Several Decadal-level questions in astrophysics, exoplanets, astrobiology, and cosmology can be addressed only at low radio frequencies inaccessible from Earth. The MegaWave Radio Surveyor would open this largely-unexplored region of the electromagnetic spectrum with a space-based interferometer to (1) Track the space weather of other stars; (2) Detect magnetically-generated emission from exoplanets to probe their interiors and assess magnetic shielding of their atmospheres; (3) Probe the Universe's evolution during the Dark Ages via the highly-redshifted HI hyperfine line; and (4) Assess the role of cosmic rays and magnetic fields in the cosmic web. An Astrophysics Strategic Technology Research Accelerator (ASTRA) Initiative concept, the MegaWave Radio Surveyor's science objectives respond to the Pathways to Discovery Decadal Survey and three other National Academies studies, and it would serve as a Formative Era mission in the Enduring Quests, Daring Visions roadmap. Developments in U.S. space industries enable this observatory to be realized. The MegaWave Radio Surveyor would offer a versatile, scalable, and resilient architecture capable of sensitive and simultaneous observations below 45 MHz and unprecedented angular resolution at these frequencies. The concept builds upon NASA's Sun Radio Interferometer Space Experiment (SunRISE), Star-Planet Activity Research CubeSat (SPARCS), and Lunar Surface Electromagnetics Experiment (LuSEE-Night). The MegaWave Radio Surveyor could leverage multiple elements of the Artemis program, such as access to and beyond cislunar space and communications, and there are opportunities to infuse new autonomy/AI modes for mission operations. By opening one of the last windows in the electromagnetic spectrum and pioneering space interferometry at unprecedented scales, the MegaWave Radio Surveyor would establish a transformational capability.

We present the characterization of two planetary systems orbiting the M dwarfs TOI-4336 A (M3.5V) and TOI-4342 (M0V), each hosting two transiting planets previously validated with TESS and ground-based observations. We refined the photometry of the TOI-4342 system using TESS and LCOGT data, and characterized the host stars with NIRPS and ESPRESSO spectroscopy. High-precision ESPRESSO radial velocities allowed us to constrain the planetary masses and investigate their potential compositions. The TOI-4336 A system is composed of a sub-Neptune with a period of 16.34 days, a radius of $2.14 \pm 0.08$ Re, and a mass of $3.33 \pm 0.36$ Me, along with an inner super-Earth on a 7.59-day orbit with a radius of $1.25 \pm 0.07$ Re and a mass of $1.55 \pm 0.13$ Me. The TOI-4342 system hosts two sub-Neptunes of similar sizes ($2.33 \pm 0.09$ Re and $2.35 \pm 0.09$ Re), with periods of 5.54 and 10.69 days. Their masses are measured to be $7.3 \pm 1.3$ Me and $4.8 \pm 1.4$ Me, respectively. The RVs also reveal a planet candidate around TOI-4342, likely non-transiting, with a period of 47.5 days and a minimum mass of $17.8 \pm 3.0$ Me. With precise radii and masses, we derived bulk densities and explored possible compositions. The TOI-4336 A sub-Neptune and super-Earth have densities of $1.87 \pm 0.30$ and $4.35 \pm 0.79$ g cm$^{-3}$, while the two similar-sized sub-Neptunes in TOI-4342 show distinct densities of $3.18 \pm 0.67$ and $2.01 \pm 0.63$ g cm$^{-3}$. All four planets are excellent targets for future atmospheric characterization with JWST, and their multi-planet nature makes them especially interesting for comparative planetology. Notably, TOI-4336 A b stands out as one of the best-known targets in its size and temperature regime, with a TSM of 138, comparable to benchmark planets such as K2-18 b and LHS 1140 b.

The interior structure and bulk composition of giant planets are not directly observable and must be inferred from models. Under the common assumption of a well-mixed, adiabatic envelope, the measured atmospheric metallicity is taken as a proxy for the metallicity of the entire envelope, and hence for the planet's heavy-element budget. JWST now provides precise atmospheric metallicities for a growing number of warm giants, allowing this assumption to be tested for the first time. We quantify the difference between envelope and bulk metallicities of warm giants to assess the evidence for compositional stratification. We assembled eleven warm giants with atmospheric metallicities from published JWST retrievals, computed tailored interior and thermal evolution model grids for each, and performed MCMC retrievals to infer the bulk metallicity consistent with the measured mass, radius, system age, and atmospheric metallicity. Envelope metallicities are smaller than bulk metallicities throughout the sample, with mixing ratios from about 0.02 to 0.90. Eight of the eleven planets have mixing ratios below 0.50, and ten are inconsistent with a fully mixed interior to within one sigma. We tentatively identify a significant anti-correlation between planetary mass and envelope metallicity, but no correlation between envelope and bulk metallicity, nor between envelope and host-star metallicity. Atmospheric metallicity is therefore not a reliable proxy for the bulk composition of warm giants, and incomplete mixing (possibly composition gradients) appears common among the planets accessible to JWST. Bulk composition estimates assuming a homogeneous envelope substantially underestimate the total heavy-element mass. That the solar-system giants are unremarkable within this sample suggests dilute or partially mixed interiors may be a generic outcome of giant planet formation.

The composition and temperature-pressure profile of the atmospheres of Uranus and Neptune are not well-determined. As observational data are limited, we often rely on chemical equilibrium computations to infer atmospheric abundances and cloud formation. The inferred atmospheric structures, however, strongly depend on several fundamental assumptions such as the elemental abundances and ratios, the condensation properties of the assumed species, or a reference temperature for the adiabatic structure. In this study we investigate the effects of different metallicities (1 to 80 solar), element ratios (C/O and S/N, from 0.1 to 2 and 0.19 to 1.6) and 1 bar temperatures (66 to 86 K) on the vertical structure of ice giant atmospheres. In particular, we use the chemical equilibrium code to derive mixing ratios and cloud structures for CH_4, NH_3, H_2S, H_2O and NH_4SH. We find that the models are very sensitive to the assumed parameters, yielding drastically different possible atmospheric structures. For the cases considered here, we find that mixing ratios and cloud deck altitudes can vary by more than an order of magnitude. Additionally, thermal profiles can differ by several tens of kelvins due to composition and 1-bar temperature. We advise that future ground-based observations and a dedicated mission to Uranus and/or Neptune are required to better characterize the atmospheric structure and composition of ice giants.

The growing sample of giant exoplanets around M dwarf stars (GEMS) helps probe the extremes of giant planet formation. Comparing the properties of this sample with their FGK counterparts can help us understand how planet formation and migration depend on stellar mass. We initiated a large Cycle 2 JWST transmission spectroscopy survey of seven GEMS. Here we present the atmospheric characterization using two JWST transits of TOI-5293 A b, a 0.5 M _Jup planet orbiting an early M dwarf with a period of ∼3 days. The two NIRSpec/PRISM transits indicate the planet is eclipsing a rapidly changing (heterogeneous) stellar photosphere. We see that Visit 1 had heterogeneity crossings across the entire transit chord, rendering inferences from it to be unreliable. The Visit 1 spectrum exhibits a downward slope at <1 μ m suggestive of stellar contamination from faculae. In contrast, for Visit 2 we are able to model the heterogeneity crossings and obtain a transmission spectrum free from stellar contamination. We therefore limit our conclusions to a detailed analysis of Visit 2, and using Bayesian free chemistry retrievals, we find a low atmospheric metallicity ( $\mathrm{log}[{\rm{M}}/{\rm{H}}]=-1.0{3}_{-0.44}^{+0.53}$ × solar) and supersolar C/O ratio ( $1.2{3}_{-0.75}^{+2.94}$ ). The retrievals yield Bayes factors that indicate strong evidence for CH _4 as well as low-significance detections of CO _2 , H _2 O, and NH _3 . Finally, using thermal evolution models we find that the radius of TOI-5293 A b is inflated above theoretical expectations (∼1.07 R _Jup ), despite it having an temperature of ∼700 K, and hence we were unable to constrain its bulk composition.

The shapes of fluid planets bear the signatures of rotational flattening and atmospheric flows. Precise knowledge of their shapes and wind profiles may therefore reveal their interior rotation rates. We reexamine this idea for the ice giants, where missions like the Uranus Orbiter and Probe could use radio occultations to measure atmospheric heights near 1 bar at multiple latitudes, complementing Voyager 2's near-equatorial occultation for Uranus. Applying geodetic calculations and considering zonal wind uncertainties, we find that only a narrow range for Uranus's 1-bar polar radius, Rpol = 24968.6 +/- 4.7 km, is consistent with Uranus's winds, occultations, and gravity field, even treating Uranus's interior spin as a free parameter. This is because the isobaric shape depends on the total rotation of the isobaric surface, which is already well constrained by observations, irrespective of what portion is attributed to bulk rotation versus winds. Occultations will, however, be valuable for testing our underlying assumption that the winds manifest the full differential rotation that sets the shape. The apparent north-south asymmetry in Uranus's winds, if permanent, produces a 5 km difference between the northern and southern polar radii, measurable with suitable radio occultations. Neptune's much more uncertain winds yield similar to 100 km variations in polar and equatorial radii. We confirm that Uranus and Neptune's magnetic rotation periods yield nonzero mean dynamical heights for their atmospheres. Accurate results for Uranus and Neptune require that the full latitude-dependent rotation be incorporated when fitting radii from occultations. Only significantly faster interior rotation-periods close to 15 hr in both Uranus and Neptune-would minimize their dynamical heights.

Context. Characterizing the masses, radii, and compositions of small planets orbiting M dwarfs is key to understanding their formation and identifying the best targets for atmospheric follow-up with facilities such as JWST. Aims. We present the characterization of two planetary systems orbiting the M dwarfs TOI-4336 A (M3.5V) and TOI-4342 (M0V), each hosting two transiting planets previously validated with TESS and ground-based observations. Methods. We refined the photometry of the TOI-4342 system using TESS and LCOGT data, and characterized the host stars with NIRPS and ESPRESSO spectroscopy. High-precision ESPRESSO radial velocities (RVs) allowed us to constrain the planetary masses and investigate their potential compositions. Results. The TOI-4336 A system is composed of a sub-Neptune with a period of 16.34 days, a radius of 2.14 ± 0.08 R⊕, and a mass of 3.33 ± 0.36 M⊕, along with an inner super-Earth on a 7.59-day orbit with a radius of 1.25 ± 0.07 R⊕ and a mass of 1.55 ± 0.13 M⊕. The TOI-4342 system hosts two sub-Neptunes of similar sizes (2.33 ± 0.09 R⊕ and 2.35 ± 0.09 R⊕), with periods of 5.54 and 10.69 days. Their masses are measured to be 7.3 ± 1.3 M⊕ and 4.8 ± 1.4 M⊕, respectively. The RVs also reveal a planet candidate around TOI-4342, most likely non-transiting, with a period of 47.5 days and a minimum mass of 17.8 ± 3.0 M⊕. Conclusions. With precise radii and masses, we derived bulk densities and explored possible compositions. The TOI-4336 A subNeptune and super-Earth have densities of 1.87 ± 0.30 and 4.35 ± 0.79 g cm−3, while the two similar-sized sub-Neptunes in TOI-4342 show distinct densities of 3.18 ± 0.67 and 2.01 ± 0.63 g cm−3. Using an inference model, we find that TOI-4336 A b, TOI-4342 b, and TOI-4342 c have an atmosphere mass fraction (AMF) of ∼3.7%, ∼1.8%, and ∼2.9%, respectively, while the super-Earth TOI-4336 A c could contain ∼2% of water or have a core-to-mass fraction (CMF) of ∼31%. All four planets are excellent targets for future atmospheric characterization with JWST, and their multi-planet nature makes them especially interesting for comparative planetology. Notably, TOI-4336 A b stands out as one of the best known targets in its size and temperature regime, with a transmission spectroscopy metric (TSM) of 138, comparable to benchmark planets such as K2-18 b and LHS 1140 b. Its inner sibling, TOI-4336 A c, may also be of interest for emission spectroscopy and exploring the “cosmic shoreline”, similarly to the Rocky Worlds DDT JWST program.

Structure models of sub-Neptunes commonly assume purely adiabatic interiors with distinct layers of homogeneous composition. We assess how allowing for more complex interiors with composition gradients affects the inferred internal structure and bulk compositions of the sub-Neptunes K2-18 b and TOI-270 d. We compare purely adiabatic models with distinct layers against models that include composition gradients and stable non-convective regions. We present solutions that are consistent with the observed masses, radii, and atmospheric boundary conditions. We find that composition gradients significantly increase the range of viable interior structures: For example, the interior degeneracy remains substantial even for fixed mass and radius and the maximum hydrogen-helium (H-He) mass fraction can increase by up to a factor of five for K2-18 b. We further show that correlations, such as those between the H-He abundance and the ice-to-rock or rock-to-iron ratios, can weaken when composition gradients are introduced. For example, the Spearman rank correlation coefficient between the H-He and iron abundances decreases from ∼0.7 to ∼0.4 for TOI-270 d in our models. Purely adiabatic models underestimate the range of plausible compositions and overestimate the impact of more precise measurements, atmosphere models, and host star constraints on a more accurate characterization. We suggest that interior models of sub-Neptunes should by default include more complex interiors, such as composition gradients and non-convective regions, when using data for interpreting the planetary structure, formation, and evolution.

Super-Earths and sub-Neptunes represent the most common class of exoplanets discovered to date in our galaxy, yet they have no direct analogues in the Solar System. Since 2014, researchers within the NCCR PlanetS have made significant contributions to understanding the origin and nature of these small planets. This chapter provides an overview of the progress made in their detection, characterization, and theoretical interpretation during the 2014-2025 period. The combined data from space-based photometric missions such as Kepler and TESS, together with ground-based radial velocity campaigns using state-of-the-art spectrographs (e.g., HARPS, ESPRESSO, NIRPS), have enabled detailed demographic analyses of these planets. These observational efforts are complemented by theoretical work exploring their internal structures, bulk compositions, formation and evolution, shedding light on the physical processes responsible for the observed diversity. As high-precision observations from facilities like JWST begin to probe the atmospheric composition of individual planets, a more complete picture of super-Earth and sub-Neptune origins is emerging, one that continues to challenge and refine current planet formation theories.

HATS-75 b is one of the recently discovered Giant Exoplanets Around M-dwarf Stars (GEMS) with a transmission spectrum shaped by both its atmosphere and the active stellar surface it transits. As part of a JWST program studying seven GEMS, we observed three transits of HATS-75 b with the NIRSpec PRISM instrument (0.6-5.3 mu m). The planet's spectra exhibit a slightly larger transit depth at shorter wavelengths, indicative of hazes or stellar contamination due to stellar heterogeneities outside the transit chord, i.e., the transit light source (TLS) effect. While both a hazy atmospheric model or TLS model can replicate the transmission spectrum, independent evidence (e.g., stellar rotation, spot-crossing events) favors a model that includes contamination from unocculted starspots and faculae. Within this stellar heterogeneity/TLS-based framework, atmospheric retrievals yield remarkably low atmospheric metallicity ( log[M/H]=-1.74-0.76+0.92 ) and supersolar carbon-to-oxygen ( C/O=1.04-0.09+0.40 ), which paired with a best-fit interior model with bulk metallicity of Zp = 0.20 +/- 0.04 implies poor vertical mixing within the planet. Retrievals also detect robust absorption signatures of CH4, CO, and CO2. We obtain only an upper limit for H2O, consistent with its atmospheric spectral features being masked by stellar contamination. These results underscore the importance of accounting for stellar heterogeneity when interpreting exoplanet transmission spectra and highlight HATS-75 b as a significant asset to our understanding of giant exoplanets around M dwarfs with JWST.

The radiative opacity plays a critical role in shaping the thermal evolution and interior structure of giant planets. Near ∼ 2,000 K, a hydrogen-transparency region creates a window of reduced opacity that can give rise to detached, deep radiative zones between two convective zones. This local opacity minimum could be deepened further by alkali depletion. While such zones have been explored for Jupiter and Saturn, their influence on cold to warm giant exoplanets remains unstudied. We investigate how opacity windows and the resulting deep radiative zones affect the cooling, radius evolution, and characterisation of giant exoplanet interiors and atmospheres. We computed thermal evolution models for cold to warm Jupiters spanning masses of 0.3 to 4.0 M_J, envelope metallicities from one to ten times solar, equilibrium temperatures of 100 to 800 K, and a parametrised reduction in the radiative opacity. Detached deep radiative zones develop in moderately irradiated Jupiters older than a few gigayears even with unmodified opacities, and earlier and more extensively when the opacity is reduced. The age and equilibrium temperature at which they appear depend on planetary mass, envelope metallicity, and opacity, with metal enrichment suppressing them at low equilibrium temperatures but promoting them at higher ones. A deep opacity window accelerates cooling, reducing predicted radii by up to 5

The evolution of giant planets depends on their formation history. While several evolutionary models self-consistently link planet formation by core accretion to long-term evolution, such models for planets formed by disk instability are lacking. We simulate the evolution of giant planets formed by disk instability and follow their evolution including the pre-collapse phase, dynamical collapse, and long-term contraction in a unified numerical framework. The evolution is simulated using the MESPA code with modifications that allow us to model gas clumps in the pre-collapse phase. We consider masses between 1 and 12 Jupiter masses and metallicities ranging from 0.5 to 2 times the protosolar value. We confirm that the pre-collapse timescale strongly depends on the planetary mass, and that after dynamical collapse the objects reach a state of long-term contraction which lasts for billions of years. We show that metallicity is a major source of uncertainty in mass estimates derived from the age-luminosity relations. For the metallicity range considered here, we find that for a given measurement of age and luminosity the difference in the inferred mass can be up to 1.5 Jupiter masses. We find that our evolution tracks predict masses that are consistent with the measured dynamical mass constraints for HR 8799 e, AF Lep b, Beta Pic b and Beta Pic c. We also show that both core accretion and disk instability can lead to very similar long-term evolutionary tracks. The agreement between our models and dynamical mass measurements suggests that disk instability remains a viable formation pathway for giant exoplanets. The luminosity evolution alone cannot distinguish between the two formation pathways. Finally, we suggest that planetary metallicity must be taken into account when inferring the masses of young giant planets from their luminosities, as it significantly affects their evolution.

Impacts play a fundamental role in shaping the physical and chemical properties of the objects in our Solar System. Given the challenges in replicating such collisions through laboratory experiments, computer simulations are an important tool to investigate their outcomes. Accurately modelling material properties such as shear strength, porosity, and the formation of cracks is crucial for understanding impacts on small bodies like asteroids and comets. Very large and massive objects are dominated by self-gravity and can be approximated as a fluid. In this regime the equation of state used to model the behaviour of the constituent materials plays a key role. However, for bodies of several hundred kilometres, which are already spheroidal due to self-gravity, shear strength must still be considered. This impact regime is most challenging to model and therefore often overlooked in publications. In this review we present different impact regimes and the relevant physics that must be included. We then discuss their application to a variety of Solar System objects and assess how recent observations and numerical simulations, focussing on the Smoothed Particle Hydrodynamics method, can be used to inform our understanding of impact processes and solar system formation.

Context. Sub-Neptunes and Neptunes are often modeled with distinct, fully convective layers. Yet, there are several arguments for composition gradients that can inhibit convection. In these regions, energy transport depends on the thermal conductivity and radiative opacity. Aims. We aim to compare three thermal-conductivity models and investigate their impact on planetary evolution, accounting for the possibility of convective mixing eroding composition gradients. Methods. Using a modified version of MESA, we modeled the evolution of planets with masses of Mp = 5, 10, 15 M⊕ and three initial entropies. We implemented thermal conductivities for pure water, fully ionized matter, and constant electron conductivity. Results. Convective mixing complicates the relation among conductivity, evolution, and radius. For hot forming planets with a large composition gradient, where the heavy-element mass fraction changes gradually from the core to the envelope, convective mixing has a significant impact on the radius evolution. In this case, the thermal conductivity is less relevant and the radii converge to similar values after billions of years. For cold forming planets or narrow composition gradients, convective mixing is less efficient. If the composition profile is not altered significantly, the thermal conductivity becomes critical. It determines how much energy can be trapped beneath a stable composition gradient. For intermediate initial entropies, high thermal conductivity inhibits convection. Conclusions. Further work is required to determine the thermal conductivity for various mixtures expected in sub-Neptune and Neptunes at high densities and temperatures. In addition, further constraints on the entropy and composition profile after formation can reduce the degeneracy of the planetary evolution, particularly the dependence of the radius with time.

First-principles modeling of dense hydrogen is crucial in materials and planetary sciences. Despite its apparent simplicity, predicting the ionic and electronic structure of hydrogen is a formidable challenge, and it is connected with the insulator-to-metal transition, a century-old problem in condensed matter. Accurate simulations of liquid hydrogen are also essential for modeling gas giant planets. Here, we perform an exhaustive study of the equation of state of hydrogen using density functional theory (DFT) and quantum Monte Carlo simulations. We find that the pressure predicted by DFT may vary qualitatively when using different functionals. The predictive power of first-principles simulations is restored by validating each functional against higher-level wavefunction theories, represented by computationally intensive variational and diffusion Monte Carlo calculations. Our simulations provide evidence that hydrogen is denser at planetary conditions, compared to currently used equations of state. For Jupiter, this implies a lower bulk metallicity (i.e., a smaller mass of heavy elements). Our results further amplify the inconsistency between Jupiter's atmospheric metallicity measured by the Galileo probe and the envelope metallicity inferred from interior models.

Giant planets orbiting low-mass stars represent a unique population of giant planets that can be studied to constrain planet formation theory. Surveys of transiting giant exoplanets around M-dwarfs (GEMS) allow for measurement of their masses and radii, which in turn can be used to estimate their bulk densities. Coupling these observations to interior structure and evolution models can yield the bulk metallicity of these planets, however there are degeneracies to this that are improved by measurements of atmospheric metallicity. Estimates of bulk metallicity can be crucial to our understanding of planet formation timescales and mass budgets, particularly in these extreme (planet-to-star) mass ratio systems. Here we show that GEMS are expected to have a low bulk metallicity as a natural result of the planet formation process, and the low efficiency of planetesimal capture post-formation. To test this empirically, we need a larger sample of GEMS with atmospheric measurements, which can reduce some of the degeneracies with the interior modelling of planets. Measuring the atmospheric composition of GEMS with JWST and Ariel will be crucial for better understanding this unique planetary type, which sits at the tail of standard planet formation conditions.

We present an algorithm to efficiently sample the full space of planetary interior density profiles. Our approach uses as few assumptions as possible to pursue an agnostic algorithm. The algorithm avoids the common Markov chain Monte Carlo method and instead uses an optimisation-based gradient-descent approach designed for computational efficiency. In this work, we use Uranus and Neptune as test cases and obtain empirical models that provide density and pressure profiles consistent with the observed physical properties (total mass, radius, and gravitational moments). We compared our findings to other work and find that while other studies are generally in line with our findings, they do not cover the entire space of solutions faithfully. Furthermore, we present guidance for modellers that construct Uranus or Neptune interior models with a fixed number of layers. We provide a statistical relation between the steepness classifying a density discontinuity and the resulting number of discontinuities to be expected. For example, if one classifies a discontinuity as a density gradient larger than 0.02 kg m(-4), then most solutions should have at most one such discontinuity. Finally, we find that discontinuities, if present, are concentrated around a planetary normalised radius of 0.65 for Uranus and 0.7 for Neptune. Our algorithm to efficiently and faithfully investigate the full space of possible interior density profiles can be used to study all planetary objects with gravitational field data.

Context. Giant planet formation requires reaching crossover mass, i.e., when the mass of the gaseous envelope becomes equal to the mass of the core, within the disc's lifetime. The formation process depends critically on the orbital distance and stellar mass. Aims. We simulate planet formation via pebble accretion up to crossover mass around stellar hosts with masses of 0.1-1.5 Msun, considering a range of formation locations, with and without Type I migration. Methods. We use a modified version of MESA that couples pebble accretion, gas accretion, and disc evolution. Results. We find that cold/warm Jupiters form, whereas in-situ formation fails at short orbital separations: viscous heating raises the isolation mass enough to assemble adequate cores, but the accompanying high disc's temperature prevents cooling and suppresses gas giant formation. This supports migration-based explanations for the origin of hot Jupiters. At large orbital distances, crossover can be reached before pebble isolation mass. This is possible due to efficient envelope contraction in the cold, low-opacity outer disc. Inferred core masses at crossover range between 0.7 and 20 M_Earth. Conclusions. Pebble accretion accommodates multiple formation pathways. Giant planets can also have very small cores. Overall, different formation conditions significantly influence planetary growth and can explain the diversity in compositions and internal structures observed in the exoplanet population.

The outgassing signatures of Io, Europa, Enceladus, Triton, and Io-like exomoons are the focus of this review chapter. The rocky volcanic world of Io is unique in our Solar System, with plumes reaching to hundreds of kilometres in altitude. Io-like exomoons could leave signatures strong enough to be detected with ground-based telescopes. The icy moons Europa and Enceladus, with their subsurface oceans, are currently the best candidates for life. Triton is different in many ways and raises unexplored questions. Our knowledge of these active moons is derived from space- and ground-based observations. To understand their origin, we discuss moon formation in general, before examining evidence and signatures of plumes on these moons. Given the accessibility of subsurface oceanic material through the occurrence of plumes, we expand on possibilities to investigate biosignatures.

Context. Characterizing the masses, radii, and compositions of small planets orbiting M dwarfs is key to understanding their formation and identifying the best targets for atmospheric follow-up with facilities such as JWST. Methods. We refined the photometry of the TOI-4342 system using TESS and LCOGT data, and characterized the host stars with NIRPS and ESPRESSO spectroscopy. High-precision ESPRESSO radial velocities (RVs) allowed us to constrain the planetary masses and investigate their potential compositions. Results. The TOI-4336 A system is composed of a sub-Neptune with a period of 16.34 days, a radius of 2.14 +/- 0.08 R-circle plus, and a mass of 3.33 +/- 0.36 M-circle plus, along with an inner super-Earth on a 7.59-day orbit with a radius of 1.25 +/- 0.07 R-circle plus and a mass of 1.55 +/- 0.13 M-circle plus. The TOI-4342 system hosts two sub-Neptunes of similar sizes (2.33 +/- 0.09 R-circle plus and 2.35 +/- 0.09 R-circle plus), with periods of 5.54 and 10.69 days. Their masses are measured to be 7.3 +/- 1.3 M-circle plus and 4.8 +/- 1.4 M-circle plus, respectively. The RVs also reveal a planet candidate around TOI-4342, most likely non-transiting, with a period of 47.5 days and a minimum mass of 17.8 +/- 3.0 M-circle plus. Conclusions. With precise radii and masses, we derived bulk densities and explored possible compositions. The TOI-4336 A subNeptune and super-Earth have densities of 1.87 +/- 0.30 and 4.35 +/- 0.79 g cm(-3), while the two similar-sized sub-Neptunes in TOI-4342 show distinct densities of 3.18 +/- 0.67 and 2.01 +/- 0.63 g cm(-3). Using an inference model, we find that TOI-4336 A b, TOI-4342 b, and TOI-4342 c have an atmosphere mass fraction (AMF) of similar to 3.7%, similar to 1.8%, and similar to 2.9%, respectively, while the super-Earth TOI-4336 A c could contain similar to 2% of water or have a core-to-mass fraction (CMF) of similar to 31%. All four planets are excellent targets for future atmospheric characterization with JWST, and their multi-planet nature makes them especially interesting for comparative planetology. Notably, TOI-4336 A b stands out as one of the best known targets in its size and temperature regime, with a transmission spectroscopy metric (TSM) of 138, comparable to benchmark planets such as K2-18 b and LHS 1140 b. Its inner sibling, TOI-4336 A c, may also be of interest for emission spectroscopy and exploring the "cosmic shoreline", similarly to the Rocky Worlds DDT JWST program.