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Register a new userTemperate super-Earths/mini-Neptunes around M/K dwarfs Consist of 2 Populations Distinguished by Their Atmospheres
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Studies of the atmospheres of hot Jupiters reveal a diversity of atmospheric composition and haze properties. Similar studies on individual smaller, temperate planets are rare due to the inherent difficulty of the observations and also to the average faintness of their host stars. To investigate their ensemble atmospheric properties, we construct a sample of 28 similar planets, all possess equilibrium temperature within 300-500K, have similar size (1-3 R_e), and orbit early M dwarfs and late K dwarfs with effective temperatures within a few hundred Kelvin of one another. In addition, NASAs Kepler/K2 and Spitzer missions gathered transit observations of each planet, producing an uniform transit data set both in wavelength and coarse planetary type. With the transits measured in Keplers broad optical bandpass and Spitzers 4.5 micron wavelength bandpass, we measure the transmission spectral slope, alpha, for the entire sample. While this measurement is too uncertain in nearly all cases to infer the properties of any individual planet, the distribution of alpha among several dozen similar planets encodes a key trend. We find that the distribution of alpha is not well-described by a single Gaussian distribution. Rather, a ratio of the Bayesian evidences between the likeliest 1-component and 2-component Gaussian models favors the latter by a ratio of 100:1. One Gaussian is centered around an average alpha=-1.3, indicating hazy/cloudy atmospheres or bare cores with atmosphere evaporated. A smaller but significant second population (20+-10% of all) is necessary to model significantly higher alpha values, which indicate atmospheres with potentially detectable molecular features. We conclude that the atmospheres of small and temperate planets are far from uniformly flat, and that a subset are particularly favorable for follow-up observation from space-based platforms like HST and JWST.
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UV radiation can induce photochemical processes in exoplanet atmospheres and produce haze particles. Recent observations suggest that haze and/or cloud layers could be present in the upper atmospheres of exoplanets. Haze particles play an important role in planetary atmospheres and may provide a source of organic material to the surface which may impact the origin or evolution of life. However, very little information is known about photochemical processes in cool, high-metallicity exoplanetary atmospheres. Previously, we investigated haze formation and particle size distribution in laboratory atmosphere simulation experiments using AC plasma as the energy source. Here, we use UV photons to initiate the chemistry rather than the AC plasma, since photochemistry driven by UV radiation is important for understanding exoplanet atmospheres. We present photochemical haze formation in current UV experiments, we investigated a range of atmospheric metallicities (100x, 1000x, and 10000x solar metallicity) at three temperatures (300 K, 400 K, and 600 K). We find that photochemical hazes are generated in all simulated atmospheres with temperature-dependent production rates: the particles produced in each metallicity group decrease as the temperature increases. The images taken with atomic force microscopy show the particle size (15-190 nm) varies with temperature and metallicity. Our laboratory experimental results provide new insight into the formation and properties of photochemical haze, which could guide exoplanet atmosphere modeling and help to analyze and interpret current and future observations of exoplanets.
We announce the discovery of two planets orbiting the M dwarfs GJ 251 ($0.360pm0.015$ M$_odot$) and HD 238090 ($0.578pm0.021$ M$_odot$) based on CARMENES radial velocity (RV) data. In addition, we independently confirm with CARMENES data the existence of Lalande 21185 b, a planet that has recently been discovered with the SOPHIE spectrograph. All three planets belong to the class of warm or temperate super-Earths and share similar properties. The orbital periods are 14.24 d, 13.67 d, and 12.95 d and the minimum masses are $4.0pm0.4$ $M_oplus$, $6.9pm0.9$ $M_oplus$, and $2.7pm0.3$ $M_oplus$ for GJ 251 b, HD 238090 b, and Lalande 21185 b, respectively. Based on the orbital and stellar properties, we estimate equilibrium temperatures of $351.0pm1.4$ K for GJ 251 b, $469.6pm2.6$ K for HD 238090 b, and $370.1pm6.8$ K for Lalande 21185 b. For the latter we resolve the daily aliases that were present in the SOPHIE data and that hindered an unambiguous determination of the orbital period. We find no significant signals in any of our spectral activity indicators at the planetary periods. The RV observations were accompanied by contemporaneous photometric observations. We derive stellar rotation periods of $122.1pm2.2$ d and $96.7pm3.7$ d for GJ 251 and HD 238090, respectively. The RV data of all three stars exhibit significant signals at the rotational period or its first harmonic. For GJ 251 and Lalande 21185, we also find long-period signals around 600 d, and 2900 d, respectively, which we tentatively attribute to long-term magnetic cycles. We apply a Bayesian approach to carefully model the Keplerian signals simultaneously with the stellar activity using Gaussian process regression models and extensively search for additional significant planetary signals hidden behind the stellar activity.
Hot super-Earths likely possess minimal atmospheres established through vapor saturation equilibrium with the ground. We solve the hydrodynamics of these tenuous atmospheres at the surface of Corot-7b, Kepler 10b and 55 Cnc-e, including idealized treatments of magnetic drag and ohmic dissipation. We find that atmospheric pressures remain close to their local saturation values in all cases. Despite the emergence of strongly supersonic winds which carry sublimating mass away from the substellar point, the atmospheres do not extend much beyond the day-night terminators. Ground temperatures, which determine the planetary thermal (infrared) signature, are largely unaffected by exchanges with the atmosphere and thus follow the effective irradiation pattern. Atmospheric temperatures, however, which control cloud condensation and thus albedo properties, can deviate substantially from the irradiation pattern. Magnetic drag and ohmic dissipation can also strongly impact the atmospheric behavior, depending on atmospheric composition and the planetary magnetic field strength. We conclude that hot super-Earths could exhibit interesting signatures in reflection (and possibly in emission) which would trace a combination of their ground, atmospheric and magnetic properties.
We investigate equilibrium chemistry between a metal-core, a silicate-mantle, and a hydrogen-rich atmosphere (reactive core model) using 18 independent reactions among 25 phase components for sub-Neptune-like exoplanets. We find hydrogen and oxygen typically comprise 1-2% and ~10% by weight of the metal-core, respectively, leading to under-dense cores and thereby offering a possible alternative explanation for the densities of the Trappist-1 planets. In addition, hydrogen occurs at about 0.1% by mass in the silicate mantle, setting a maximum limit to the hydrogen-budget for out-gassing by future super-Earths. The total hydrogen-budget of most sub-Neptunes can be, to first order, well estimated from their atmospheres alone, as more than ~93% of all H resides in their atmospheres. However, reactions with the magma ocean produce significant amounts of SiO and H_2O in the atmospheres which increase the mean molecular weight averaged over the whole atmosphere, by about a factor of two, to ~4 amu. We also investigated the case where metal is excluded from the equilibrium chemistry (unreactive core model). In this case, we find most noticeably that, as the hydrogen mass fraction is reduced from 2% to 1%, the atmosphere becomes water dominated and large fractions of H are absorbed by the magma. As water dominated atmospheres appear inconsistent with observations, we conclude that either the unreactive core model does not apply to sub-Neptunes and that their evolution is better described by a reactive core, or that in-gassing of hydrogen into the mantle is much less efficient than permitted by equilibrium chemistry.
In a few years, space telescopes will investigate our Galaxy to detect evidence of life, mainly by observing rocky planets. In the last decade, the observation of exoplanet atmospheres and the theoretical works on biosignature gasses have experienced a considerable acceleration. The~most attractive feature of the realm of exoplanets is that 40% of M dwarfs host super-Earths with a minimum mass between 1 and 30 Earth masses, orbital periods shorter than 50 days, and radii between those of the Earth and Neptune (1--3.8 R$_oplus$). Moreover, the recent finding of cyanobacteria able to use far-red (FR) light for oxygenic photosynthesis due to the synthesis of chlorophylls $d$ and $f$, extending in vivo light absorption up to 750 nm, suggests the possibility of exotic photosynthesis in planets around M dwarfs. Using innovative laboratory instrumentation, we exposed different cyanobacteria to an M dwarf star simulated irradiation, comparing their responses to those under solar and FR simulated lights.~As expected, in FR light, only the cyanobacteria able to synthesize chlorophyll $d$ and $f$ could grow. Surprisingly, all strains, both able or unable to use FR light, grew and photosynthesized under the M dwarf generated spectrum in a similar way to the solar light and much more efficiently than under the FR one. Our findings highlight the importance of simulating both the visible and FR light components of an M dwarf spectrum to correctly evaluate the photosynthetic performances of oxygenic organisms exposed under such an exotic light~condition.
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