ruling out any variations greater than 0.6 scale heights (assuming a H/He dominated atmosphere), thus showing no significant water absorption features. We interpreted the flat spectra as the result of a high-altitude aerosol layer (pressure $<$3 mbar) on each planet. Adding this new result to the collection of flat spectra that have been observed for other sub-Neptune planets, we find support for one of the two hypotheses introduced by Crossfield and Kreidberg (2017), that planets with cooler equilibrium temperatures have more high-altitude aerosols. We strongly disfavor their other hypothesis that the H/He mass fraction drives the appearance of large amplitude transmission features.
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Spatial heterogeneity and temporal variability are general features in planetary weather and climate, due to the effects of planetary rotation, uneven stellar flux distribution, fluid motion instability, etc. In this study, we investigate the asymmetry and variability in the transmission spectra of 1:1 spin--orbit tidally locked (or called synchronously rotating) planets around low-mass stars. We find that for rapidly rotating planets, the transit atmospheric thickness on the evening terminator (east of the substellar region) is significantly larger than that of the morning terminator (west of the substellar region). The asymmetry is mainly related to the spatial heterogeneity in ice clouds, as the contributions of liquid clouds and water vapor are smaller. The underlying mechanism is that there are always more ice clouds on the evening terminator, due to the combined effect of coupled Rossby--Kelvin waves and equatorial superrotation that advect vapor and clouds to the east, especially at high levels of the atmosphere. For slowly rotating planets, the asymmetry reverses (the morning terminator has a larger transmission depth than the evening terminator) but the magnitude is small or even negligible. For both rapidly and slowly rotating planets, there is strong variability in the transmission spectra. The asymmetry signal is nearly impossible to be observed by the James Webb Space Telescope (JWST), because the magnitude of the asymmetry (about 10 ppm) is smaller than the instrumental noise and the high variability further increases the challenge.
GJ 436b is a warm-- approximately 800 K--extrasolar planet 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 have indicated that its atmosphere has a methane-to-CO ratio that is 100,000 times smaller than predicted by models for hydrogen-dominated atmospheres at these temperatures. A recent study proposed that this unusual chemistry could be explained if the planets atmosphere is significantly enhanced in elements heavier than H and He. In this study we present complementary observations of GJ 436bs atmosphere obtained during transit. Our observations indicate that the planets transmission spectrum is effectively featureless, ruling out cloud-free, hydrogen-dominated atmosphere models with an extremely high significance of 48 sigma. The measured spectrum is consistent with either a high cloud or haze layer located at a pressure of approximately 1 mbar or with a relatively hydrogen-poor (three percent hydrogen and helium mass fraction) atmospheric composition.
Spectral slopes in optical transmission spectra of exoplanetary atmospheres encapsulate information on the properties of exotic clouds. The slope is usually attributed to the Rayleigh scattering caused by tiny aerosol particles, whereas recent retrieval studies have suggested that the slopes are often steeper than the canonical Rayleigh slopes. Here, we propose that photochemical haze formed in vigorously mixing atmospheres can explain such super-Rayleigh slopes. We first analytically show that the spectral slope can be steepened by the vertical opacity gradient in which atmospheric opacity increases with altitude. Using a microphysical model, we demonstrate that such opacity gradient can be naturally generated by photochemical haze, especially when the eddy mixing is substantially efficient. The transmission spectra of hazy atmospheres can be demarcated into four typical regimes in terms of the haze mass flux and eddy diffusion coefficient. We find that the transmission spectrum can have the spectral slope 2--4 times steeper than the Rayleigh slope if the eddy diffusion coefficient is sufficiently high and the haze mass flux falls into a moderate value. Based on the eddy diffusion coefficient suggested by a recent study of atmospheric circulations, we suggest that photochemical haze preferentially generates super-Rayleigh slopes at planets with equilibrium temperature of 1000--1500 K, which might be consistent with results of recent retrieval studies. Our results would help to interpret the observations of spectral slopes from the perspective of haze formation.
The short period ($0.94$-day) transiting exoplanet WASP-19b is an exceptional target for transmission spectroscopy studies, due to its relatively large atmospheric scale-height ($sim 500$ km) and equilibrium temperature ($sim 2100$ K). Here we report on six precise spectroscopic Magellan/IMACS observations, five of which target the full optical window from $0.45-0.9mu$m and one targeting the $0.4-0.55mu$m blue-optical range. Five of these datasets are consistent with a transmission spectrum without any significant spectral features, while one shows a significant slope as a function of wavelength, which we interpret as arising from photospheric heterogeneities in the star. Coupled with HST/WFC3 infrared observations, our optical/near-infrared measurements point to the presence of high altitude clouds in WASP-19bs atmosphere in agreement with previous studies. Using a semi-analytical retrieval approach, considering both planetary and stellar spectral features, we find a water abundance consistent with solar for WASP-19b and strong evidence for sub-solar abundances for optical absorbers such as TiO and Na; no strong optical slope is detected, which suggests that if hazes are present, they are much weaker than previously suggested. In addition, two spot-crossing events are observed in our datasets and analyzed, including one of the first unambiguously detected bright spot-crossing events on an exoplanet host star.
Super-Earths, objects slightly larger than Earth and slightly smaller than Uranus, have found a special place in exoplanetary science. As a new class of planetary bodies, these objects have challenged models of planet formation at both ends of the spectrum and have triggered a great deal of research on the composition and interior dynamics of rocky planets in connection to their masses and radii. Being relatively easier to detect than an Earth-sized planet at 1 AU around a G star, super-Earths have become the focus of worldwide observational campaigns to search for habitable planets. With a range of masses that allows these objects to retain moderate atmospheres and perhaps even plate tectonics, super-Earths may be habitable if they maintain long-term orbits in the habitable zones of their host stars. Given that in the past two years a few such potentially habitable super-Earths have in fact been discovered, it is necessary to develop a deep understanding of the formation and dynamical evolution of these objects. This article reviews the current state of research on the formation of super-Earths and discusses different models of their formation and dynamical evolution.
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