Planetary formation and water delivery in the habitable zone around solar-type stars in different dynamical environments

Planetary formation and water delivery in the habitable zone around solar-type stars in different dynamical environments 

Authors:

Zain et al

Abstract:

Aims.

We study the formation and water delivery of planets in the habitable zone (HZ) around solar-type stars. In particular, we study different dynamical environments that are defined by the most massive body in the system.

Methods.

First of all, a semi-analytical model was used to define the mass of the protoplanetary disks that produce each of the five dynamical scenarios of our research. Then, we made use of the same semi-analytical model to describe the evolution of embryos and planetesimals during the gaseous phase. Finally, we carried out N-body simulations of planetary accretion in order to analyze the formation and water delivery of planets in the HZ in the different dynamical environments.

Results.

Water worlds are efficiently formed in the HZ in different dynamical scenarios. In systems with a giant planet analog to Jupiter or Saturn around the snow line, super-Earths tend to migrate into the HZ from outside the snow line as a result of interactions with other embryos and accrete water only during the gaseous phase. In systems without giant planets, Earths and super-Earths with high water by mass contents can either be formed in situ in the HZ or migrate into it from outer regions, and water can be accreted during the gaseous phase and in collisions with water-rich embryos and planetesimals.

Conclusions.

The formation of planets in the HZ with very high water by mass contents seems to be a common process around Sun- like stars. Our research suggests that such planets are still very efficiently produced in different dynamical environments. Moreover, our study indicates that the formation of planets in the HZ with masses and water contents similar to those of Earth seems to be a rare process around solar-type stars in the systems under consideration.

Transfer, loss and physical processing of water in hit-and-run collisions of planetary embryos

Transfer, loss and physical processing of water in hit-and-run collisions of planetary embryos

Authors:

Burger et al

Abstract:

Collisions between large, similar-sized bodies are believed to shape the final characteristics and composition of terrestrial planets. Their inventories of volatiles such as water, are either delivered or at least significantly modified by such events. Besides the transition from accretion to erosion with increasing impact velocity, similar-sized collisions can also result in hit-and-run outcomes for sufficiently oblique impact angles and large enough projectile-to-target mass ratios. We study volatile transfer and loss focusing on hit-and-run encounters by means of Smooth Particle Hydrodynamics simulations, including all main parameters: impact velocity, impact angle, mass ratio, and also the total colliding mass. We find a broad range of overall water losses, up to 75% in the most energetic hit-and-run events, and confirm the much more severe consequences for the smaller body also for stripping of volatile layers. Transfer of water between projectile and target inventories is found to be mostly rather inefficient, and final water contents are dominated by pre-collision inventories reduced by impact losses, for similar pre-collision water mass fractions. Comparison with our numerical results shows that current collision outcome models are not accurate enough to reliably predict these composition changes in hit-and-run events. To also account for non-mechanical losses we estimate the amount of collisionally vaporized water over a broad range of masses, and find that these contributions are particularly important in collisions of ~Mars-sized bodies, with sufficiently high impact energies, but still relatively low gravity. Our results clearly indicate that the cumulative effect of several (hit-and-run) collisions can efficiently strip protoplanets of their volatile layers, especially the smaller body, as it might be common e.g. for Earth-mass planets in systems with Super-Earths.

Water, Methane Depletion, and High-Altitude Condensates in the Atmosphere of the Warm Super-Neptune WASP-107b

Water, Methane Depletion, and High-Altitude Condensates in the Atmosphere of the Warm Super-Neptune WASP-107b

Authors:

Kreidberg et al

Abstract:

The super-Neptune exoplanet WASP-107b is an exciting target for atmosphere characterization. It has an unusually large atmospheric scale height and a small, bright host star, raising the possibility of precise constraints on its current nature and formation history. We report the first atmospheric study of WASP-107b, a Hubble Space Telescope measurement of its near-infrared transmission spectrum. We determined the planet's composition with two techniques: atmospheric retrieval based on the transmission spectrum and interior structure modeling based on the observed mass and radius. The interior structure models set a 3σ upper limit on the atmospheric metallicity of 30× solar. The transmission spectrum shows strong evidence for water absorption (6.5σ confidence), and we infer a water abundance consistent with expectations for a solar abundance pattern. On the other hand, methane is depleted relative to expectations (at 3σ confidence), suggesting a low carbon-to-oxygen ratio or high internal heat flux. The water features are smaller than predicted for a cloudless atmosphere, crossing less than one scale height. A thick condensate layer at high altitudes (0.1 - 3 mbar) is needed to match the observations; however, we find that it is challenging for physically motivated cloud and haze models to produce opaque condensates at these pressures. Taken together, these findings serve as an illustration of the diversity and complexity of exoplanet atmospheres. The community can look forward to more such results with the high precision and wide spectral coverage afforded by future observing facilities.

Detecting Water's Dissociation in Protoplanetary Disks

The 1600 Angstrom Emission Bump in Protoplanetary Disks: A Spectral Signature of H2O Dissociation

Authors:

France et al

Abstract:

The FUV continuum spectrum of many accreting pre-main sequence stars, Classical T Tauri Stars (CTTSs), does not continue smoothly from the well-studied Balmer continuum emission in the NUV, suggesting that additional processes contribute to the short-wavelength emission in these objects. The most notable spectral feature in the FUV continuum of some CTTSs is a broad emission approximately centered at 1600~\AA, which has been referred to as the "1600 A Bump". The origin of this feature remains unclear. We have assembled archival FUV spectra of 37 disk-hosting systems observed by the {\it Hubble Space Telescope}. Clear 1600 A Bump emission is observed above the smooth, underlying 1100-1800 A continuum spectrum in 19/37 Classical T Tauri disks in the HST sample, with the detection rate in transition disks (8/8) being much higher than in primordial or non-transition sources (11/29). We describe a spectral deconvolution analysis to separate the Bump (spanning 1490~--~1690 A) from the underlying FUV continuum, finding an average Bump luminosity, L(Bump7 x 10^{29} erg s^{-1}. We find that the 1600 A Bump is characterized by a peak wavelength of 1598.6 +/- 3.3 A.
Contrary to previous studies, we find that this feature is inconsistent with models of H2 excited by electron-impact. We show that this Bump emits roughly 10-80% of the total fluorescent H2 luminosity for stars with well-defined Bump features. Energetically, this suggests that the carrier of the 1600 A Bump emission is powered by Ly-a photons. We argue that the most likely mechanism is Ly-a-driven dissociation of H2O in the inner disk, r < 2 AU. We demonstrate that non-thermally populated H2O fragments can qualitatively account for the observed emission (discrete and continuum), and find that the average Ly-a-driven H2O dissociation rate is 1.7x 10^{42} water molecules s^{-1}.

Water Clouds in hot Jupiter HAT-P-32b's Atmosphere

Near-IR Transmission Spectrum of HAT-P-32b using HST/WFC3

Authors:

Damiano et al

Abstract:
We report here the analysis of the near-infrared transit spectrum of the hot Jupiter HAT-P-32b, which was recorded with the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope. HAT-P-32b is one of the most inflated exoplanets discovered, making it an excellent candidate for transit spectroscopic measurements. To obtain the transit spectrum, we have adopted different analysis methods, both parametric and non-parametric (Independent Component Analysis, ICA), and compared the results. The final spectra are all consistent within 0.5σ. The uncertainties obtained with ICA are larger than those obtained with the parametric method by a factor of ~1.6–1.8. This difference is the trade-off for higher objectivity due to the lack of any assumption about the instrument systematics compared to the parametric approach. The ICA error bars are therefore worst-case estimates. To interpret the spectrum of HAT-P-32b we used ${ \mathcal T }$-REx, our fully Bayesian spectral retrieval code. As for other hot Jupiters, the results are consistent with the presence of water vapor (${\mathrm{log}{\rm{H}}}_{2}{\rm{O}}=-{3.45}_{-1.65}^{+1.83}$), clouds (top pressure between 5.16 and 1.73 bar). Spectroscopic data over a broader wavelength range are needed to de-correlate the mixing ratio of water vapor from clouds and identify other possible molecular species in the atmosphere of HAT-P-32b.

The Influence of Orbital Resonances on the Water Transport to Objects in the Circumprimary Habitable Zone of Binary Star Systems

The Influence of Orbital Resonances on the Water Transport to Objects in the Circumprimary Habitable Zone of Binary Star Systems

Authors:

Bancelin et al

Abstract:
We investigate the role of secular and mean motion resonances on the water transport from a belt of icy asteroids onto planets or embryos orbiting inside the circumprimary habitable zone (HZ) of a binary star system. In addition, the host-star has an accompanying gas giant planet. For a comparison, we perform two case studies where a secular resonance (SR) is located either inside the HZ close to 1.0 au (causing eccentric motion of a planet or embryos therein) or in the asteroid belt, beyond the snow line. In the latter case, a higher flux of icy objects moving toward the HZ is expected. Collisions between asteroids and objects in the HZ are treated analytically. Our purely dynamical study shows that the SR in the HZ boosts the water transport; however, collisions can occur at very high impact speeds. In this paper, we treat for the first time, realistic collisions using a GPU 3D-SPH code to assess the water loss in the projectile. Including the water loss into the dynamical results, we get more realistic values for the water mass fraction of the asteroid during an impact. We highlight that collisions occurring at high velocities greatly reduce the water content of the projectile and thus the amount of water transported to planets or embryos orbiting inside the HZ. Moreover, we discuss other effects that could modify our results, namely the asteroid's surface rate recession due to ice sublimation and the atmospheric drag contribution on the asteroids' mass loss.

Water in SuperEarth 55 Cancri e's Atmosphere

A Search for Water in a Super-Earth Atmosphere: High-resolution Optical Spectroscopy of 55Cancri e 

Authors: 

Esteves et al

Abstract:

We present the analysis of high-resolution optical spectra of four transits of 55Cnc e, a low-density super-Earth that orbits a nearby Sun-like star in under 18 hr. The inferred bulk density of the planet implies a substantial envelope, which, according to mass–radius relationships, could be either a low-mass extended or a high-mass compact atmosphere. Our observations investigate the latter scenario, with water as the dominant species. We take advantage of the Doppler cross-correlation technique, high-spectral resolution, and the large wavelength coverage of our observations to search for the signature of thousands of optical water absorption lines. Using our observations with HDS on the Subaru telescope and ESPaDOnS on the Canada–France–Hawaii Telescope, we are able to place a 3σ lower limit of 10 g mol−1 on the mean-molecular weight of 55Cnc e's water-rich (volume mixing ratio >10%), optically thin atmosphere, which corresponds to an atmospheric scale-height of ~80 km. Our study marks the first high-spectral resolution search for water in a super-Earth atmosphere, and demonstrates that it is possible to recover known water-vapor absorption signals in a nearby super-Earth atmosphere, using high-resolution transit spectroscopy with current ground-based instruments.

Hunting for Water Vapor in hot Jupiter HD 189733b's Atmosphere

Search for water vapor in the high-resolution transmission spectrum of HD189733b in the visible
Authors:

Allart et al

Abstract:

Ground-based telescopes equipped with state-of-the-art spectrographs are able to obtain high-resolution transmission and emission spectra of exoplanets that probe the structure and composition of their atmospheres. Various atomic and molecular species, such as Na, CO, H2O have been already detected. Molecular species have been observed only in the near-infrared while atomic species have been observed in the visible. In particular, the detection and abundance determination of water vapor bring important constraints to the planet formation process. We search for water vapor in the atmosphere of the exoplanet HD189733b using a high-resolution transmission spectrum in the visible obtained with HARPS. We use Molecfit to correct for telluric absorption features. Then we compute the high-resolution transmission spectrum of the planet using 3 transit datasets. We finally search for water vapor absorption using a cross-correlation technique that combines the signal of 800 individual lines. Telluric features are corrected to the noise level. We place a 5-sigma upper limit of 100 ppm on the strength of the 6500 A water vapor band. The 1-sigma precision of 20 ppm on the transmission spectrum demonstrates that space-like sensitivity can be achieved from the ground. This approach opens new perspectives to detect various atomic and molecular species with future instruments such as ESPRESSO at the VLT. Extrapolating from our results, we show that only 1 transit with ESPRESSO would be sufficient to detect water vapor on HD189733b-like hot Jupiter with a cloud-free atmosphere. Upcoming near-IR spectrographs will be even more efficient and sensitive to a wider range of molecular species. Moreover, the detection of the same molecular species in different bands (e.g. visible and IR) is key to constrain the structure and composition of the atmosphere, such as the presence of Rayleigh scattering or aerosols.

SuperEarth Gliese 1132b's Atmosphere has been Detected

Detection of the atmosphere of the 1.6 Earth mass exoplanet GJ 1132b

Authors:

Southworth et al

Abstract:

Detecting the atmospheres of low-mass low-temperature exoplanets is a high-priority goal on the path to ultimately detect biosignatures in the atmospheres of habitable exoplanets. High-precision HST observations of several super-Earths with equilibrium temperatures below 1000 K have to date all resulted in featureless transmission spectra, which have been suggested to be due to high-altitude clouds. We report the detection of an atmospheric feature in the atmosphere of a 1.6 Mearth transiting exoplanet, GJ 1132b, with an equilibrium temperature of ~600 K and orbiting a nearby M dwarf. We present observations of nine transits of the planet obtained simultaneously in the griz and JHK passbands. We find an average radius of 1.44 +/- 0.21 Rearth for the planet, averaged over all the passbands, which can be decomposed into a "surface radius" at ~1.35 Rearth, and higher contributions in the z and K bands. The z-band radius is 4 sigma higher than the continuum, suggesting a strong detection of an atmosphere. We deploy a suite of tests to verify the reliability of the transmission spectrum, which are greatly helped by the existence of repeat observations. The large z-band transit depth indicates strong opacity from H2O and/or CH4 or an hitherto unconsidered opacity. A surface radius of 1.35 +/- 0.21 Rearth allows for a wide range of interior compositions ranging from a nearly Earth-like rocky interior, with ~70% silicate and ~30% Fe, to a substantially H2O-rich water world. New observations with HST and existing ground-based facilities would be able to confirm the present detection and further constrain the atmospheric composition of the planet.

Kepler's Ultra-Short-Period Planets Formed Water-Poor

Born Dry in the Photo-Evaporation Desert: Kepler's Ultra-Short-Period Planets Formed Water-Poor

Author:

Lopez

Abstract:

Recent surveys have uncovered an exciting new population of ultra-short-period (USP) planets with orbital periods less than a day. These planets typically have radii ≲1.5 R⊕, indicating that they likely have rocky compositions. This stands in contrast to the overall distribution of planets out to ∼100 days, which is dominated by low-density sub-Neptunes above 2 R⊕, which must have gaseous envelopes to explain their size. However, on ultra-short-period orbits, planets are bombarded by intense levels of photo-ionizing radiation and consequently gaseous sub-Neptunes are extremely vulnerable to losing their envelopes to atmospheric photo-evaporation. Using models of planet evolution, I show that the rocky USP planets can easily be produced as the evaporated remnants of sub-Neptunes with H/He envelopes and that we can therefore understand the observed dearth of USP sub-Neptunes as a natural consequence of photo-evaporation. Critically however, planets on USP orbits could often retain their envelopes if they formed with very high-metallicity water dominated envelopes. Such water-rich planets would commonly be ≳2 R⊕ today, which is inconsistent with the observed evaporation desert, indicating that most USP planets likely formed from water-poor material within the snow-line. Finally, I examine the special case of 55 Cancri e and its possible composition in the light of recent observations, and discuss the prospects for further characterizing this population with future observations.

GJ 1132b Predicted to Have a Water Rich Atmosphere & Magma Ocean

PREDICTIONS OF THE ATMOSPHERIC COMPOSITION OF GJ 1132b

Authors:

Schaefer et al

Abstract:

GJ 1132b is a nearby Earth-sized exoplanet transiting an M dwarf, and is among the most highly characterizable small exoplanets currently known. In this paper, we study the interaction of a magma ocean with a water-rich atmosphere on GJ 1132b and determine that it must have begun with more than 5 wt% initial water in order to still retain a water-based atmosphere. We also determine the amount of O2 that can build up in the atmosphere as a result of hydrogen dissociation and loss. We find that the magma ocean absorbs at most ~10% of the O2 produced, whereas more than 90% is lost to space through hydrodynamic drag. The most common outcome for GJ 1132b from our simulations is a tenuous atmosphere dominated by O2, though, for very large initial water abundances, atmospheres with several thousands of bars of O2 are possible. A substantial steam envelope would indicate either the existence of an earlier H2 envelope or low XUV flux over the system's lifetime. A steam atmosphere would also imply the continued existence of a magma ocean on GJ 1132b. Further modeling is needed to study the evolution of CO2 or N2-rich atmospheres on GJ 1132b.

Gas Giants Need Lots of Icy Planetesimals to Form

Planet formation with envelope enrichment: new insights on planetary diversity

Authors:

Venturini et al

Abstract:

We compute, for the first time, self-consistent models of planet growth including the effect of envelope enrichment. The change of envelope metallicity is assumed to be the result of planetesimal disruption or icy pebble sublimation. We solve internal structure equations taking into account global energy conservation for the envelope to compute in-situ planetary growth. We consider different opacities and equations of state suited for a wide range of metallicities. We find that envelope enrichment speeds up the formation of gas giants. It also explains naturally the formation of low and intermediate mass objects with large fractions of H-He (~ 20 - 30 % in mass). High opacity models explain well the metallicity of the giant planets of the solar system, whereas low opacity models are suited for forming small mass objects with thick H-He envelopes and gas giants with sub-solar envelope metallicities. We find good agreement between our models and the estimated water abundance for WASP-43b. For HD 189733b, HD 209458b and WASP-12b we predict fractions of water larger than what is estimated from observations, by at least a factor ~ 2. Envelope enrichment by icy planetesimals is the natural scenario to explain the formation of a large variety of objects, ranging from mini-Neptunes, to gas giants. We predict that the total and envelope metallicity decrease with planetary mass.

Water, Carbon Monoxide & Carbon Dioxide Distributions in hot Jupiter WASP-43b's Atmosphere

Spitzer Phase Curve Constraints for WASP-43b at 3.6 and 4.5 microns

Authors:

Stevenson et al

Abstract:

Previous measurements of heat redistribution efficiency (the ability to transport energy from a planet's highly-irradiated dayside to its eternally-dark nightside) show considerable variation between exoplanets. Theoretical models predict a correlation between heat redistribution efficiency and temperature for tidally-locked planets; however, recent Hubble Space Telescope (HST) WASP-43b spectroscopic phase curve results are inconsistent with current predictions. Using the Spitzer Space Telescope, we obtained a total of three phase curve observations of WASP-43b at 3.6 and 4.5 microns. The first 3.6 micron visit exhibits spurious nightside emission that requires invoking unphysical conditions in our atmospheric retrievals. The two other visits exhibit strong day-night contrasts that are consistent with the HST data. To reconcile the departure from theoretical predictions, WASP-43b would need to have a high-altitude, nightside cloud/haze layer blocking its thermal emission. Clouds/hazes could be produced within the planet's cool, nearly-retrograde mid-latitude flows before dispersing across its nightside at high altitudes. Since mid-latitude flows only materialize in fast-rotating (≲1 day) planets, this may explain an observed trend connecting measured day-night contrast with planet rotation rate that matches all current Spitzer phase curve results. Combining independent planetary emission measurements from multiple phases, we obtain a precise dayside hemisphere H2O abundance (2.5×10−5−1.1×10−4 at 1σ confidence) and, assuming chemical equilibrium and a scaled solar abundance pattern, we derive a corresponding metallicity estimate that is consistent with being solar (0.4 -- 1.7). Using the retrieved global CO+CO2 abundance under the same assumptions, we estimate a comparable metallicity of 0.3 - 1.7× solar.

There is Considerable Uncertainty About the Inner Edge of the Habitable Zone

DIFFERENCES IN WATER VAPOR RADIATIVE TRANSFER AMONG 1D MODELS CAN SIGNIFICANTLY AFFECT THE INNER EDGE OF THE HABITABLE ZONE

Authors:

Yang et al

Abstract:

An accurate estimate of the inner edge of the habitable zone is critical for determining which exoplanets are potentially habitable and for designing future telescopes to observe them. Here, we explore differences in estimating the inner edge among seven one-dimensional radiative transfer models: two line-by-line codes (SMART and LBLRTM) as well as five band codes (CAM3, CAM4_Wolf, LMDG, SBDART, and AM2) that are currently being used in global climate models. We compare radiative fluxes and spectra in clear-sky conditions around G and M stars, with fixed moist adiabatic profiles for surface temperatures from 250 to 360 K. We find that divergences among the models arise mainly from large uncertainties in water vapor absorption in the window region (10 μm) and in the region between 0.2 and 1.5 μm. Differences in outgoing longwave radiation increase with surface temperature and reach 10–20 W m−2; differences in shortwave reach up to 60 W m−2, especially at the surface and in the troposphere, and are larger for an M-dwarf spectrum than a solar spectrum. Differences between the two line-by-line models are significant, although smaller than among the band models. Our results imply that the uncertainty in estimating the insolation threshold of the inner edge (the runaway greenhouse limit) due only to clear-sky radiative transfer is ≈10% of modern Earth's solar constant (i.e., ≈34 W m−2 in global mean) among band models and ≈3% between the two line-by-line models. These comparisons show that future work is needed that focuses on improving water vapor absorption coefficients in both shortwave and longwave, as well as on increasing the resolution of stellar spectra in broadband models.

Hot and dense water in the inner 25 AU of SVS13-A

Hot and dense water in the inner 25 AU of SVS13-A

Authors:

Codella et al

Abstract:

In the context of the ASAI (Astrochemical Surveys At IRAM) project, we carried out an unbiased spectral survey in the millimeter window towards the well known low-mass Class I source SVS13-A. The high sensitivity reached (3-12 mK) allowed us to detect at least 6 HDO broad (FWHM ~ 4-5 km/s) emission lines with upper level energies up to Eu = 837 K. A non-LTE LVG analysis implies the presence of very hot (150-260 K) and dense (> 3 10^7 cm-3) gas inside a small radius (∼ 25 AU) around the star, supporting, for the first time, the occurrence of a hot corino around a Class I protostar.

The temperature is higher than expected for water molecules are sublimated from the icy dust mantles (~ 100 K). Although we cannot exclude we are observig the effects of shocks and/or winds at such small scales, this could imply that the observed HDO emission is tracing the water abundance jump expected at temperatures ~ 220-250 K, when the activation barrier of the gas phase reactions leading to the formation of water can be overcome. We derive X(HDO) ~ 3 10-6, and a H2O deuteration > 1.5 10-2, suggesting that water deuteration does not decrease as the protostar evolves from the Class 0 to the Class I stage.

Velocity-resolved hot water emission detected toward HL Tau with the Submillimeter Array

Velocity-resolved hot water emission detected toward HL Tau with the Submillimeter Array

Authors:

Kristensen et al

Abstract:

Using the Submillimeter Array (SMA) on Mauna Kea, the H2-16O 10_2,9-9_3,6 transition (E_up=1863K) at 321.2 GHz has been detected toward the embedded low-mass protostar HL Tau. The line centroid is blue-shifted by 15 km/s with respect to the source velocity, and it has a FWHM of 20 km/s. The emission is tentatively resolved and extends ~3-4" over the sky (~2 beams), or ~500 AU at the distance of Taurus. The velocity offset, and to a lesser degree the spatial extent of the emission, shows that the line originates in the protostellar jet or wind. This result suggests that at least some water emission observed toward embedded sources, and perhaps also disk sources, with Herschel and Spitzer contains a wind or jet component, which is crucial for interpreting these data. These pathfinder observations done with the SMA opens a new window to studying the origin of water emission with e.g. ALMA, thus providing new insights into where water is in protostellar systems.

Water ice at the surface of HD 100546 disk

Water ice at the surface of HD 100546 disk

Authors:

Honda et al

Abstract:

We made near infrared multicolor imaging observations of a disk around Herbig Be star HD100546 using Gemini/NICI. K (2.2\,μm), H2O ice (3.06\,μm), and L'(3.8\,μm) disk images were obtained and we found the 3.1\,μm absorption feature in the scattered light spectrum, likely due to water ice grains at the disk surface. We compared the observed depth of the ice absorption feature with the disk model based on \cite{Oka2012} including water ice photodesorption effect by stellar UV photons. The observed absorption depth can be explained by the both disk models with/without photodesorption effect within the measurement accuracy, but slightly favors the model with photodesorption effects, implying that the UV photons play an important role on the survival/destruction of ice grains at the Herbig Ae/Be disk surface. Further improvement on the accuracy of the observations of the water ice absorption depth is needed to constrain the disk models.

How Much Water is TOO Much to be a Habitable World?

Water-rich planets: How habitable is a water layer deeper than on Earth?

Authors:

Noack et al

Abstract:

Water is necessary for the origin and survival of life as we know it. In the search for life-friendly worlds, water-rich planets therefore are obvious candidates and have attracted increasing attention in recent years. The surface H2O layer on such planets (containing a liquid water ocean and possibly high-pressure ice below a specific depth) could potentially be hundreds of kilometres deep depending on the water content and the evolution of the proto-atmosphere.

We study possible constraints for the habitability of deep water layers and introduce a new habitability classification relevant for water-rich planets (from Mars-size to super-Earth-size planets). A new ocean model has been developed that is coupled to a thermal evolution model of the mantle and core. Our interior structure model takes into account depth-dependent thermodynamic properties and the possible formation of high-pressure ice.

We find that heat flowing out of the silicate mantle can melt an ice layer from below (in some cases episodically), depending mainly on the thickness of the ocean-ice shell, the mass of the planet, the surface temperature and the interior parameters (e.g. radioactive mantle heat sources). The high pressure at the bottom of deep water–ice layers could also impede volcanism at the water–mantle boundary for both stagnant lid and plate tectonics silicate shells.

We conclude that water-rich planets with a deep ocean, a large planet mass, a high average density or a low surface temperature are likely less habitable than planets with an Earth-like ocean.

Water Rich SuperEarths Probably Form Around Oxygen Depleted Stars

Influence of the water content in protoplanetary discs on planet migration and formation

Authors:

Bitsh et al

Abstract:

The temperature and density profiles of protoplanetary discs depend crucially on the mass fraction of micrometre-sized dust grains and on their chemical composition. A larger abundance of micrometre-sized grains leads to an overall heating of the disc, so that the water ice line moves further away from the star. An increase in the water fraction inside the disc, maintaining a fixed dust abundance, increases the temperature in the icy regions of the disc and lowers the temperature in the inner regions. Discs with a larger silicate fraction have the opposite effect. Here we explore the consequence of the dust composition and abundance for the formation and migration of planets. We find that discs with low water content can only sustain outwards migration for planets up to 4 Earth masses, while outwards migration in discs with a larger water content persists up to 8 Earth masses in the late stages of the disc evolution. Icy planetary cores that do not reach run-away gas accretion can thus migrate to orbits close to the host star if the water abundance is low. Our results imply that hot and warm super-Earths found in exoplanet surveys could have formed beyond the ice line and thus contain a significant fraction in water. These water-rich super-Earths should orbit primarily around stars with a low oxygen abundance, where a low oxygen abundance is caused by either a low water-to-silicate ratio or by overall low metallicity.

Can The Trappist-1 Worlds Retain Water?

Water loss from Earth-sized planets in the habitable zones of ultracool dwarfs: Implications for the planets of TRAPPIST-1

Authors:

Bolmont et al

Abstract:

Ultracool dwarfs (UCD) encompass the population of extremely low mass stars (later than M6-type) and brown dwarfs. Because UCDs cool monotonically, their habitable zone (HZ) sweeps inward in time. Assuming they possess water, planets found in the HZ of UCDs have experienced a runaway greenhouse phase too hot for liquid water prior to entering the HZ. It has been proposed that such planets are desiccated by this hot early phase and enter the HZ as dry, inhospitable worlds. Here we model the water loss during this pre-HZ hot phase taking into account recent upper limits on the XUV emission of UCDs and using 1D radiation-hydrodynamic simulations. We address the whole range of UCDs but also focus on the planets b, c and d recently found around the 0.08 M⊙ dwarf TRAPPIST-1.

Despite assumptions maximizing the FUV-photolysis of water and the XUV-driven escape of hydrogen, we find that planets can retain significant amounts of water in the HZ of UCDs, with a sweet spot in the 0.04-0.06 M⊙ range. With our assumptions, TRAPPIST-1b and c can lose as much as 4 Earth Ocean but planet d - which may be inside the HZ depending on its actual period - may have kept enough water to remain habitable depending on its initial content. TRAPPIST-1 planets are key targets for atmospheric characterization and could provide strong constraints on the water erosion around UCDs.