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Proxima Centauri b provides an unprecedented opportunity to understand the evolution and nature of terrestrial planets orbiting M dwarfs. Although Proxima Cen b orbits within its star's habitable zone, multiple plausible evolutionary paths could have generated different environments that may or may not be habitable.

48 ar av vm langtan snart over, we use 1-D coupled climate-photochemical models to generate self-consistent atmospheres for several evolutionary scenarios, including high-O 2high-CO 2and more Earth-like atmospheres, with both oxic and anoxic compositions. We show that these modeled environments can be habitable or uninhabitable at Proxima Cen b's position in the habitable zone. We use radiative transfer models to generate synthetic spectra and thermal phase curves for these simulated environments, and use instrument models to explore our ability to discriminate between possible planetary states.

These results are applicable not only to Proxima Cen b but to other terrestrial planets orbiting M dwarfs. Thermal phase curves may provide the first constraint on the existence of an atmosphere. Detection of ocean glint is unlikely with JWST but may be within the reach of larger-aperture telescopes.

Direct imaging spectra may detect O 4 absorption, which is diagnostic of massive water loss and O 2 retention, rather than a photosynthetic biosphere. Similarly, strong CO 2 and CO bands at wavelengths shortward of 2. If the planet is habitable and volatile-rich, direct imaging will be the best means of detecting habitability. Earth-like planets with microbial biospheres may be identified by the presence of CH 4 —which has a longer atmospheric lifetime under Proxima Centauri's incident UV—and either photosynthetically produced O 2 or a hydrocarbon haze layer.

Planetary habitability and biosignatures—Planetary atmospheres—Exoplanets—Spectroscopic biosignatures—Planetary science—Proxima Centauri b. If confirmed, Proxima Cen b is the closest potentially habitable planet and one of the most accessible examples orbiting a late-type M dwarf host.

However, although Proxima Cen b has several promising attributes, including its possibly small size minimum mass of 1. A determination of the degree to which this planet is habitable, or any other insights into the fate of terrestrial planets orbiting M dwarfs, awaits follow-up information on its orbit, the planet's phase- and time-dependent photometry, and on spectra of the atmosphere and surface. Conversely, Proxima Cen b will ultimately provide an excellent observational laboratory for our current understanding of terrestrial planet evolution, which will in turn inform our knowledge of the prevalence of habitability and life in our galaxy.

In advance of these data, we can synthesize what has been learned over the past several decades on star-planet and planetary environmental interactions for terrestrial planets to make a broad, preliminary assessment of plausible evolutionary 48 ar av vm langtan snart over and current environmental states for Proxima Cen b.

In our companion paper Barnes et al. In this paper, we model several of these possible planetary environmental outcomes with coupled climate-photochemical models that are self-consistently forced by the spectrum of Proxima Cen. We then use radiative transfer and instrument simulation models to generate synthetic phase-dependent light curves, and transmission and direct imaging spectra, which are relevant to missions such as the James Webb Space Telescope JWST 1 ; e.

These spectra can be used to predict possible attributes and necessary measurements to identify discriminants for habitable and uninhabitable environments, and to explore the potential detectability of environmental signs of habitability and life. We consider scenarios for atmospheric composition driven by stellar, orbital, and planetary evolution, and use our models to explore the photochemical and climatic outcomes, and observable attributes.

Our first scenario is an oxygen-dominated atmosphere, generated by the loss of oceans of water during Proxima Cen's pre-main sequence phase Luger and Barnes, In this scenario, the planet can be either desiccated or have retained some of its initial water inventory.

1. Introduction

In the second class of scenario, the atmosphere is CO 2 -dominated and progressively desiccated, and is formed when the majority of O 2 from ocean loss is either "48 ar av vm langtan snart over" to space or sequestered in the planetary crust or interior Schaefer et al. Two cases are presented for this habitable scenario: In Section 2we review the possible evolutionary scenarios for Proxima Centauri b and briefly discuss observable parameters for identification of habitability and biosignatures for terrestrial planets.

In Section 3we describe the radiative transfer, instrument, climate, and photochemical models, along with their model inputs. In Section 4we present our results. In Section 5we discuss the implications of our modeling, including an assessment of the scenarios that might lead to habitability for Proxima Cen b.

We also identify future observations that will help discriminate between the proposed evolutionary paths, and potentially identify signs of habitability and life. These results are summarized in our conclusions in Section 6.

In this section, we briefly review the factors that can affect terrestrial planet habitability and the plausible evolutionary paths for Proxima Cen b as explored in our companion paper by Barnes et al. We describe how characteristics of the planet and planetary system could affect Proxima Cen b's current climate. To motivate the detectability simulation results and discussion that follow, we will also briefly review relevant knowledge on observations to identify signs of habitability and life for terrestrial planets.

Although Proxima Cen b is possibly small enough to be terrestrial and sits in the habitable zone of its 48 ar av vm langtan snart over star, many factors other than planetary mass and star-planet distance affect a planet's volatile inventory, atmosphere, and surface environment.

These factors, in conjunction with the evolution of the star and planet, can maintain or destroy habitability. Classically, exoplanet habitability is defined as the ability to maintain liquid water on the surface of a rocky, terrestrial planet, and the habitable zone is that range of distances from the star over which an Earth-like planet can maintain surface liquid water Kasting et al. However, if Proxima Cen b formed in situ, then planet formation models suggest that it may be water-poor Lissauer et al.

These interactions could drive volatile and atmospheric loss processes and orbital changes that could preclude habitability for this planet. These processes are examined in more detail in our companion paper, and an overview of the scope of plausible scenarios is given here to motivate the climate and spectral modeling work that follows.

Perhaps the greatest threat to Proxima Cen b's habitability is the luminosity evolution of its M dwarf host Luger and Barnes, ; Barnes et al. Lower-mass M dwarfs can experience extended pre-main sequence phases of up to a billion years, in which they are more luminous than they will ultimately be when they settle on to the main sequence Baraffe et al.

During this superluminous pre-main "48 ar av vm langtan snart over" phase, any terrestrial planet that forms in what will become the main sequence habitable zone is subjected to extremely high levels of radiation, which can vaporize and photolyze oceans and strip lighter elements from the atmosphere Luger and Barnes, Consequently, if Proxima Cen b is a terrestrial planet, it may have been in a runaway greenhouse state for the first million years.

The final amount of oxygen will depend on the initial water inventory, the stellar XUV flux, atmospheric losses through hydrodynamic escape and other top-of-atmosphere processes Collinson et al. Note 48 ar av vm langtan snart over Barnes et al. If the planet also outgasses SO 2photochemical processes can result in the formation of a planetwide sulfate cloud and haze deck.

If the atmosphere is desiccated, with H 2 O abundances of the order of tens of parts per million, HDO may be enhanced as an indicator of early water loss, as is also the case for Venus de Bergh et al.

However, Proxima Cen b may be more likely to be habitable if it formed at its current position with a dense hydrogen envelope, or formed in a region of the protoplanetary disk rich in ices—and then migrated inward.

For these scenarios, rather than stripping water from the planet, the superluminous pre-main sequence phase of the star may have removed enough of the primordial hydrogen Owen and Mohanty, to reveal a habitable evaporated core—a potentially volatile-rich planet without the dense hydrogen envelope that may otherwise preclude habitability Luger et al.

Our calculations suggest that, if Proxima Cen b started with 0. At this point, the planet would have been safe from further H 2 O loss if atmospheric water vapor were cold trapped in the troposphere by a sufficient inventory of noncondensable gases, such as N 2 Wordsworth and Pierrehumbert, Even if sufficient N 2 were not initially available, the buildup of 48 ar av vm langtan snart over 2 also a noncondensable gas from the loss of H 2 O could potentially reestablish the cold trap and inhibit subsequent water loss Wordsworth and Pierrehumbert, If the planet formed beyond the habitable zone, orbital disruption could have allowed it to arrive in its current orbit after the pre-main sequence phase.

In this scenario, the planet would not need an initial thick hydrogen envelope to protect it from desiccation and could start off as a terrestrial body. Despite its relatively long rotation period Earlier modeling suggested that Earth-mass planets in the habitable zones of M dwarf stars would suffer continuous exposure to strong stellar winds originating from coronal mass ejection events with a subsequent rapid loss of planetary atmosphere Lammer et al.

This loss could be exacerbated by the shutdown of magnetic dynamo production due to tidal heating of the planet Driscoll and Barnes, or higher initial radiogenic abundances than Earth Barnes et al.

Other processes that may inhibit atmospheric loss also need to be considered, such as cooling to space from upper-atmosphere CO 2 Tian, or shielding via formation of ozone from high-O 2 atmospheres.

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In all cases for atmospheric loss, the resultant atmosphere is dependent on a number of factors including the initial atmospheric inventory, and atmospheric replenishment processes, such as cometary volatile delivery and volcanic outgassing, over the planet's lifetime. If a similar process works for terrestrial "48 ar av vm langtan snart over" orbiting M dwarfs, then these planets would be susceptible to significant loss of water and atmosphere early on, but may, over billions of years, accumulate a surface ocean and atmosphere from volcanic outgassing, after the M dwarf has settled into its more benign main sequence phase.

However, if enough atmosphere is lost via interaction with the star, then the entire atmosphere could potentially be removed by condensation on the cold nightside.

This is more likely if the planet is synchronously rotating, and atmospheric transport processes are weak or inefficient Joshi et al. However, Turbet et al. Significantly less CO 2 as low as a few hundred parts per million is required if a plausible background gas such as N 2 were present Turbet et al. If, however, the atmosphere survived and is currently Earth-like, protons released via repeated flaring events would destroy any incipient ozone layer, resulting in high surface UV fluxes during flare activity Segura et al.

Proxima Cen b's close-in orbit also makes the planet more vulnerable to gravitational tidal interaction with the star Jackson et al. Over time, the star could have circularized Proxima Cen b's orbit, trapped it into synchronous rotation, reduced the semimajor axis, and set the obliquity to zero Barnes et al.

Sufficiently high levels of tidal heating could result 48 ar av vm langtan snart over surface heat fluxes on the planet that could evaporate oceans of water on the planetary surface Barnes et al.

Our initial simulations suggest that by 3 billion years ago, for a starting eccentricity close to 0. However, these forces would have been much more significant in the past, exceeding that of the volcanically active Io for the first billion years of the planet's 48 ar av vm langtan snart over Barnes et al. However, these heat fluxes fall far short of those required to trigger a runaway greenhouse Barnes et al.

Instead, the heat deposited into the mantle of the planet may have reduced circulation in the planet's interior and shut down the generation of a protective magnetic field Driscoll and Barnes,which could have left the atmosphere more vulnerable to erosion.

Wertheimer and Laughlin, ; Matvienko and Orlov, If this orbit takes Proxima Cen closer to its two companions, then the orbit of Proxima Cen b could have been significantly perturbed, resulting in large changes in eccentricity.

The existing RV data allows for eccentricity values up to 0. Tidally locked exoplanets with eccentric orbits have a much higher probability of being captured into orbital resonance than synchronous rotation e. An eccentric orbit would have made Proxima Centauri b vulnerable to climatic swings Williams and Kasting, ; Williams and Pollard, ; Dressing et al.

This disruption of Proxima Cen b's orbit could have occurred at any time in the past and is difficult to predict from the current position of the three stars.

Perturbations from stellar encounters could also enhance habitability via impacts, which may generate atmospheric blowoff of a dense H 2 envelope, or deliver volatiles to the planet after formation or after the pre-main sequence phase.

Differences in temperature and O 3 profiles are primarily driven by the presence or absence of water vapor. In summary, based on our current knowledge of M dwarf—planet interactions, there are several plausible scenarios for the environmental state of Proxima Cen b: Note that early total loss of an atmosphere—without generation or retention of a secondary atmosphere from outgassing—may also be a possible outcome, but we do not consider it for our atmospheric modeling activities.

In the abiotic O 2 -rich scenarios, Proxima Cen b formed at or close to its current position and suffered catastrophic water loss during the star's superluminous pre-main sequence phase.

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The resulting steam atmosphere was photolyzed, and H was lost to space. O 2and possibly remnant water, was left behind, so there could be two cases from this scenario: O 2 -rich without water and O 2 -rich with water Luger and Barnes, Similarly, if massive water loss occurs, and O 2 is lost either via hydrodynamic escape or sequestration in the planet's crust or mantle, or via a magma ocean, then CO 2 may be the dominant gas that persists in the atmosphere.

That H 2 envelope could have been sufficiently thick to protect the volatile-rich planet underneath during the superluminous pre-main sequence phase but not thick enough to remain and compromise the planet's habitability Owen and Mohanty, For these cases, the resultant planet could have had a strongly oxidizing atmosphere, or one that was more reducing, depending on where in the planetary system it formed and how it evolved.

A planet in the habitable zone can be impacted by the host star's incident spectrum, activity levels, and orbital and tidal interactions. Each of these agents, when interacting with the planet's environment, can strongly impact the current environmental state of the planet, including its atmospheric composition, climate, and potential habitability.

For example, the planet's current climate and potentially enhanced ability to maintain surface liquid water are strongly impacted by the interaction of the spectral energy distribution SED of the M dwarf star 48 ar av vm langtan snart over the planet's atmospheric and surface composition Shields et al. In particular, the UV spectrum of the star is critically important for understanding the planet's photochemistry—and therefore the atmospheric composition and climate Segura et al.

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It is also the key to interpretation of any spectra obtained from the planet. The stellar UV also affects whether or not a UV-absorbing haze will form in a reducing atmosphere Arney et al. The presence and strength of these UV shields will affect the resultant surface UV flux, which could in turn strongly impact habitability. IN FURNITURE we maintain the lead and pride ourselves on having the largest and best.

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