Water vapour in the atmosphere of the habitable-zone eight Earth-mass planet K2-18 b

Abstract

In the past decade, observations from space and ground have found H${}_2$O to be the most abundant molecular species, after hydrogen, in the atmospheres of hot, gaseous, extrasolar planets. Being the main molecular carrier of oxygen, H${}_2$O is a tracer of the origin and the evolution mechanisms of planets. For temperate, terrestrial planets, the presence of H${}_2$O is of great significance as an indicator of habitable conditions. Being small and relatively cold, these planets and their atmospheres are the most challenging to observe, and therefore no atmospheric spectral signatures have so far been detected. Super-Earths -- planets lighter than ten M${}_{\oplus }$ -- around later-type stars may provide our first opportunity to study spectroscopically the characteristics of such planets, as they are best suited for transit observations. Here we report the detection of an H${}_2$O spectroscopic signature in the atmosphere of \planet\ -- an eight M${}_{\oplus }$ planet in the habitable-zone of an M-dwarf -- with high statistical confidence (ADI = 5.0, $\sim$3.6$\sigma$). In addition, the derived mean molecular weight suggests an atmosphere still containing some hydrogen. The observations were recorded with the Hubble Space Telescope/WFC3 camera, and analysed with our dedicated, publicly available, algorithms. While the suitability of M-dwarfs to host habitable worlds is still under discussion, \planet\ offers an unprecedented opportunity to get insight into the composition and climate of habitable-zone planets.

Published in Nature Astronomy

References in corpus (10)

• Achieving better than 1 minute accuracy in the Heliocentric and Barycentric Julian Dates
• Strong Water Absorption in the Dayside Emission Spectrum of the Planet HD 189733b
• Tau-REx I: A next generation retrieval code for exoplanetary atmospheres
• Water Vapour Absorption in the Clear Atmosphere of an exo-Neptune
• The habitability of Proxima Centauri b II. Possible climates and Observability
• Hubble Space Telescope Near-IR Transmission Spectroscopy of the Super-Earth HD 97658b
• Spitzer Observations Confirm and Rescue the Habitable-Zone Super-Earth K2-18b for Future Characterization
• Characterization of the K2-18 multi-planetary system with HARPS: A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit
• Infrared Transmission Spectra for Extrasolar Giant Planets
• HST PanCET program: A Cloudy Atmosphere for the promising JWST target WASP-101b

Cited by in corpus (136)

• Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars
• Water Vapor and Clouds on the Habitable-Zone Sub-Neptune Exoplanet K2-18b
• The 2020 release of the ExoMol database: molecular line lists for exoplanet and other hot atmospheres
• TauREx III: A fast, dynamic and extendable framework for retrievals
• Habitability and Biosignatures of Hycean Worlds
• The nature and origins of sub-Neptune size planets
• The interior and atmosphere of the habitable-zone exoplanet K2-18b
• Aerosols in Exoplanet Atmospheres
• Causality for Machine Learning
• Atmospheric Regimes and Trends on Exoplanets and Brown Dwarfs
• Five key exoplanet questions answered via the analysis of 25 hot Jupiter atmospheres in eclipse
• 1D atmospheric study of the temperate sub-Neptune K2-18b
• Distinguishing oceans of water from magma on mini-Neptune K2-18b
• ARES II: Characterising the Hot Jupiters WASP-127 b, WASP-79 b and WASP-62 b with HST
• TRIDENT: A Rapid 3D Radiative Transfer Model for Exoplanet Transmission Spectra
• Hubble WFC3 Spectroscopy of the Habitable-zone Super-Earth LHS 1140 b
• Aurora: A Generalised Retrieval Framework for Exoplanetary Transmission Spectra
• Detection of an Atmosphere on a Rocky Exoplanet
• How to identify exoplanet surfaces using atmospheric trace species in hydrogen-dominated atmospheres
• Unveiling shrouded oceans on temperate sub-Neptunes via transit signatures of solubility equilibria vs. gas thermochemistry
• Clouds will likely prevent the detection of water vapor in JWST transmission spectra of terrestrial exoplanets
• The Pyrat Bay Framework for Exoplanet Atmospheric Modeling: A Population Study of Hubble/WFC3 Transmission Spectra
• How Deep Is the Ocean? Exploring the phase structure of water-rich sub-Neptunes
• ARES V: No Evidence For Molecular Absorption in the HST WFC3 Spectrum of GJ 1132 b
• Hydrodynamic escape of water vapor atmospheres near very active stars
• TESS Hunt for Young and Maturing Exoplanets (THYME) IV: Three small planets orbiting a 120 Myr-old star in the Pisces--Eridanus stream
• Non-detection of Helium in the Upper Atmospheres of Three Sub-Neptune Exoplanets
• Formation and dynamics of water clouds on temperate sub-Neptunes: The example of K2-18b
• Chemistry of Temperate Super-Earth and Mini-Neptune Atmospheric Hazes from Laboratory Experiments
• On The Compatibility of Ground-based and Space-based Data: WASP-96 b, An Example
• Stellar surface inhomogeneities as a potential source of the atmospheric signal detected in the K2-18 b transmission spectrum
• A comparison of chemical models of exoplanet atmospheres enabled by TauREx 3.1
• Dry or water world? How the water contents of inner sub-Neptunes constrain giant planet formation and the location of the water ice line
• A Heavy Molecular Weight Atmosphere for the Super-Earth π Men c
• On the Temperature Profiles and Emission Spectra of Mini-Neptune Atmospheres
• Haze Evolution in Temperate Exoplanet Atmospheres Through Surface Energy Measurements
• ARES III: Unveiling the Two Faces of KELT-7 b with HST WFC3
• The Ariel Target List: The Impact of TESS and the Potential for Characterising Multiple Planets Within a System
• Nitrogen Dioxide Pollution as a Signature of Extraterrestrial Technology
• The runaway greenhouse on subNeptune waterworlds
• The White Dwarf Opportunity: Robust Detections of Molecules in Earth-like Exoplanet Atmospheres with the James Webb Space Telescope
• Distinguishing between wet and dry atmospheres of TRAPPIST-1 e and f
• Arid or Cloudy: Characterizing the Atmosphere of the super-Earth 55 Cancri e using High-Resolution Spectroscopy
• The high-energy environment and atmospheric escape of the mini-Neptune K2-18 b
• CHES: a space-borne astrometric mission for the detection of habitable planets of the nearby solar-type stars
• Consistently Simulating a Wide Range of Atmospheric Scenarios for K2-18b with a Flexible Radiative Transfer Module
• Alfnoor: A Retrieval Simulation of the Ariel Target List
• Super-Earths and Earth-like Exoplanets
• Methane as a dominant absorber in the habitable-zone sub-Neptune K2-18 b
• ARES IV: Probing the atmospheres of the two warm small planets HD 106315 c and HD 3167 c with the HST/WFC3 camera
• TOI-269 b: An eccentric sub-Neptune transiting a M2 dwarf revisited with ExTrA
• The First Near-Infrared Transmission Spectrum of HIP 41378 f, a Low-Mass Temperate Jovian World in a Multi-Planet System
• Exploring and Validating Exoplanet Atmospheric Retrievals with Solar System Analog Observations
• TOI-1759 b: a transiting sub-Neptune around a low mass star characterized with SPIRou and TESS
• Escaping Helium and a Highly Muted Spectrum Suggest a Metal-Enriched Atmosphere on Sub-Neptune GJ3090b from JWST Transit Spectroscopy
• The future of astronomy with small satellites
• A sub-Neptune and a non-transiting Neptune-mass companion unveiled by ESPRESSO around the bright late-F dwarf HD 5278 (TOI-130)
• KELT-11 b: Abundances of water and constraints on carbon-bearing molecules from the Hubble transmission spectrum
• Is the atmosphere of the ultra-hot Jupiter WASP-121b variable?
• Retrieval study of cool, directly imaged exoplanet 51 Eri b
• Optical Properties of Organic Haze Analogues in Water-rich Exoplanet Atmospheres Observable with JWST
• Cleaning our Hazy Lens: Exploring Trends in Transmission Spectra of Warm Exoplanets
• The Physics of Falling Raindrops in Diverse Planetary Atmospheres
• Formation of secondary atmospheres on terrestrial planets by late disk accretion
• Phase curve pollution of exoplanet transmission spectra
• Detectability of Chlorofluorocarbons in the Atmospheres of Habitable M-dwarf Planets
• TOI-1231 b: A Temperate, Neptune-Sized Planet Transiting the Nearby M3 Dwarf NLTT 24399
• Inefficient water degassing inhibits ocean formation on rocky planets: An insight from self-consistent mantle degassing models
• 3D convection-resolving model of temperate, tidally-locked exoplanets
• Possibilities for an Aerial Biosphere in Temperate Sub Neptune-Sized Exoplanet Atmospheres
• A transiting, temperate mini-Neptune orbiting the M dwarf TOI-1759 unveiled by TESS
• Possible Atmospheric Diversity of Low Mass Exoplanets, some Central Aspects
• WASP-117 b: an eccentric hot-Saturn as a future complex chemistry laboratory
• A transmission spectrum of the sub-Earth planet L98-59~b in 1.1-1.7 $\mu$m
• Hubble WFC3 Spectroscopy of the Terrestrial Planets L~98-59~c \& d: No Evidence for a Clear Hydrogen Dominated Primary Atmosphere
• On spectroscopic phase-curve retrievals: H2 dissociation and thermal inversion in the atmosphere of the ultra-hot Jupiter WASP-103 b
• A retrieval challenge exercise for the Ariel mission
• Hubble WFC3 Spectroscopy of the Rocky Planet L 98-59 b: No Evidence for a Cloud-Free Primordial Atmosphere
• Insufficient evidence for DMS and DMDS in the atmosphere of K2-18 b. From a joint analysis of JWST NIRISS, NIRSpec, and MIRI observations
• Characterising Atmospheres of Cloudy Temperate Mini-Neptunes with JWST
• A Framework for Characterizing Transmission Spectra of Exoplanets with Circumplanetary Rings
• Heating and ionization by non-thermal electrons in the upper atmospheres of water-rich exoplanets
• Challenges in the detection of gases in exoplanet atmospheres
• Dynamics and Clouds in Planetary Atmospheres from Telescopic Observations
• Microphysics of Water Clouds in the Atmospheres of Y Dwarfs and Temperate Giant Planets
• Revisiting Radial Velocity Measurements of the K2-18 System with the Line-by-Line Framework
• The transmission spectrum of WASP-17 b from the optical to the near-infrared wavelengths: combining STIS, WFC3 and IRAC datasets
• Water content trends in K2-138 and other low-mass multiplanetary systems
• Precise mass determination for the keystone sub-Neptune planet transiting the mid-type M dwarf G 9-40
• GJ 3090 b: one of the most favourable mini-Neptune for atmospheric characterisation
• HATS-47b, HATS-48Ab, HATS-49b and HATS-72b: Four Warm Giant Planets Transiting K Dwarfs
• Prebiosignature Molecules Can Be Detected in Temperate Exoplanet Atmospheres with JWST
• Atmospheric dynamics of temperate sub-Neptunes. Part I: dry dynamics
• Analysis of the planetary mass uncertainties on the accuracy of atmospherical retrieval
• YunMa: Enabling Spectral Retrievals of Exoplanetary Clouds
• Prospects for Detecting Signs of Life on Exoplanets in the JWST Era
• Hazy with a chance of star spots: constraining the atmosphere of the young planet, K2-33b
• TESS and CHEOPS Discover Two Warm Sub-Neptunes Transiting the Bright K-dwarf HD 15906
• From Super-Earths to Mini-Neptunes: Implications of a Surface on Atmospheric Circulation
• A New Definition of Exoplanet Habitability: Introducing the Photosynthetic Habitable Zone
• On the Effect of Stellar Activity on Low-resolution Transit Spectroscopy and the Use of High Resolution as Mitigation
• A sub-Neptune planet around TOI-1695 discovered and characterized with SPIRou and TESS
• Instrumentation prospects for rocky exoplanet atmospheres studies with high resolution spectroscopy
• Observational constraints on the formation and evolution of Neptune-class exoplanets
• A Population Analysis of 20 Exoplanets Observed from the Optical to the Near-infrared Wavelengths with HST: Evidence for Widespread Stellar Contamination
• JexoSim 2.0: End-to-End JWST Simulator for Exoplanet Spectroscopy -- Implementation and Case Studies
• TOI-4336 A b: A temperate sub-Neptune ripe for atmospheric characterization in a nearby triple M-dwarf system
• Detecting molecules in Ariel low resolution transmission spectra
• On The Synergy Between Ariel And Ground-Based High-Resolution Spectroscopy
• Unraveling the non-equilibrium chemistry of the temperate sub-Neptune K2-18 b
• A formation pathway for terrestrial planets with moderate water content involving atmospheric-volatile recycling
• HST Transmission Spectra of the Hot-Neptune HD 219666 b: Detection of Water and the Challenge of Constraining Both Water and Methane
• The effect of spectroscopic binning on atmospheric retrievals
• Planetary albedo is limited by the above-cloud atmosphere: Implications for sub-Neptune climate
• A Coronagraph with a Sub-$\lambda /D$ IWA and a Moderate Spectral Bandwidth
• Spectroscopic fourth-order coronagraph for the characterization of terrestrial planets at small angular separations from host stars
• A re-analysis of equilibrium chemistry in five hot Jupiters
• Inferring Shallow Surfaces on sub-Neptune Exoplanets with JWST
• Distinguishing multicellular life on exoplanets by testing Earth as an exoplanet
• Polycyclic Aromatic Hydrocarbons (PAHs) as an Extraterrestrial Atmospheric Technosignature
• Is the high-energy environment of K2-18b special?
• RAPOC : the Rosseland and Planck opacity converter. A user-friendly and fast opacity program for Python
• Diversity of Exoplanets
• The atmosphere of K2-18 b: The role of hazes, clouds and photoelectrons
• Ocean signatures in the total flux and polarization spectra of Earth-like exoplanets
• Exploring Synergies between Twinkle and Ariel: a Pilot Study
• Disentangling Atmospheric Compositions of K2-18 b with Next Generation Facilities
• Effect of clouds on emission spectra for Super Venus
• L 98-59: a Benchmark System of Small Planets for Future Atmospheric Characterization
• A Ride to Exoplanets: Finding Potentially Habitable Worlds
• Reflected spectroscopy of small exoplanets I: determining the atmospheric composition of sub-Neptunes planets
• Information content of JWST transmission spectroscopy of the exoplanet HAT-P-12b from the optical to the mid-infrared
• Convection modeling of pure-steam atmospheres
• Thermoelastic Contraction as a Suppressor of Atmospheric Escape in Close-in Exoplanets
• Searching for technosignatures in exoplanetary systems with current and future missions
• Photochemistry and Spectral Characterization of Temperate and Gas-Rich Exoplanets