arXiv subjects

Nikku Madhusudhan

Publications and source records attributed to Nikku Madhusudhan.

At least 55 records · Page 3Linked to original sources

arXiv2023

Robustness Measures for Molecular Detections using High-Resolution Transmission Spectroscopy of Exoplanets

Ground-based high-resolution transmission spectroscopy has emerged as a promising technique for detecting chemicals in transiting exoplanetary atmospheres. Despite chemical inferences in several exoplanets and previous robustness studies, a robust and consistent detrending method to remove telluric and stellar features from transmission spectra has yet to be agreed upon. In this work we investigate the robustness of metrics used to optimise PCA-based detrending for high-resolution transmission spectra of exoplanets in the near-infrared. As a case study, we consider observations of the hot Jupiter HD 189733 b obtained using the CARMENES spectrograph on the 3.5 m CAHA telescope. We confirm that optimising the detrending parameters to maximise the S/N of a cross-correlation signal in the presence of noise has the potential to bias the detection significance at the planetary velocity of optimisation. However, we find that optimisation using the difference between a signal-injected cross-correlation function and the direct cross-correlation function (CCF) is more robust against over-optimisation of noise and spurious signals. We additionally examine the robustness of weighting the contribution of each order to the final CCF, and of S/N calculations. Using a prescribed robust methodology, we confirm H2O in the atmosphere of HD 189733 b (S/N = 6.1). We then investigate two further case studies, of exoplanets HD 209458 b and WASP-76 b, confirming OH in the atmosphere of WASP-76 b (S/N = 4.7), and demonstrating how non-robust methods may induce false positive or inflated detections. Our findings pave the way towards a robust framework for homogeneous characterisation of exoplanetary atmospheres using high-resolution transmission spectroscopy in the near-infrared.

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arXiv2023

A precise blue-optical transmission spectrum from the ground: Evidence for haze in the atmosphere of WASP-74b

We report transmission spectroscopy of the bloated hot Jupiter WASP-74b in the wavelength range from 4000 to 6200 Å. We observe two transit events with the Very Large Telescope FOcal Reducer and Spectrograph (VLT FORS2) and present a new method to measure the exoplanet transit depth as a function of wavelength. The new method removes the need for a reference star in correcting the spectroscopic light curves for the impact of atmospheric extinction. It also provides improved precision, compared to other techniques, reaching an average transit depth uncertainty of 211 ppm for a solar-type star of V=9.8 mag and over wavelength bins of 80 Å. The VLT transmission spectrum is analysed both individually and in combination with published data from Hubble Space Telescope (HST) and Spitzer. The spectrum is found to exhibit a mostly featureless slope and equilibrium chemistry retrievals with PLATON favour hazes in the upper atmosphere of the exoplanet. Free chemistry retrievals with AURA further support the presence of hazes. While additional constraints are possible depending on the choice of atmospheric model, they are not robust and may be influenced by residual systematics in the data sets. Our results demonstrate the utility of new techniques in the analysis of optical, ground-based spectroscopic data and can be highly complementary to follow-up observations in the infrared with JWST.

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arXiv2023

Early Insights for Atmospheric Retrievals of Exoplanets using JWST Transit Spectroscopy

We have entered the era of the James Webb Space Telescope (JWST). We use the first JWST transmission spectrum of the hot Saturn-mass exoplanet, WASP-39 b, obtained with the NIRSpec instrument in the 3-5 $μ$m range to investigate (a) what atmospheric constraints are possible with JWST-quality data in this spectral range, (b) requirements for atmospheric models used in retrievals, (c) effect of differences between data reduction pipelines on retrieved atmospheric properties, and (d) complementarity between JWST data in the 3-5 $μ$m range and HST observations at shorter wavelengths. JWST spectra in the 3-5 $μ$m range provide a promising avenue for chemical detections while encompassing a window in cloud opacity for several prominent aerosols. We confirm recent inferences of CO$_2$, SO$_2$, H$_2$O, and CO in WASP-39 b, report tentative evidence for H$_2$S, and retrieve elemental abundances consistent with Saturn's metallicity. We retrieve molecular abundances with $\sim$0.3-0.6 dex precision with this relatively limited spectral range. When considering the 3-5 $μ$m data alone, reported differences in spectra with different reduction pipelines can affect abundance estimates by up to $\sim$1 dex and the detectability of less prominent species. Complementing with data at shorter wavelengths, e.g. with other JWST instruments or HST WFC3 ($\sim$0.8-1.7 $μ$m), can significantly improve the accuracy and precision of the abundance estimates. The high data quality enables constraints on aerosol properties, including their composition, modal size and extent, motivating their consideration in retrievals. Our results highlight the promise of JWST exoplanet spectroscopy, while underscoring the importance of robust data reduction and atmospheric retrieval approaches in the JWST era.

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arXiv2022

Hubble Space Telescope transmission spectroscopy for the temperate sub-Neptune TOI-270d: a possible hydrogen-rich atmosphere containing water vapour

TOI-270d is a temperate sub-Neptune discovered by the Transiting Exoplanet Survey Satellite (TESS) around a bright (J=9.1mag) M3V host star. With an approximate radius of 2RE and equilibrium temperature of 350K, TOI-270d is one of the most promising small exoplanets for atmospheric characterisation using transit spectroscopy. Here we present a primary transit observation of TOI-270d made with the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) spectrograph across the 1.126-1.644 micron wavelength range, and a 95% credible upper limit of $8.2 \times 10^{-14}$ erg s$^{-1}$ cm$^{-2}$ A$^{-1}$ arcsec$^{-2}$ for the stellar Ly-alpha emission obtained using the Space Telescope Imaging Spectrograph (STIS). The transmission spectrum derived from the TESS and WFC3 data provides evidence for molecular absorption by a hydrogen-rich atmosphere at 4-sigma significance relative to a featureless spectrum. The strongest evidence for any individual absorber is obtained for H2O, which is favoured at 3-sigma significance. When retrieving on the WFC3 data alone and allowing for the possibility of a heterogeneous stellar brightness profile, the detection significance of H2O is reduced to 2.8-sigma. Further observations are therefore required to robustly determine the atmospheric composition of TOI-270d and assess the impact of stellar heterogeneity. If confirmed, our findings would make TOI-270d one of the smallest and coolest exoplanets to date with detected atmospheric spectral features.

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arXiv2022

A First Look at CRIRES+: Performance Assessment and Exoplanet Spectroscopy

High-resolution spectroscopy has proven to be a powerful avenue for atmospheric remote sensing of exoplanets. Recently, ESO commissioned the CRIRES+ high-resolution infrared spectrograph at VLT. CRIRES+ is a cross-dispersed spectrograph with high throughput and wide wavelength coverage across the near-infrared (0.95-5.3 $μ$m), designed to be particularly suited for atmospheric characterisation of exoplanets. In this work, we report early insights into the performance of CRIRES+ for exoplanet spectroscopy and conduct a detailed assessment of the data reduction procedure. Because of the novelty of the instrument, we perform two independent data reduction strategies, using the official CR2RES pipeline and our new custom-built ExoRES pipeline. Using science verification observations we find that the spectral resolving power of CRIRES+ can reach $R \gtrsim 100,000$ for optimal observing conditions. Similarly, we find the signal-to-noise ratio (S/N) to be consistent with expected and empirical estimates for the observations considered. As a case study, we perform the first application of CRIRES+ to the atmospheric characterisation of an exoplanet - the ultra-hot Jupiter MASCARA-1 b. We detect CO and H$_2$O in the atmosphere of MASCARA-1 b at a S/N of 12.9 and 5.3, respectively, and a temperature inversion revealed through the CO and H$_2$O emission lines, the first for an exoplanet. We find a combined S/N of 13.8 for CO and H$_2$O together, with a preference for lower H$_2$O abundance compared to CO. Our findings demonstrate the scientific potential of CRIRES+ and highlight the excellent opportunity for high-resolution atmospheric spectroscopy of diverse exoplanets.

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arXiv2022

Aura-3D: A Three-dimensional Atmospheric Retrieval Framework for Exoplanet Transmission Spectra

Atmospheric retrievals of exoplanet transmission spectra allow constraints on the composition and structure of the day-night terminator region. Such retrievals in the past have typically assumed one-dimensional temperature structures which were adequate to explain extant observations. However, the increasing data quality expected from exoplanet spectroscopy with JWST motivates considerations of multidimensional atmospheric retrievals. We present AURA-3D, a three-dimensional atmospheric retrieval framework for exoplanet transmission spectra. AURA-3D includes a forward model that enables rapid computation of transmission spectra in 3D geometry for a given atmospheric structure and can, therefore, be used for atmospheric retrievals as well as for computing spectra from General Circulation Models (GCMs). In order to efficiently explore the space of possible 3D temperature structures in retrievals, we develop a parametric 3D pressure-temperature profile which can accurately represent azimuthally-averaged temperature structures of a range of hot Jupiter GCMs. We apply our retrieval framework to simulated JWST observations of hot Jupiter transmission spectra, obtaining accurate estimates of the day-night temperature variation across the terminator as well as the abundances of chemical species. We demonstrate an example of a model hot Jupiter transmission spectrum for which a traditional 1D retrieval of JWST-quality data returns biased abundance estimates, whereas a retrieval including a day-night temperature gradient can accurately retrieve the true abundances. Our forward model also has the capability to include inhomogeneous chemistry as well as variable clouds/hazes. This new retrieval framework opens the field to detailed multidimensional atmospheric characterisation using transmission spectra of exoplanets in the JWST era.

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arXiv2022

Molecular tracers of planet formation in the atmospheres of hot Jupiters

The atmospheric chemical composition of a hot Jupiter can lead to insights into where in its natal protoplanetary disk it formed and its subsequent migration pathway. We use a 1-D chemical kinetics code to compute a suite of models across a range of elemental abundances to investigate the resultant abundances of key molecules in hot jupiter atmospheres. Our parameter sweep spans metallicities between 0.1x and 10x solar values for the C/H, O/H and N/H ratios, and equilibrium temperatures of 1000K and 2000K. We link this parameter sweep to the formation and migration models from previous works to predict connections between the atmospheric molecular abundances and formation pathways, for the molecules \ce{H2O}, \ce{CO}, \ce{CH4}, \ce{CO2}, \ce{HCN} and \ce{NH3}. We investigate atmospheric \ce{H2O} abundances in eight hot Jupiters reported in the literature. All eight planets fall within our predicted ranges for various formation models, however six of them are degenerate between multiple models and, hence, require additional molecular detections for constraining their formation histories. The other two planets, HD 189733~b and HD 209458~b, have water abundances that fall within ranges expected from planets that formed beyond the \ce{CO2} snowline. Finally, we investigate the detections of \ce{H2O}, \ce{CO}, \ce{CH4}, \ce{CO2}, \ce{HCN} and \ce{NH3} in the atmosphere of HD 209458~b and find that, within the framework of our model, the abundances of these molecules best match with a planet that formed between the \ce{CO2} and \ce{CO} snowlines and then underwent disk-free migration to reach its current location.

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arXiv2022

A survey of sodium absorption in ten giant exoplanets with high-resolution transmission spectroscopy

The alkali metal sodium (Na) is one of the most commonly detected chemical species in the upper atmospheres of giant exoplanets. In this work we conducted a homogeneous survey of Na in a diverse sample of ten highly irradiated giant exoplanets using high-resolution transmission spectroscopy. Our sample includes nine planets with previous Na detections and one new detection. We confirm previous detections and assess multiple approaches for deriving Na line properties from high-resolution transmission spectra. The homogeneously measured sodium line depths were used to constrain the atmospheric heights ($H_{\text{Na}}$) with respect to the planetary radii ($R_{\text{p}}$). We assess an empirical trend describing the relative atmospheric height ($H_{\text{Na}}/R_{\text{p}}$) as a function of planetary equilibrium temperature ($T_{\text{eq}}$) and surface gravity ($g$), in which $H_{\text{Na}}/R_{\text{p}}$ decreases exponentially with $ξ\propto gT_{\text{eq}}$, approaching a constant at large $ξ$. We also report the sodium D2/D1 line ratios across our sample and find that seven targets have line ratios that are consistent with unity. Finally, we measured net blueshifted offsets of the sodium absorption lines from their rest frame wavelengths for all ten planets, corresponding to day-night wind velocities of a few km s$^{-1}$. This suggests that the broad sample of exoplanets share common underlying processes that govern atmospheric dynamics. Our study highlights a promising avenue for using high-resolution transmission spectroscopy to further our understanding of how atmospheric characteristics vary over a diverse sample of exoplanets.

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arXiv2022

Characterising Atmospheres of Cloudy Temperate Mini-Neptunes with JWST

The upcoming James Webb Space Telescope (JWST) promises a generational shift in the study of temperate mini-Neptune atmospheres using transit spectroscopy. High-altitude clouds however threaten to impede their atmospheric characterisation by muting spectral features. In this study, we systematically investigate JWST instrument configurations for characterising cloudy mini-Neptune atmospheres, assessing the importance of instrument choice and wavelength coverage, focusing on NIRISS and NIRSpec. We consider two temperate mini-Neptunes orbiting nearby M dwarfs, K2-18 b and TOI-732 c, with equilibrium temperatures below 400 K, as case studies and assess observations using different instrument configurations with one transit per instrument. We find that their JWST transmission spectra with modest observing time and adequate wavelength coverage can provide precise abundance constraints of key molecules H2O, CH4, and NH3 even in the presence of clouds at significantly high altitudes. The best constraints are obtained by combining all three high-resolution NIRSpec gratings (G140H+G235H+G395H) that together span the ~1-5 $μ$m range. Single-transit observations with this three-instrument configuration allow precise abundance constraints for cloud-top pressures as low as 3 mbar and 0.1 mbar for K2-18 b and TOI-732 c, respectively, assuming a nominal 10x solar metallicity. The constraints vary with instrument combinations. We find that NIRSpec G235H+G395H is the optimal two-instrument configuration, while NIRISS or NIRSpec G235H is optimal for single-instrument observations. Absent high-altitude clouds, even single-instrument observations can provide good abundance constraints for these planets. Our findings underscore the promise of JWST transmission spectroscopy for characterising temperate mini-Neptunes orbiting nearby M dwarfs.

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arXiv2022

On Atmospheric Retrievals of Exoplanets with Inhomogeneous Terminators

The complexity of atmospheric retrieval models is largely data-driven and one-dimensional models have generally been considered adequate with current data quality. However, recent studies have suggested that using 1D models in retrievals can result in anomalously cool terminator temperatures and biased abundance estimates even with existing transmission spectra of hot Jupiters. Motivated by these claims and upcoming high-quality transmission spectra we systematically explore the limitations of 1D models using synthetic and current observations. We use 1D models of varying complexity, both analytic and numerical, to revisit claims of biases when interpreting transmission spectra of hot Jupiters with inhomogeneous terminator compositions. Overall, we find the reported biases to be resulting from specific model assumptions rather than intrinsic limitations of 1D atmospheric models in retrieving current observations of asymmetric terminators. Additionally, we revise atmospheric retrievals of the hot Jupiter WASP-43b ($T_{\rm eq}=1440$ K) and the ultra-hot Jupiter WASP-103b ($T_{\rm eq}=2484$ K ) for which previous studies inferred abnormally cool atmospheric temperatures. We retrieve temperatures consistent with expectations. We note, however, that in the limit of extreme terminator inhomogeneities and high data quality some atmospheric inferences may conceivably be biased, although to a lesser extent than previously claimed. To address such cases, we implement a 2D retrieval framework for transmission spectra which allows accurate constraints on average atmospheric properties and provides insights into the spectral ranges where the imprints of atmospheric inhomogeneities are strongest. Our study highlights the need for careful considerations of model assumptions and data quality before attributing biases in retrieved estimates to unaccounted atmospheric inhomogeneities.

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arXiv2022

Constraints on TESS albedos for five hot Jupiters

Photometric observations of occultations of transiting exoplanets can place important constraints on the thermal emission and albedos of their atmospheres. We analyse photometric measurements and derive geometric albedo ($A_\mathrm{g}$) constraints for five hot Jupiters observed with TESS in the optical: WASP-18 b, WASP-36 b, WASP-43 b, WASP-50 b and WASP-51 b. For WASP-43 b, our results are complemented by a VLT/HAWK-I observation in the near-infrared at $2.09~μ$m. We derive the first geometric albedo constraints for WASP-50 b and WASP-51 b: $A_\mathrm{g}<0.445$ and $A_\mathrm{g}<0.368$, respectively. We find that WASP-43 b and WASP-18 b are both consistent with low geometric albedos ($A_\mathrm{g}<0.16$) even though they lie at opposite ends of the hot Jupiter temperature range with equilibrium temperatures of $\sim1400$ K and $\sim2500$ K, respectively. We report self-consistent atmospheric models which explain broadband observations for both planets from TESS, \HST, \Spitzer and VLT/HAWK-I. We find that the data of both hot Jupiters can be explained by thermal emission alone and inefficient day-night energy redistribution. The data do not require optical scattering from clouds/hazes, consistent with the low geometric albedos observed.

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arXiv2021

HyDRo: Atmospheric Retrieval of Rocky Exoplanets in Thermal Emission

Emission spectroscopy is a promising technique to observe atmospheres of rocky exoplanets, probing both their chemistry and thermal profiles. We present HyDRo, an atmospheric retrieval framework for thermal emission spectra of rocky exoplanets. HyDRo does not make prior assumptions about the background atmospheric composition, and can therefore be used to interpret spectra of secondary atmospheres with unknown compositions. We use HyDRo to assess the chemical constraints which can be placed on rocky exoplanet atmospheres using JWST. Firstly, we identify the best currently-known rocky exoplanet candidates for spectroscopic observations in thermal emission with JWST, finding >30 known rocky exoplanets whose thermal emission will be detectable by JWST/MIRI in fewer than 10 eclipses at R~10. We then consider the observations required to characterise the atmospheres of three promising rocky exoplanets across the ~400-800 K equilibrium temperature range: Trappist-1 b, GJ 1132 b, and LHS 3844 b. Considering a range of CO_2- to H_2O-rich atmospheric compositions, we find that as few as 8 eclipses of LHS 3844 b or GJ 1132 b with MIRI will be able to place important constraints on the chemical compositions of their atmospheres. This includes confident detections of CO_2 and H_2O in the case of a cloud-free CO_2-rich composition, besides ruling out a bare rock scenario. Similarly, 30 eclipses of Trappist-1 b with MIRI/LRS can allow detections of a cloud-free CO_2-rich or CO_2-H_2O atmosphere. HyDRo will allow important atmospheric constraints for rocky exoplanets using JWST observations, providing clues about their geochemical environments.

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arXiv2021

Atmospheric Modelling and Retrieval

This brief review focuses on methods and applications of modeling exoplanetary atmospheres. We discuss various kinds of state of the art self-consistent and retrieval models in 1D and multi-D with a focus on open questions and short- and long-term goals in the field. Expertise previously developed in modeling cool stellar atmospheres and in modeling solar system planetary atmospheres has proven valuable to the field, and will continue to do so. We described upcoming opportunities for making progress in our understanding of atmospheres, and close with what we see as the field's challenges.

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arXiv2021

JexoSim 2.0: End-to-End JWST Simulator for Exoplanet Spectroscopy -- Implementation and Case Studies

The recently developed JWST Exoplanet Observation Simulator (JexoSim) simulates transit spectroscopic observations of exoplanets by JWST with each of its four instruments using a time-domain approach. Previously we reported the validation of JexoSim against Pandexo and instrument team simulators. In the present study, we report a substantially enhanced version, JexoSim 2.0, which improves on the original version through incorporation of new noise sources, enhanced treatment of stellar and planetary signals and instrumental effects, as well as improved user-operability and optimisations for increased speed and efficiency. A near complete set of instrument modes for exoplanet time-series observations is now included. In this paper we report the implementation of JexoSim 2.0 and assess performance metrics for JWST in end-member scenarios using the hot Jupiter HD 209458 b and the mini-Neptune K2-18 b. We show how JexoSim can be used to compare performance across the different JWST instruments, selecting an optimal combination of instrument and subarray modes, producing synthetic transmission spectra for each planet. These studies indicate that the 1.4 μm water feature detected in the atmosphere of K2-18 b using the Hubble WFC3 might be observable in just one transit observation with JWST with either NIRISS or NIRSpec. JexoSim 2.0 can be used to investigate the impact of complex noise and systematic effects on the final spectrum, plan observations and test the feasibility of novel science cases for JWST. It can also be customised for other astrophysical applications beyond exoplanet spectroscopy. JexoSim 2.0 is now available for use by the scientific community.

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arXiv2021

Habitability and Biosignatures of Hycean Worlds

We investigate a new class of habitable planets composed of water-rich interiors with massive oceans underlying H2-rich atmospheres, referred to here as Hycean worlds. With densities between those of rocky super-Earths and more extended mini-Neptunes, Hycean planets can be optimal candidates in the search for exoplanetary habitability and may be abundant in the exoplanet population. We investigate the bulk properties (masses, radii, and temperatures), potential for habitability, and observable biosignatures of Hycean planets. We show that Hycean planets can be significantly larger compared to previous considerations for habitable planets, with radii as large as 2.6 Earth radii (2.3 Earth radii) for a mass of 10 Earth masses (5 Earth masses). We construct the Hycean habitable zone (HZ), considering stellar hosts from late M to sun-like stars, and find it to be significantly wider than the terrestrial-like HZ. While the inner boundary of the Hycean HZ corresponds to equilibrium temperatures as high as ~500 K for late M dwarfs, the outer boundary is unrestricted to arbitrarily large orbital separations. Our investigations include tidally locked `Dark Hycean' worlds that permit habitable conditions only on their permanent nightsides and `Cold Hycean' worlds that see negligible irradiation. Finally, we investigate the observability of possible biosignatures in Hycean atmospheres. We find that a number of trace terrestrial biomarkers which may be expected to be present in Hycean atmospheres would be readily detectable using modest observing time with the James Webb Space Telescope (JWST). We identify a sizable sample of nearby potential Hycean planets that can be ideal targets for such observations in search of exoplanetary biosignatures.

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arXiv2021

Transmission spectroscopy with VLT FORS2: a featureless spectrum for the low-density transiting exoplanet WASP-88b

We present ground-based optical transmission spectroscopy of the low-density hot Jupiter WASP-88b covering the wavelength range 4413-8333 Å with the FORS2 spectrograph on the Very Large Telescope. The FORS2 white light curves exhibit a significant time-correlated noise which we model using a Gaussian Process and remove as a wavelength-independent component from the spectroscopic light curves. We analyse complementary photometric observations from the Transiting Exoplanet Survey Satellite and refine the system properties and ephemeris. We find a featureless transmission spectrum with increased absorption towards shorter wavelengths. We perform an atmospheric retrieval analysis with the AURA code, finding tentative evidence for haze in the upper atmospheric layers and a lower likelihood for a dense cloud deck. Whilst our retrieval analysis results point toward clouds and hazes, further evidence is needed to definitively reject a clear-sky scenario.

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arXiv2021

Sulfur Chemistry in the Atmospheres of Warm and Hot Jupiters

We present and validate a new network of atmospheric thermo-chemical and photo-chemical sulfur reactions. We use a 1-D chemical kinetics model to investigate these reactions as part of a broader HCNO chemical network in a series of hot and warm Jupiters. We find that temperatures approaching $1400\,\mathrm{K}$ are favourable for the production of H2S and HS around $\mathrm{10^{-3}\,bar}$, the atmospheric level where detection by transit spectroscopy may be possible, leading to mixing ratios of around $10^{-6}$. At lower temperatures, down to $1000\,\mathrm{K}$, the abundance of S2 can be up to a mixing ratio of $10^{-5}$ at the same pressure, at the expense of H2S and HS, which are depleted down to a mixing ratio of $10^{-6}$. We also investigate how the inclusion of sulfur can manifest in an atmosphere indirectly, by its effect on the abundance of non-sulfur-bearing species. We find that in a model of the atmosphere of HD 209458 b, the inclusion of sulfur can lower the abundance of NH3, CH4 and HCN by up to two orders of magnitude around $\mathrm{10^{-3}\,bar}$. In the atmosphere of the warm Jupiter 51 Eri b, we additionally find the inclusion of sulphur depletes the peak abundance of CO2 by a factor of five, qualitatively consistent with prior models. We note that many of the reactions used in the network have poorly determined rates, especially at higher temperatures. To obtain a truly accurate idea of the impact of sulfur chemistry in hot and warm Jupiter atmospheres, new measurements of these reaction rates must take place.

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arXiv2021

How Deep Is the Ocean? Exploring the phase structure of water-rich sub-Neptunes

Understanding the internal structures of planets with a large H$_2$O component is important for the characterisation 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 characterisation 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 H$_2$O 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 H$_2$O, noting that the liquid phase can persist at temperatures up to 647 K at high pressures of $218$-$7\times10^4$ 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.

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