A cool runaway greenhouse without surface magma ocean.

Water vapour atmospheres with content equivalent to the Earth's oceans, resulting from impacts1 or a high insolation2,3, were found to yield a surface magma ocean4,5. This was, however, a consequence of assuming a fully convective structure2-11. Here, we report using a consistent climate model that pure steam atmospheres are commonly shaped by radiative layers, making their thermal structure strongly dependent on the stellar spectrum and internal heat flow. The surface is cooler when an adiabatic profile is not imposed; melting Earth's crust requires an insolation several times higher than today, which will not happen during the main sequence of the Sun. Venus's surface can solidify before the steam atmosphere escapes, which is the opposite of previous works4,5. Around the reddest stars (Teff < 3,000 K), surface magma oceans cannot form by stellar forcing alone, whatever the water content. These findings affect observable signatures of steam atmospheres and exoplanet mass-radius relationships, drastically changing current constraints on the water content of TRAPPIST-1 planets. Unlike adiabatic structures, radiative-convective profiles are sensitive to opacities. New measurements of poorly constrained high-pressure opacities, in particular far from the H2O absorption bands, are thus necessary to refine models of steam atmospheres, which are important stages in terrestrial planet evolution.

No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c.

Seven rocky planets orbit the nearby dwarf star TRAPPIST-1, providing a unique opportunity to search for atmospheres on small planets outside the Solar System1. Thanks to the recent launch of the James Webb Space Telescope (JWST), possible atmospheric constituents such as carbon dioxide (CO2) are now detectable2,3. Recent JWST observations of the innermost planet TRAPPIST-1 b showed that it is most probably a bare rock without any CO2 in its atmosphere4. Here we report the detection of thermal emission from the dayside of TRAPPIST-1 c with the Mid-Infrared Instrument (MIRI) on JWST at 15 µm. We measure a planet-to-star flux ratio of fp/f⁎ = 421 ± 94 parts per million (ppm), which corresponds to an inferred dayside brightness temperature of 380 ± 31 K. This high dayside temperature disfavours a thick, CO2-rich atmosphere on the planet. The data rule out cloud-free O2/CO2 mixtures with surface pressures ranging from 10 bar (with 10 ppm CO2) to 0.1 bar (pure CO2). A Venus-analogue atmosphere with sulfuric acid clouds is also disfavoured at 2.6σ confidence. Thinner atmospheres or bare-rock surfaces are consistent with our measured planet-to-star flux ratio. The absence of a thick, CO2-rich atmosphere on TRAPPIST-1 c suggests a relatively volatile-poor formation history, with less than [Formula: see text] Earth oceans of water. If all planets in the system formed in the same way, this would indicate a limited reservoir of volatiles for the potentially habitable planets in the system.

Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1.

One aim of modern astronomy is to detect temperate, Earth-like exoplanets that are well suited for atmospheric characterization. Recently, three Earth-sized planets were detected that transit (that is, pass in front of) a star with a mass just eight per cent that of the Sun, located 12 parsecs away. The transiting configuration of these planets, combined with the Jupiter-like size of their host star-named TRAPPIST-1-makes possible in-depth studies of their atmospheric properties with present-day and future astronomical facilities. Here we report the results of a photometric monitoring campaign of that star from the ground and space. Our observations reveal that at least seven planets with sizes and masses similar to those of Earth revolve around TRAPPIST-1. The six inner planets form a near-resonant chain, such that their orbital periods (1.51, 2.42, 4.04, 6.06, 9.1 and 12.35 days) are near-ratios of small integers. This architecture suggests that the planets formed farther from the star and migrated inwards. Moreover, the seven planets have equilibrium temperatures low enough to make possible the presence of liquid water on their surfaces.

Atmospheric characterization of terrestrial exoplanets in the mid-infrared: biosignatures, habitability, and diversity.

Exoplanet science is one of the most thriving fields of modern astrophysics. A major goal is the atmospheric characterization of dozens of small, terrestrial exoplanets in order to search for signatures in their atmospheres that indicate biological activity, assess their ability to provide conditions for life as we know it, and investigate their expected atmospheric diversity. None of the currently adopted projects or missions, from ground or in space, can address these goals. In this White Paper, submitted to ESA in response to the Voyage 2050 Call, we argue that a large space-based mission designed to detect and investigate thermal emission spectra of terrestrial exoplanets in the mid-infrared wavelength range provides unique scientific potential to address these goals and surpasses the capabilities of other approaches. While NASA might be focusing on large missions that aim to detect terrestrial planets in reflected light, ESA has the opportunity to take leadership and spearhead the development of a large mid-infrared exoplanet mission within the scope of the "Voyage 2050" long-term plan establishing Europe at the forefront of exoplanet science for decades to come. Given the ambitious science goals of such a mission, additional international partners might be interested in participating and contributing to a roadmap that, in the long run, leads to a successful implementation. A new, dedicated development program funded by ESA to help reduce development and implementation cost and further push some of the required key technologies would be a first important step in this direction. Ultimately, a large mid-infrared exoplanet imaging mission will be needed to help answer one of humankind's most fundamental questions: "How unique is our Earth?"

Water vapour in the atmosphere of a transiting extrasolar planet.

Water is predicted to be among the most abundant (if not the most abundant) molecular species after hydrogen in the atmospheres of close-in extrasolar giant planets ('hot Jupiters'). Several attempts have been made to detect water on such planets, but have either failed to find compelling evidence for it or led to claims that should be taken with caution. Here we report an analysis of recent observations of the hot Jupiter HD 189733b (ref. 6) taken during the transit, when the planet passed in front of its parent star. We find that absorption by water vapour is the most likely cause of the wavelength-dependent variations in the effective radius of the planet at the infrared wavelengths 3.6 mum, 5.8 mum (both ref. 7) and 8 mum (ref. 8). The larger effective radius observed at visible wavelengths may arise from either stellar variability or the presence of clouds/hazes. We explain the report of a non-detection of water on HD 189733b (ref. 4) as being a consequence of the nearly isothermal vertical profile of the planet's atmosphere.

Extraterrestrial meteors: a martian meteor and its parent comet.

Regular meteor showers occur when a planet approaches the orbit of a periodic comet--for example, the Leonid shower is evident around 17 November every year as Earth skims past the dusty trail of comet Tempel-Tuttle. Such showers are expected to occur on Mars as well, and on 7 March last year, the panoramic camera of Spirit, the Mars Exploration Rover, revealed a curious streak across the martian sky. Here we show that the timing and orientation of this streak, and the shape of its light curve, are consistent with the existence of a regular meteor shower associated with the comet Wiseman-Skiff, which could be characterized as martian Cepheids.

Coronal mass ejection (CME) activity of low mass M stars as an important factor for the habitability of terrestrial exoplanets. I. CME impact on expected magnetospheres of Earth-like exoplanets in close-in habitable zones.

Low mass M- and K-type stars are much more numerous in the solar neighborhood than solar-like G-type stars. Therefore, some of them may appear as interesting candidates for the target star lists of terrestrial exoplanet (i.e., planets with mass, radius, and internal parameters identical to Earth) search programs like Darwin (ESA) or the Terrestrial Planet Finder Coronagraph/Inferometer (NASA). The higher level of stellar activity of low mass M stars, as compared to solar-like G stars, as well as the closer orbital distances of their habitable zones (HZs), means that terrestrial-type exoplanets within HZs of these stars are more influenced by stellar activity than one would expect for a planet in an HZ of a solar-like star. Here we examine the influences of stellar coronal mass ejection (CME) activity on planetary environments and the role CMEs may play in the definition of habitability criterion for the terrestrial type exoplanets near M stars. We pay attention to the fact that exoplanets within HZs that are in close proximity to low mass M stars may become tidally locked, which, in turn, can result in relatively weak intrinsic planetary magnetic moments. Taking into account existing observational data and models that involve the Sun and related hypothetical parameters of extrasolar CMEs (density, velocity, size, and occurrence rate), we show that Earth-like exoplanets within close-in HZs should experience a continuous CME exposure over long periods of time. This fact, together with small magnetic moments of tidally locked exoplanets, may result in little or no magnetospheric protection of planetary atmospheres from a dense flow of CME plasma. Magnetospheric standoff distances of weakly magnetized Earth-like exoplanets at orbital distances

Coronal mass ejection (CME) activity of low mass M stars as an important factor for the habitability of terrestrial exoplanets. II. CME-induced ion pick up of Earth-like exoplanets in close-in habitable zones.

Atmospheric erosion of CO2-rich Earth-size exoplanets due to coronal mass ejection (CME)-induced ion pick up within close-in habitable zones of active M-type dwarf stars is investigated. Since M stars are active at the X-ray and extreme ultraviolet radiation (XUV) wave-lengths over long periods of time, we have applied a thermal balance model at various XUV flux input values for simulating the thermospheric heating by photodissociation and ionization processes due to exothermic chemical reactions and cooling by the CO2 infrared radiation in the 15 microm band. Our study shows that intense XUV radiation of active M stars results in atmospheric expansion and extended exospheres. Using thermospheric neutral and ion densities calculated for various XUV fluxes, we applied a numerical test particle model for simulation of atmospheric ion pick up loss from an extended exosphere arising from its interaction with expected minimum and maximum CME plasma flows. Our results indicate that the Earth-like exoplanets that have no, or weak, magnetic moments may lose tens to hundreds of bars of atmospheric pressure, or even their whole atmospheres due to the CME-induced O ion pick up at orbital distances

Spiders: water-driven erosive structures in the southern hemisphere of Mars.

Recent data from space missions reveal that there are ongoing climatic changes and erosive processes that continuously modify surface features of Mars. We have investigated the seasonal dynamics of a number of morphological features located at Inca City, a representative area at high southern latitude that has undergone seasonal processes. By integrating visual information from the Mars Orbiter Camera on board the Mars Global Surveyor and climatic cycles from a Mars' General Circulation Model, and considering the recently reported evidence for the presence of water-ice and aqueous precipitates on Mars, we propose that a number of the erosive features identified in Inca City, among them spiders, result from the seasonal melting of aqueous salty solutions.

Solar UV irradiation conditions on the surface of Mars.

The UV radiation environment on planetary surfaces and within atmospheres is of importance in a wide range of scientific disciplines. Solar UV radiation is a driving force of chemical and organic evolution and serves also as a constraint in biological evolution. In this work we modeled the transmission of present and early solar UV radiation from 200 to 400 nm through the present-day and early (3.5 Gyr ago) Martian atmosphere for a variety of possible cases, including dust loading, observed and modeled O3 concentrations. The UV stress on microorganisms and/or molecules essential for life was estimated by using DNA damaging effects (specifically bacteriophage T7 killing and uracil dimerization) for various irradiation conditions on the present and ancient Martian surface. Our study suggests that the UV irradiance on the early Martian surface 3.5 Gyr ago may have been comparable with that of present-day Earth, and though the current Martian UV environment is still quite severe from a biological viewpoint, we show that substantial protection can still be afforded under dust and ice.

An Earth-sized planet in the habitable zone of a cool star.

The quest for Earth-like planets is a major focus of current exoplanet research. Although planets that are Earth-sized and smaller have been detected, these planets reside in orbits that are too close to their host star to allow liquid water on their surfaces. We present the detection of Kepler-186f, a 1.11 ± 0.14 Earth-radius planet that is the outermost of five planets, all roughly Earth-sized, that transit a 0.47 ± 0.05 solar-radius star. The intensity and spectrum of the star's radiation place Kepler-186f in the stellar habitable zone, implying that if Kepler-186f has an Earth-like atmosphere and water at its surface, then some of this water is likely to be in liquid form.

Co-evolution of atmospheres, life, and climate.

After Earth's origin, our host star, the Sun, was shining 20-25% less brightly than today. Without greenhouse-like conditions to warm the atmosphere, our early planet would have been an ice ball, and life may never have evolved. But life did evolve, which indicates that greenhouse gases must have been present on early Earth to warm the planet. Evidence from the geological record indicates an abundance of the greenhouse gas CO(2). CH(4) was probably present as well; and, in this regard, methanogenic bacteria, which belong to a diverse group of anaerobic prokaryotes that ferment CO(2) plus H(2) to CH(4), may have contributed to modification of the early atmosphere. Molecular oxygen was not present, as is indicated by the study of rocks from that era, which contain iron carbonate rather than iron oxide. Multicellular organisms originated as cells within colonies that became increasingly specialized. The development of photosynthesis allowed the Sun's energy to be harvested directly by life-forms. The resultant oxygen accumulated in the atmosphere and formed the ozone layer in the upper atmosphere. Aided by the absorption of harmful UV radiation in the ozone layer, life colonized Earth's surface. Our own planet is a very good example of how life-forms modified the atmosphere over the planets' lifetime. We show that these facts have to be taken into account when we discover and characterize atmospheres of Earth-like exoplanets. If life has originated and evolved on a planet, then it should be expected that a strong co-evolution occurred between life and the atmosphere, the result of which is the planet's climate.

A roadmap for the detection and characterization of other Earths.

The European Space Agency and other space agencies such as NASA recognize that the question with regard to life beyond Earth in general, and the associated issue of the existence and study of exoplanets in particular, is of paramount importance for the 21(st) century. The new Cosmic Vision science plan, Cosmic Vision 2015-2025, which is built around four major themes, has as its first theme: "What are the conditions for planet formation and the emergence of life?" This main theme is addressed through further questions: 1) How do gas and dust give rise to stars and planets? 2) How will the search for and study of exoplanets eventually lead to the detection of life outside Earth (biomarkers)? 3) How did life in the Solar System arise and evolve? Although ESA has busied itself with these issues since the beginning of the Darwin study in 1996, it has become abundantly clear that, as these topics have evolved, only a very large effort, addressed from the ground and from space with the utilization of different instruments and space missions, can provide the empirical results required for a complete understanding. The good news is that the problems can be addressed and solved within a not-too-distant future. In this short essay, we present the present status of a roadmap related to projects that are related to the key long-term goal of understanding and characterizing exoplanets, in particular Earth-like planets.

The far future of exoplanet direct characterization.

We describe future steps in the direct characterization of habitable exoplanets subsequent to medium and large mission projects currently underway and investigate the benefits of spectroscopic and direct imaging approaches. We show that, after third- and fourth-generation missions have been conducted over the course of the next 100 years, a significant amount of time will lapse before we will have the capability to observe directly the morphology of extrasolar organisms.

The search for worlds like our own.

The direct detection of Earth-like exoplanets orbiting nearby stars and the characterization of such planets-particularly, their evolution, their atmospheres, and their ability to host life-constitute a significant problem. The quest for other worlds as abodes of life has been one of mankind's great questions for several millennia. For instance, as stated by Epicurus approximately 300 BC: "Other worlds, with plants and other living things, some of them similar and some of them different from ours, must exist." Demokritos from Abdera (460-370 BC), the man who invented the concept of indivisible small parts-atoms-also held the belief that other worlds exist around the stars and that some of these worlds may be inhabited by life-forms. The idea of the plurality of worlds and of life on them has since been held by scientists like Johannes Kepler and William Herschel, among many others. Here, one must also mention Giordano Bruno. Born in 1548, Bruno studied in France and came into contact with the teachings of Nicolas Copernicus. He wrote the book De l'Infinito, Universo e Mondi in 1584, in which he claimed that the Universe was infinite, that it contained an infinite amount of worlds like Earth, and that these worlds were inhabited by intelligent beings. At the time, this was extremely controversial, and eventually Bruno was arrested by the church and burned at the stake in Rome in 1600, as a heretic, for promoting this and other equally confrontational issues (though it is unclear exactly which idea was the one that ultimately brought him to his end). In all the aforementioned cases, the opinions and results were arrived at through reasoning-not by experiment. We have only recently acquired the technological capability to observe planets orbiting stars other than 6 our Sun; acquisition of this capability has been a remarkable feat of our time. We show in this introduction to the Habitability Primer that mankind is at the dawning of an age when, by way of the scientific method and 21(st)-century technology, we will be able to answer this fascinating controversial issue that has persisted for at least 2500 years.

Origin and evolution of life on terrestrial planets.

The ultimate goal of terrestrial planet-finding missions is not only to discover terrestrial exoplanets inside the habitable zone (HZ) of their host stars but also to address the major question as to whether life may have evolved on a habitable Earth-like exoplanet outside our Solar System. We note that the chemical evolution that finally led to the origin of life on Earth must be studied if we hope to understand the principles of how life might evolve on other terrestrial planets in the Universe. This is not just an anthropocentric point of view: the basic ingredients of terrestrial life, that is, reduced carbon-based molecules and liquid H(2)O, have very specific properties. We discuss the origin of life from the chemical evolution of its precursors to the earliest life-forms and the biological implications of the stellar radiation and energetic particle environments. Likewise, the study of the biological evolution that has generated the various life-forms on Earth provides clues toward the understanding of the interconnectedness of life with its environment.

Radiative habitable zones in martian polar environments.

The biologically damaging solar ultraviolet (UV) radiation (quantified by the DNA-weighted dose) reaches the martian surface in extremely high levels. Searching for potentially habitable UV-protected environments on Mars, we considered the polar ice caps that consist of a seasonally varying CO2 ice cover and a permanent H2O ice layer. It was found that, though the CO2 ice is insufficient by itself to screen the UV radiation, at approximately 1 m depth within the perennial H2O ice the DNA-weighted dose is reduced to terrestrial levels. This depth depends strongly on the optical properties of the H2O ice layers (for instance snow-like layers). The Earth-like DNA-weighted dose and Photosynthetically Active Radiation (PAR) requirements were used to define the upper and lower limits of the northern and southern polar Radiative Habitable Zone (RHZ) for which a temporal and spatial mapping was performed. Based on these studies we conclude that photosynthetic life might be possible within the ice layers of the polar regions. The thickness varies along each martian polar spring and summer between approximately 1.5 and 2.4 m for H2O ice-like layers, and a few centimeters for snow-like covers. These martian Earth-like radiative habitable environments may be primary targets for future martian astrobiological missions. Special attention should be paid to planetary protection, since the polar RHZ may also be subject to terrestrial contamination by probes.

Dynamic co-evolution of peptides and chemical energetics, a gateway to the emergence of homochirality and the catalytic activity of peptides.

We propose a scenario for the dynamic co-evolution of peptides and energy on the primitive Earth. From a multi component system consisting of hydrogen cyanide, several carbonyl compounds, ammonia, alkyl amine, carbonic anhydride, borate and isocyanic acid, we show that the reversibility of this system leads to several intermediate nitriles, that irreversibly evolve to alpha-amino acids and N-carbamoyl amino acids via selective catalytic processes. On the primitive Earth these N-carbamoyl amino acids combined with energetic molecules (NOx) may have been the core of a molecular engine producing peptides permanently and assuring their recycling and evolution. We present this molecular engine, a production example, and its various selectivities. The perspectives for such a dynamic approach to the emergence of peptides are evoked in the conclusion.