Water-rich Planets Research Articles

TESS and ESPRESSO discover a super-Earth and a mini-Neptune orbiting the K-dwarf TOI-238

The number of super-Earth and mini-Neptune planet discoveries has increased significantly in the last two decades thanks to transit and radial velocity (RV) surveys. When it is possible to apply both techniques, we can characterise the internal composition of exoplanets, which in turn provides unique insights on their architecture, formation and evolution. We performed a combined photometric and RV analysis of TOI-238 (TYC 6398-132-1), which has one short-orbit super-Earth planet candidate announced by NASA’s TESS team. We aim to confirm its planetary nature using radial velocities taken with the ESPRESSO and HARPS spectrographs, to measure its mass, and to detect the presence of other possible planetary companions. We carried out a joint analysis by including Gaussian processes and Keplerian orbits to account for the stellar activity and planetary signals simultaneously. We detected the signal induced by TOI-238 b in the RV time series, and the presence of a second transiting planet, TOI-238 c, whose signal appears in RV and TESS data. TOI-238 b is a planet with a radius of 1.402−0.086+0.084 R⊕ and a mass of 3.40−0.45+0.46 M⊕. It orbits at a separation of 0.02118 ± 0.00038 au of its host star, with an orbital period of 1.2730988 ± 0.0000029 days, and has an equilibrium temperature of 1311 ± 28 K. TOI-238 c has a radius of 2.18 ± 0.18 R⊕ and a mass of 6.7 ± 1.1 M⊕. It orbits at a separation of 0.0749 ± 0.0013 au of its host star, with an orbital period of 8.465652 ± 0.000031 days, and has an equilibrium temperature of 696 ± 15 K. The mass and radius of planet b are fully consistent with an Earth-like composition, making it a likely rocky super-Earth. Planet c could be a water-rich planet or a rocky planet with a small H-He atmosphere.

TOI-2266 b: A keystone super-Earth at the edge of the M dwarf radius valley

We validate the Transiting Exoplanet Survey Satellite (TESS) object of interest TOI-2266.01 (TIC 8348911) as a small transiting planet (most likely a super-Earth) orbiting a faint M5 dwarf (V = 16.54) on a 2.33 d orbit. The validation is based on an approach where multicolour transit light curves are used to robustly estimate the upper limit of the transiting object's radius. Our analysis uses SPOC-pipeline TESS light curves from Sectors 24, 25, 51, and 52, simultaneous multicolour transit photometry observed with MuSCAT2, MuSCAT3' and HiPERCAM, and additional transit photometry observed with the LCOGT telescopes. TOI-2266 b is found to be a planet with a radius of 1.54 ± 0.09 R⊕, which locates it at the edge of the transition zone between rocky planets, water-rich planets, and sub-Neptunes (the so-called M dwarf radius valley). The planet is amenable to ground-based radial velocity mass measurement with red-sensitive spectrographs installed in large telescopes, such as MAROON-X and Keck Planet Finder (KPF), which makes it a valuable addition to a relatively small population of planets that can be used to probe the physics of the transition zone. Further, the planet's orbital period of 2.33 days places it inside a ‘keystone planet’ wedge in the period-radius plane where competing planet formation scenarios make conflicting predictions on how the radius valley depends on the orbital period. This makes the planet also a welcome addition to the small population of planets that can be used to test small-planet formation scenarios around M dwarfs.

A radius valley between migrated steam worlds and evaporated rocky cores

AbstractThe radius valley (or gap) in the observed distribution of exoplanet radii, which separates smaller super-Earths from larger sub-Neptunes, is a key feature that theoretical models must explain. Conventionally, it is interpreted as the result of the loss of primordial hydrogen and helium (H/He) envelopes atop rocky cores. However, planet formation models predict that water-rich planets migrate from cold regions outside the snowline towards the star. Assuming water to be in the form of solid ice in their interior, many of these planets would be located in the radius gap contradicting observations. Here we use an advanced coupled formation and evolution model that describes the planets’ growth and evolution starting from solid, moon-sized bodies in the protoplanetary disk to mature Gyr-old planetary systems. Employing new equations of state and interior structure models to treat water as vapour mixed with H/He, we naturally reproduce the valley at the observed location. The model results demonstrate that the observed radius valley can be interpreted as the separation of less massive, rocky super-Earths formed in situ from more massive, ex situ, water-rich sub-Neptunes. Furthermore, the occurrence drop at larger radii, the so-called radius cliff, is matched by planets with water-dominated envelopes. Our statistical approach shows that the synthetic distribution of radii quantitatively agrees with observations for close-in planets, but only if low-mass planets initially containing H/He lose their atmosphere due to photoevaporation, which populates the super-Earth peak with evaporated rocky cores. Therefore, we provide a hybrid theoretical explanation of the radius gap and cliff caused by both planet formation (orbital migration) as well as evolution (atmospheric escape).

Spright: a probabilistic mass–density–radius relation for small planets

ABSTRACT We present spright, a python package that implements a fast and lightweight mass–density–radius relation for small planets. The relation represents the joint planetary radius and bulk density probability distribution as a mean posterior predictive distribution of an analytical three-component mixture model. The analytical model, in turn, represents the probability for the planetary bulk density as three generalized Student’s t-distributions with radius-dependent weights and means based on theoretical composition models. The approach is based on Bayesian inference and aims to overcome the rigidity of simple parametric mass–radius relations and the danger of overfitting of non-parametric mass–radius relations. The package includes a set of pre-trained and ready-to-use relations based on two M-dwarf catalogues, one catalogue containing stars of spectral types F, G, and K (FGK stars), and two theoretical composition models for water-rich planets. The inference of new models is easy and fast, and the package includes a command line tool that allows for coding-free use of the relation, including the creation of publication-quality plots. Additionally, we study whether the current mass and radius observations of small exoplanets support the presence of a population of water-rich planets positioned between rocky planets and sub-Neptunes. The study is based on Bayesian model comparison and shows somewhat strong support against the existence of a water-world population around M dwarfs. However, the results of the study depend on the chosen theoretical water-world density model. A more conclusive result requires a larger sample of precisely characterized planets and community consensus on a realistic water-world interior structure and atmospheric composition model.

Planet formation throughout the Milky Way

As stellar compositions evolve over time in the Milky Way, so will the resulting planet populations. In order to place planet formation in the context of Galactic chemical evolution, we made use of a large (N = 5325) stellar sample representing the thin and thick discs, defined chemically, and the halo, and we simulated planet formation by pebble accretion around these stars. We built a chemical model of their protoplanetary discs, taking into account the relevant chemical transitions between vapour and refractory minerals, in order to track the resulting compositions of formed planets. We find that the masses of our synthetic planets increase on average with increasing stellar metallicity [Fe/H] and that giant planets and super-Earths are most common around thin-disc (α-poor) stars since these stars have an overall higher budget of solid particles. Giant planets are found to be very rare (≲1%) around thick-disc (α-rich) stars and nearly non-existent around halo stars. This indicates that the planet population is more diverse for more metal-rich stars in the thin disc. Water-rich planets are less common around low-metallicity stars since their low metallicity prohibits efficient growth beyond the water ice line. If we allow water to oxidise iron in the protoplanetary disc, this results in decreasing core mass fractions with increasing [Fe/H]. Excluding iron oxidation from our condensation model instead results in higher core mass fractions, in better agreement with the core-mass fraction of Earth, that increase with increasing [Fe/H]. Our work demonstrates how the Galactic chemical evolution and stellar parameters, such as stellar mass and chemical composition, can shape the resulting planet population.

Where are the Water Worlds?: Self-consistent Models of Water-rich Exoplanet Atmospheres

It remains to be ascertained whether sub-Neptune exoplanets primarily possess hydrogen-rich atmospheres or whether a population of H2O-rich water worlds lurks in their midst. Addressing this question requires improved modeling of water-rich exoplanetary atmospheres, both to predict and interpret spectroscopic observations and to serve as upper boundary conditions on interior structure calculations. Here, we present new models of hydrogen-helium-water atmospheres with water abundances ranging from solar to 100% water vapor. We improve upon previous models of high-water-content atmospheres by incorporating updated prescriptions for water self-broadening and a nonideal gas equation of state. Our model grid (https://umd.box.com/v/water-worlds) includes temperature–pressure profiles in radiative-convective equilibrium, along with their associated transmission and thermal emission spectra. We find that our model updates primarily act at high pressures, significantly impacting bottom-of-atmosphere temperatures, with implications for the accuracy of interior structure calculations. Upper-atmosphere conditions and spectroscopic observables are less impacted by our model updates, and we find that, under most conditions, retrieval codes built for hot Jupiters should also perform well on water-rich planets. We additionally quantify the observational degeneracies among both thermal emission and transmission spectra. We recover standard degeneracies with clouds and mean molecular weight for transmission spectra, and we find thermal emission spectra to be more readily distinguishable from one another in the water-poor (i.e., near-solar) regime.

Electrolyte Permeability in Plastic Ice VII.

Deep brines in water-rich planets form when electrolytes diffuse from the rocky interior through layers of thick dense ice such as ice VII and the hypothesized plastic ice VII. We perform classical molecular dynamics simulations of Li+, Na+, and K+ alkali ions and F- and Cl- halide ions in plastic ice VII at conditions similar to water-rich super-Earths, icy moons, and ocean worlds. We find that plastic ice VII is permeable to electrolytes on geological timescales. Diffusion occurs via jumps between adjacent voids in the bcc crystal structure and is governed by molecular rotations. An exception to this mechanism is Na+ which, at variance with other ions, can substitute water molecules on lattice positions. The bulk modulus of pristine plastic ice VII is dependent on the pace of molecular rotations: when the rotations are slow, the bulk modulus is 1 order of magnitude lower compared to the bulk modulus at conditions of fast rotations, hence providing direct evidence of the role of molecular rotations in determining elastic properties. Electrolytes affect the bulk modulus only at high-concentration conditions and slow molecular rotations. Our results show that plastic ice VII may facilitate the development of brines in water-rich planets and ocean worlds, with a clear significance for their potential to support exobiology and for the chemical evolution of their aqueous reservoirs.

Traveling Planetary-scale Waves Cause Cloud Variability on Tidally Locked Aquaplanets

Cloud cover at the planetary limb of water-rich Earth-like planets is likely to weaken chemical signatures in transmission spectra, impeding attempts to characterize these atmospheres. However, based on observations of Earth and Solar System worlds, exoplanets with atmospheres should have both short-term weather and long-term climate variability, implying that cloud cover may be less during some observing periods. We identify and describe a mechanism driving periodic clear sky events at the terminators in simulations of tidally locked Earth-like planets. A feedback between dayside cloud–radiative effects, incoming stellar radiation and heating, and the dynamical state of the atmosphere, especially the zonal wavenumber 1 Rossby wave identified in past work on tidally locked planets, leads to oscillations in Rossby wave phase speeds and in the position of Rossby gyres, and this results in advection of clouds to or away from the planet’s eastern terminator. We study this oscillation in simulations of Proxima Centauri b, TRAPPIST-1e, and rapidly rotating versions of these worlds located at the inner edge of their stars’ habitable zones. We simulate time series of the transit depths of the 1.4 μm water feature and 2.7 μm carbon dioxide feature. The impact of atmospheric variability on the transmission spectra is sensitive to the structure of the dayside cloud cover and the location of the Rossby gyres, but none of our simulations have variability significant enough to be detectable with current methods.

TOI-1695 b: A Water World Orbiting an Early-M Dwarf in the Planet Radius Valley

Characterizing the bulk compositions of transiting exoplanets within the M dwarf radius valley offers a unique means to establish whether the radius valley emerges from an atmospheric mass-loss process or is imprinted by planet formation itself. We present the confirmation of such a planet orbiting an early-M dwarf (T mag = 11.0294 ± 0.0074, M s = 0.513 ± 0.012 M ⊙, R s = 0.515 ± 0.015 R ⊙, and T eff = 3690 ± 50 K): TOI-1695 b (P = 3.13 days and ). TOI-1695 b’s radius and orbital period situate the planet between model predictions from thermally driven mass loss versus gas depleted formation, offering an important test case for radius valley emergence models around early-M dwarfs. We confirm the planetary nature of TOI-1695 b based on five sectors of TESS data and a suite of follow-up observations including 49 precise radial velocity measurements taken with the HARPS-N spectrograph. We measure a planetary mass of 6.36 ± 1.00 M ⊕, which reveals that TOI-1695 b is inconsistent with a purely terrestrial composition of iron and magnesium silicate, and instead is likely a water-rich planet. Our finding that TOI-1695 b is not terrestrial is inconsistent with the planetary system being sculpted by thermally driven mass loss. We present a statistical analysis of seven well-characterized planets within the M dwarf radius valley demonstrating that a thermally driven mass-loss scenario is unlikely to explain this population.

Terminator Habitability: The Case for Limited Water Availability on M-dwarf Planets

Rocky planets orbiting M-dwarf stars are among the most promising and abundant astronomical targets for detecting habitable climates. Planets in the M-dwarf habitable zone are likely synchronously rotating, such that we expect significant day–night temperature differences and potentially limited fractional habitability. Previous studies have focused on scenarios where fractional habitability is confined to the substellar or “eye” region, but in this paper we explore the possibility of planets with terminator habitability, defined by the existence of a habitable band at the transition between a scorching dayside and a glacial nightside. Using a global climate model, we show that for water-limited planets it is possible to have scorching temperatures in the “eye” and freezing temperatures on the nightside, while maintaining a temperate climate in the terminator region, due to reduced atmospheric energy transport. On water-rich planets, however, increasing the stellar flux leads to increased atmospheric energy transport and a reduction in day–night temperature differences, such that the terminator does not remain habitable once the dayside temperatures approach runaway or moist greenhouse limits. We also show that while water-abundant simulations may result in larger fractional habitability, they are vulnerable to water loss through cold trapping on the nightside surface or atmospheric water vapor escape, suggesting that even if planets were formed with abundant water, their climates could become water-limited and subject to terminator habitability.

Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars.

Exoplanets smaller than Neptune are common around red dwarf stars (M dwarfs), with those that transit their host star constituting the bulk of known temperate worlds amenable for atmospheric characterization. We analyze the masses and radii of all known small transiting planets around M dwarfs, identifying three populations: rocky, water-rich, and gas-rich. Our results are inconsistent with the previously known bimodal radius distribution arising from atmospheric loss of a hydrogen/helium envelope. Instead, we propose that a density gap separates rocky from water-rich exoplanets. Formation models that include orbital migration can explain the observations: Rocky planets form within the snow line, whereas water-rich worlds form outside it and later migrate inward.

Temperature- and pressure-dependence of the hydrogen bond network in plastic ice VII.

We model, via classical molecular dynamics simulations, the plastic phase of ice VII across a wide range of the phase diagram of interest for planetary investigations. Although structural and dynamical properties of plastic ice VII are mostly independent on the thermodynamic conditions, the hydrogen bond network (HBN) acquires a diverse spectrum of topologies distinctly different from that of liquid water and of ice VII simulated at the same pressure. We observe that the HBN topology of plastic ice carries some degree of similarity with the crystal phase, stronger at thermodynamic conditions proximal to ice VII, and gradually lessening when approaching the liquid state. Our results enrich our understanding of the properties of water at high pressure and high temperature and may help in rationalizing the geology of water-rich planets.

On the Stability of Low-mass Planets with Supercritical Hydrospheres

Short-period, low-mass water-rich planets are subject to strong irradiation from their host star, resulting in hydrospheres in a supercritical state. In this context, we explore the role of irradiation on small terrestrial planets that are moderately wet in the low-mass regime (0.2–1 M ⊕). We investigate their bulk properties for water content in the 0.01–5% range by making use of an internal structure model that is coupled to an atmosphere model. This coupling allows us to take into account both the compression of the interior due to the weight of the hydrosphere and the possibility of atmospheric instability in the low-mass regime. We show that, even for low masses and low water content, these planets display inflated atmospheres. For extremely low planetary masses and high irradiation temperatures, we find that steam atmospheres become gravitationally unstable when the ratio η of their scale height to planetary radius exceeds a critical value of ∼0.1. This result is supported by observational data, as all currently detected exoplanets exhibit values of η smaller than 0.013. Depending on their water content, our results show that highly irradiated, low-mass planets up to 0.9 M ⊕ with significative hydrospheres are not in a stable form and should lose their volatile envelope.

Internal structures and magnetic moments of rocky planets

Context. For a planet to be considered habitable on its surface, it is an important advantage for it to have a magnetic field that protects its atmosphere from stellar winds as well as cosmic rays. The magnetic protection of potentially habitable planets plays a key role in determining the chances of detecting atmospheric biosignatures. In making an estimate of a planet’s magnetic field, its internal structure must first be known. Aims. This study proposes to use the Preliminary Reference Earth Model (PREM) internal structure as the base of a numerical model, as it is considered the best available option for estimating the internal structure of rocky exoplanets. With this model, we estimate the magnetic properties of dry and water-rich Earth-like and Super-Earth-like planets and we apply it to the first 176 planets of this kind confirmed by the Transiting Exoplanet Survey Satellite exoplanet mission (TESS). Methods. Using PREM as a reference, we estimated the internal structure of dry and water-rich rocky planets. This model provides an estimation of the average density and core size of the planet, which are the key ingredients for estimating its magnetic moment, depending on whether it is tidally locked or not. Our model estimates the thermodynamic variables as a function of pressure and including saltwater as a component of water-rich exoplanets. In addition, we did not use the perfect layer differentiation approximation. We validated our model based on the selection of planets and satellites in the Solar System with similar characteristics. Results. Using our model, we estimated the internal structure and magnetic moment of selected dry and water-rich rocky planets and satellites in the Solar system. The differences with the observed values in the internal structure characteristics, mass, average density, moment of inertia factor, and local Rossby number are remarkably low or even negligible. The estimated magnetic moments are also very similar to the observed ones. We applied the model to the first dry and water-rich rocky planets confirmed by TESS, 176 in total, and we find, from an astrobiological perspective, that TOI-700 d and TOI-2257 b are the most interesting examples, as they are located in the habitable zone. However, we note that their magnetic moments are only about 0.01 of the Earth’s magnetic moment.

The Radiation Environment of Ceres and Implications for Surface Sampling.

Ceres is a large water-rich dwarf planet located within the asteroid belt. Its surface displays evidence of material sourced from a deep subsurface liquid brine layer within recent geologic time, making it a candidate ocean world with possible present-day activity. However, Ceres lacks a substantial atmosphere and likely does not possess a global magnetic field. Therefore, any material emplaced or exposed on the surface will be subject to weathering by charged particles of solar and galactic origin. We have evaluated the effect of charged particle radiation on material within the near-surface of Ceres and find that the timescale for radiation-induced modification and destruction of organics and endogenic material is ∼100 Myr to 1 Gyr within the top 10-20 cm of the surface. Furthermore, we find that the timescale for sterilization of any putative living organisms contained within material at these depths is <500 kyr. Future missions to the surface may therefore consider targeting regions with geologic ages that fall between these two timescales to avoid the risk of backward contamination while ensuring that sampled material is not heavily radiation processed.

Structure and properties of two superionic ice phases

In the phase diagram of water, superionic ices with highly mobile protons within the stable oxygen sublattice have been predicted at high pressures. However, the existence of superionic ices and the location of the melting line have been challenging to determine from both theory and experiments, yielding contradictory results depending on the employed techniques and the interpretation of the data. Here we report high-pressure and high-temperature synchrotron X-ray diffraction and optical spectroscopy measurements of water in a laser-heated diamond anvil cell and reveal first-order phase transitions to ices with body-centred and face-centred cubic oxygen lattices. Based on the distinct density, increased optical conductivity and the greatly decreased fusion enthalpies, we assign these observed structures to the theoretically predicted superionic ice phases. Our measurements determine the pressure–temperature stability fields of superionic ice phases and the melting line, suggesting the presence of face-centred cubic superionic ice in water-rich giant planets, such as Neptune and Uranus. The melting line determined here is at higher temperatures than previously determined in static compression experiments, but it is in agreement with theoretical calculations and data from shock-wave experiments.

How deep is the ocean? Exploring the phase structure of water-rich sub-Neptunes

ABSTRACT Understanding the internal structures of planets with a large H2O component is important for the characterization of sub-Neptune planets. The finding that the mini-Neptune K2-18b could host a liquid water ocean beneath a mostly hydrogen envelope motivates a detailed examination of the phase structures of water-rich planets. To this end, we present new internal structure models for super-Earths and mini-Neptunes that enable detailed characterization of a planet’s water component. We use our models to explore the possible phase structures of water worlds and find that a diverse range of interiors are possible, from oceans sandwiched between two layers of ice to supercritical interiors beneath steam atmospheres. We determine how the bulk properties and surface conditions of a water world affect its ocean depth, finding that oceans can be up to hundreds of times deeper than on Earth. For example, a planet with a 300 K surface can possess H2O oceans with depths from 30–500 km, depending on its mass and composition. We also constrain the region of mass–radius space in which planets with H/He envelopes could host liquid H2O, noting that the liquid phase can persist at temperatures up to 647 K at high pressures of 218–$7\times 10^4$ bar. Such H/He envelopes could contribute significantly to the planet radius while retaining liquid water at the surface, depending on the planet mass and temperature profile. Our findings highlight the exciting possibility that habitable conditions may be present on planets much larger than Earth.

Mass–Radius Relationships for Irradiated Ocean Planets

Abstract Massive and water-rich planets should be ubiquitous in the universe. Many of these worlds are expected to be subject to important irradiation from their host star, and display supercritical water layers surrounded by extended steam atmospheres. Irradiated ocean planets with such inflated hydrospheres have been recently shown to be good candidates for matching the mass–radius distribution of sub-Neptunes. Here we describe a model that computes a realistic structure for water-rich planets by combining an interior model with an updated equation of state (EOS) for water, and an atmospheric model that takes into account radiative transfer. We find that the use of inappropriate EOSs can lead to the overestimation of the planetary radius by up to ∼10%, depending on the planet size and composition. Our model has been applied to the GJ 9827 system as a test case and indicates Earth- or Venus-like interiors for planets b and c, respectively. Planet d could be an irradiated ocean planet with a water mass fraction (WMF) of ∼20% ± 10%. We also provide fits for the mass–radius relationships, allowing one to directly retrieve a wide range of planetary compositions, without the requirement to run the model. Our calculations finally suggest that highly irradiated planets lost their H/He content through atmospheric loss processes, and that the leftover material led to either super-Earths or sub-Neptunes, depending on the WMF.

Atomic-scale mixing between MgO and H2O in the deep interiors of water-rich planets

Water-rich planets exist in our Solar System (Uranus and Neptune) and are found to be common in the extrasolar systems (some of the sub-Neptunes). In conventional models of these planets a thick water-rich layer is underlain by a separate rocky interior. Here we report experimental results on two rock-forming minerals, olivine ((Mg,Fe)2SiO4) and ferropericlase ((Mg,Fe)O), in water at the pressure and temperature conditions expected for the water-rich planets. Our data indicate a selective leaching of MgO, which peaks between 20 and 40 GPa and above 1,500 K. For water-rich planets with 1–6 Earth masses (>50 wt% H2O), the chemical reaction at the deep water–rock interface would lead to high concentrations of MgO in the H2O layer. For Uranus and Neptune, the top ~3% of the H2O layer would have a large storage capacity for MgO. If an early dynamic process enables the rock–H2O reaction, the topmost H2O layer may be rich in MgO, possibly affecting the thermal history of the planet.

A fuzzy Multi-Criteria Decision Making approach for Exo-Planetary Habitability

Nowadays, we know thousands of exoplanets, some of them potentially habitable. Next technological facilities (JWST, for example) have exoplanet atmosphere analysis capabilities, but they also have limits in terms of how many targets can be studied. Therefore, there is a need to rank and prioritize these exoplanets with the aim of searching for biomarkers. Some criteria involved, such as the habitability potential of a dry-rock planet versus a water-rich planet, or a potentially-locked planet versus a tidally-locked planet, are often vague and the use of the fuzzy set theory is advisable. We have applied a combination of Multi-Criteria Decision-Making methodologies with fuzzy logic, the Fuzzy Reference Ideal Method (FRIM), to this problem. We have analyzed the habitability potential of 1798 exoplanets from TEPCat database based on a set of criteria (composition, atmosphere, energy, tidal locking, type of planet and liquid water), in terms of their similarity to the only ideal alternative, The Earth. Our results, when compared with the probability index SEPHI, indicate that Kepler-442b, Kepler-062e/f, and LHS_1140b are the best exoplanets for searching for biomarkers, regardless its technical difficulty. If we take into account current technical feasibility, the best candidate is TRAPPIST-1e.