Abstract
The recent discovery and initial characterization of sub-Neptune-sized exoplanets that receive stellar irradiance of approximately Earth’s raised the prospect of finding habitable planets in ...the coming decade, because some of these temperate planets may support liquid-water oceans if they do not have massive H
2
/He envelopes and are thus not too hot at the bottom of the envelopes. For planets larger than Earth, and especially planets in the 1.7–3.5
R
⊕
population, the mass of the H
2
/He envelope is typically not sufficiently constrained to assess the potential habitability. Here we show that the solubility equilibria versus thermochemistry of carbon and nitrogen gases typically results in observable discriminators between small H
2
atmospheres versus massive ones, because the condition to form a liquid-water ocean and that to achieve the thermochemical equilibrium are mutually exclusive. The dominant carbon and nitrogen gases are typically CH
4
and NH
3
due to thermochemical recycling in a massive atmosphere of a temperate planet, and those in a small atmosphere overlying a liquid-water ocean are most likely CO
2
and N
2
, followed by CO and CH
4
produced photochemically. NH
3
is depleted in the small atmosphere by dissolution into the liquid-water ocean. These gases lead to distinctive features in the planet’s transmission spectrum, and a moderate number of transit observations with the James Webb Space Telescope should tell apart a small atmosphere versus a massive one on planets like K2-18 b. This framework thus points to a way to use near-term facilities to constrain the atmospheric mass and habitability of temperate sub-Neptune exoplanets.
ABSTRACT We investigate the chemical stability of CO2-dominated atmospheres of desiccated M dwarf terrestrial exoplanets using a one-dimensional photochemical model. Around Sun-like stars, CO2 ...photolysis by Far-UV (FUV) radiation is balanced by recombination reactions that depend on water abundance. Planets orbiting M dwarf stars experience more FUV radiation, and could be depleted in water due to M dwarfs' prolonged, high-luminosity pre-main sequences. We show that, for water-depleted M dwarf terrestrial planets, a catalytic cycle relying on H2O2 photolysis can maintain a CO2 atmosphere. However, this cycle breaks down for atmospheric hydrogen mixing ratios <1 ppm, resulting in ∼40% of the atmospheric CO2 being converted to CO and O2 on a timescale of 1 Myr. The increased O2 abundance leads to high O3 concentrations, the photolysis of which forms another CO2-regenerating catalytic cycle. For atmospheres with <0.1 ppm hydrogen, CO2 is produced directly from the recombination of CO and O. These catalytic cycles place an upper limit of ∼50% on the amount of CO2 that can be destroyed via photolysis, which is enough to generate Earth-like abundances of (abiotic) O2 and O3. The conditions that lead to such high oxygen levels could be widespread on planets in the habitable zones of M dwarfs. Discrimination between biological and abiotic O2 and O3 in this case can perhaps be accomplished by noting the lack of water features in the reflectance and emission spectra of these planets, which necessitates observations at wavelengths longer than 0.95 m.
ABSTRACT Kepler has detected numerous exoplanet transits by measuring stellar light in a single visible-wavelength band. In addition to detection, the precise photometry provides phase curves of ...exoplanets, which can be used to study the dynamic processes on these planets. However, the interpretation of these observations can be complicated by the fact that visible-wavelength phase curves can represent both thermal emission and scattering from the planets. Here we present a semi-analytical model framework that can be applied to study Kepler and future visible-wavelength phase curve observations of exoplanets. The model efficiently computes reflection and thermal emission components for both rocky and gaseous planets, considering both homogeneous and inhomogeneous surfaces or atmospheres. We analyze the phase curves of the gaseous planet Kepler- 7 b and the rocky planet Kepler- 10 b using the model. In general, we find that a hot exoplanet's visible-wavelength phase curve having a significant phase offset can usually be explained by two classes of solutions: one class requires a thermal hot spot shifted to one side of the substellar point, and the other class requires reflective clouds concentrated on the same side of the substellar point. Particularly for Kepler- 7 b, reflective clouds located on the west side of the substellar point can best explain its phase curve. The reflectivity of the clear part of the atmosphere should be less than 7% and that of the cloudy part should be greater than 80%, and the cloud boundary should be located at 11° 3° to the west of the substellar point. We suggest single-band photometry surveys could yield valuable information on exoplanet atmospheres and surfaces.
Abstract
In our solar system, the densely cloud-covered atmosphere of Venus stands out as an example of how polarimetry can be used to gain information on cloud composition and particle mean radius. ...With current interest running high on discovering and characterizing extrasolar planets in the habitable zone where water exists in the liquid state, making use of spectropolarimetric measurements of directly imaged exoplanets could provide key information unobtainable through other means. In principle, spectropolarimetric measurements can determine if acidity causes water activities in the clouds to be too low for life. To this end, we show that a spectropolarimeter measurement over the range 400–1000 nm would need to resolve linear polarization to a precision of about 1% or better for reflected starlight from an optically thick cloud-enshrouded exoplanet. We assess the likelihood of achieving this goal by simulating measurements from a notional spectropolarimeter as part of a starshade configuration for a large space telescope (a HabEx design, but for a 6 m diameter primary mirror). Our simulations include consideration of noise from a variety of sources. We provide guidance on limits that would need to be levied on instrumental polarization to address the science issues we discuss. For photon-limited noise, integration times would need to be of order 1 hr for a large radius (10 Earth radii) planet to more than 100 hr for smaller exoplanets depending on the star–planet separation, planet radius, phase angle, and desired uncertainty. We discuss implications for surface chemistry and habitability.
Transmission spectroscopy
of exoplanets has revealed signatures of water vapour, aerosols and alkali metals in a few dozen exoplanet atmospheres
. However, these previous inferences with the Hubble ...and Spitzer Space Telescopes were hindered by the observations' relatively narrow wavelength range and spectral resolving power, which precluded the unambiguous identification of other chemical species-in particular the primary carbon-bearing molecules
. Here we report a broad-wavelength 0.5-5.5 µm atmospheric transmission spectrum of WASP-39b
, a 1,200 K, roughly Saturn-mass, Jupiter-radius exoplanet, measured with the JWST NIRSpec's PRISM mode
as part of the JWST Transiting Exoplanet Community Early Release Science Team Program
. We robustly detect several chemical species at high significance, including Na (19σ), H
O (33σ), CO
(28σ) and CO (7σ). The non-detection of CH
, combined with a strong CO
feature, favours atmospheric models with a super-solar atmospheric metallicity. An unanticipated absorption feature at 4 µm is best explained by SO
(2.7σ), which could be a tracer of atmospheric photochemistry. These observations demonstrate JWST's sensitivity to a rich diversity of exoplanet compositions and chemical processes.
Carbon dioxide (CO
) is a key chemical species that is found in a wide range of planetary atmospheres. In the context of exoplanets, CO
is an indicator of the metal enrichment (that is, elements ...heavier than helium, also called 'metallicity')
, and thus the formation processes of the primary atmospheres of hot gas giants
. It is also one of the most promising species to detect in the secondary atmospheres of terrestrial exoplanets
. Previous photometric measurements of transiting planets with the Spitzer Space Telescope have given hints of the presence of CO
, but have not yielded definitive detections owing to the lack of unambiguous spectroscopic identification
. Here we present the detection of CO
in the atmosphere of the gas giant exoplanet WASP-39b from transmission spectroscopy observations obtained with JWST as part of the Early Release Science programme
. The data used in this study span 3.0-5.5 micrometres in wavelength and show a prominent CO
absorption feature at 4.3 micrometres (26-sigma significance). The overall spectrum is well matched by one-dimensional, ten-times solar metallicity models that assume radiative-convective-thermochemical equilibrium and have moderate cloud opacity. These models predict that the atmosphere should have water, carbon monoxide and hydrogen sulfide in addition to CO
, but little methane. Furthermore, we also tentatively detect a small absorption feature near 4.0 micrometres that is not reproduced by these models.
Abstract
Efforts to spectrally characterize the atmospheric compositions of temperate terrestrial exoplanets orbiting M dwarf stars with JWST are now underway. Key molecular targets of such searches ...include O
2
and CO, which are potential indicators of life. Recently, it was proposed that CO
2
photolysis generates abundant (≳0.1 bar) abiotic O
2
and CO in the atmospheres of habitable M dwarf planets with CO
2
-rich atmospheres, constituting a strong false positive for O
2
as a biosignature and further complicating efforts to use CO as a diagnostic of surface biology. Importantly, this implied that TRAPPIST-1e and TRAPPIST-1f, now under observation with JWST, would abiotically accumulate abundant O
2
and CO, if habitable. Here, we use a multi-model approach to reexamine photochemical O
2
and CO accumulation on planets orbiting M dwarf stars. We show that photochemical O
2
remains a trace gas on habitable CO
2
-rich M dwarf planets, with earlier predictions of abundant O
2
and CO due to an atmospheric model top that was too low to accurately resolve the unusually high CO
2
photolysis peak on such worlds. Our work strengthens the case for O
2
as a biosignature gas, and affirms the importance of CO as a diagnostic of photochemical O
2
production. However, observationally relevant false-positive potential remains, especially for O
2
's photochemical product O
3
, and further work is required to confidently understand O
2
and O
3
as biosignature gases on M dwarf planets.
Launched in 2018 April, NASA's Transiting Exoplanet Survey Satellite (TESS) has been performing a wide-field survey for exoplanets orbiting bright stars with a goal of producing a rich database for ...follow-on studies. Here we present estimates of the detected exoplanet orbital periods in the 2 minute cadence mode during the TESS mission. For a two-transit detection criterion, the expected mean value of the most frequently detected orbital period is 5.01 days, with the most frequently detected range of 2.12-11.82 days in the region with observation of 27 days. Near the poles where the observational duration is 351 days, the expected mean orbital period is 10.93 days, with the most frequently detected range being from 3.35 to 35.65 days. For one transit, the most frequently detected orbital period is 8.17 days in the region with observation of 27 days and 11.25 days in the region near the poles. For the entire TESS mission containing several sectors, we estimate that the mean value of orbital period is 8.47 days for two-transit detection criterion and 10.09 days for one-transit detection criterion. If TESS yields a planet population substantially different from what is predicted here, the underlying planet occurrence rates are likely different between the stellar sample probed by TESS and that by Kepler.
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