50 research outputs found
The Prospect of Detecting Volcanic Signatures on an ExoEarth Using Direct Imaging
The James Webb Space Telescope (JWST) has provided the first opportunity to
study the atmospheres of terrestrial exoplanets and estimate their surface
conditions. Earth-sized planets around Sun-like stars are currently
inaccessible with JWST however, and will have to be observed using the next
generation of telescopes with direct imaging capabilities. Detecting active
volcanism on an Earth-like planet would be particularly valuable as it would
provide insight into its interior, and provide context for the commonality of
the interior states of Earth and Venus. In this work we used a climate model to
simulate four exoEarths over eight years with ongoing large igneous province
eruptions with outputs ranging from 1.8-60 Gt of sulfur dioxide. The
atmospheric data from the simulations were used to model direct imaging
observations between 0.2-2.0 m, producing reflectance spectra for every
month of each exoEarth simulation. We calculated the amount of observation time
required to detect each of the major absorption features in the spectra, and
identified the most prominent effects that volcanism had on the reflectance
spectra. These effects include changes in the size of the O, O, and
HO absorption features, and changes in the slope of the spectrum. Of these
changes, we conclude that the most detectable and least ambiguous evidence of
volcanism are changes in both O absorption and the slope of the spectrum.Comment: 13 pages, 5 figures, 4 tables, Accepted for publication in AJ
(September 26, 2023
Sensitive Probing of Exoplanetary Oxygen via Mid Infrared Collisional Absorption
The collision-induced fundamental vibration-rotation band at 6.4 um is the
most significant absorption feature from O2 in the infrared (Timofeyev and
Tonkov, 1978; Rinslandet al., 1982, 1989), yet it has not been previously
incorporated into exoplanet spectral analyses for several reasons. Either CIAs
were not included or incomplete/obsolete CIA databases were used. Also, the
current version of HITRAN does not include CIAs at 6.4 um with other collision
partners (O2-X). We include O2-X CIA features in our transmission spectroscopy
simulations by parameterizing the 6.4 um O2-N2 CIA based on Rinsland et
al.(1989) and the O2-CO2 CIA based on Baranov et al. (2004). Here we report
that the O2-X CIA may be the most detectable O2 feature for transit
observations. For a potentialTRAPPIST-1e analogue system within 5 pc of the
Sun, it could be the only O2 detectable signature with JWST (using MIRI LRS)
for a modern Earth-like cloudy atmosphere with biological quantities of O2.
Also, we show that the 6.4 um O2-X CIA would be prominent for O2-rich
desiccated atmospheres (Luger and Barnes, 2015) and could be detectable with
JWST in just a few transits. For systems beyond 5 pc, this feature could
therefore be a powerful discriminator of uninhabited planets with
non-biological "false positive" O2 in their atmospheres - as they would only be
detectable at those higher O2 pressures.Comment: Published in Nature Astronom
Evaluating the Plausible Range of N2O Biosignatures on Exo-Earths: An Integrated Biogeochemical, Photochemical, and Spectral Modeling Approach
Nitrous oxide (N2O) -- a product of microbial nitrogen metabolism -- is a
compelling exoplanet biosignature gas with distinctive spectral features in the
near- and mid-infrared, and only minor abiotic sources on Earth. Previous
investigations of N2O as a biosignature have examined scenarios using Earthlike
N2O mixing ratios or surface fluxes, or those inferred from Earth's geologic
record. However, biological fluxes of N2O could be substantially higher, due to
a lack of metal catalysts or if the last step of the denitrification metabolism
that yields N2 from N2O had never evolved. Here, we use a global biogeochemical
model coupled with photochemical and spectral models to systematically quantify
the limits of plausible N2O abundances and spectral detectability for Earth
analogs orbiting main-sequence (FGKM) stars. We examine N2O buildup over a
range of oxygen conditions (1%-100% present atmospheric level) and N2O fluxes
(0.01-100 teramole per year; Tmol = 10^12 mole) that are compatible with
Earth's history. We find that N2O fluxes of 10 [100] Tmol yr would lead
to maximum N2O abundances of ~5 [50] ppm for Earth-Sun analogs, 90 [1600] ppm
for Earths around late K dwarfs, and 30 [300] ppm for an Earthlike TRAPPIST-1e.
We simulate emission and transmission spectra for intermediate and maximum N2O
concentrations that are relevant to current and future space-based telescopes.
We calculate the detectability of N2O spectral features for high-flux scenarios
for TRAPPIST-1e with JWST. We review potential false positives, including
chemodenitrification and abiotic production via stellar activity, and identify
key spectral and contextual discriminants to confirm or refute the biogenicity
of the observed N2O.Comment: 22 pages, 17 figures; ApJ, 937, 10
Water Condensation Zones around Main Sequence Stars
Understanding the set of conditions that allow rocky planets to have liquid
water on their surface -- in the form of lakes, seas or oceans -- is a major
scientific step to determine the fraction of planets potentially suitable for
the emergence and development of life as we know it on Earth. This effort is
also necessary to define and refine the so-called "Habitable Zone" (HZ) in
order to guide the search for exoplanets likely to harbor remotely detectable
life forms. Until now, most numerical climate studies on this topic have
focused on the conditions necessary to maintain oceans, but not to form them in
the first place. Here we use the three-dimensional Generic Planetary Climate
Model (PCM), historically known as the LMD Generic Global Climate Model (GCM),
to simulate water-dominated planetary atmospheres around different types of
Main-Sequence stars. The simulations are designed to reproduce the conditions
of early ocean formation on rocky planets due to the condensation of the
primordial water reservoir at the end of the magma ocean phase. We show that
the incoming stellar radiation (ISR) required to form oceans by condensation is
always drastically lower than that required to vaporize oceans. We introduce a
Water Condensation Limit, which lies at significantly lower ISR than the inner
edge of the HZ calculated with three-dimensional numerical climate simulations.
This difference is due to a behavior change of water clouds, from low-altitude
dayside convective clouds to high-altitude nightside stratospheric clouds.
Finally, we calculated transit spectra, emission spectra and thermal phase
curves of TRAPPIST-1b, c and d with H2O-rich atmospheres, and compared them to
CO2 atmospheres and bare rock simulations. We show using these observables that
JWST has the capability to probe steam atmospheres on low-mass planets, and
could possibly test the existence of nightside water clouds.Comment: Accepted for publication in Astronomy & Astrophysic
Modeling Atmospheric Lines By the Exoplanet Community (MALBEC) version 1.0: A CUISINES radiative transfer intercomparison project
Radiative transfer (RT) models are critical in the interpretation of
exoplanetary spectra, in simulating exoplanet climates and when designing the
specifications of future flagship observatories. However, most models differ in
methodologies and input data, which can lead to significantly different
spectra. In this paper, we present the experimental protocol of the MALBEC
(Modeling Atmospheric Lines By the Exoplanet Community) project. MALBEC is an
exoplanet model intercomparison project (exoMIP) that belongs to the CUISINES
(Climates Using Interactive Suites of Intercomparisons Nested for Exoplanet
Studies) framework which aims to provide the exoplanet community with a large
and diverse set of comparison and validation of models. The proposed protocol
tests include a large set of initial participating RT models, a broad range of
atmospheres (from Hot Jupiters to temperate terrestrials) and several
observation geometries, which would allow us to quantify and compare the
differences between different RT models used by the exoplanetary community. Two
types of tests are proposed: transit spectroscopy and direct imaging modeling,
with results from the proposed tests to be published in dedicated follow-up
papers. To encourage the community to join this comparison effort and as an
example, we present simulation results for one specific transit case (GJ-1214
b), in which we find notable differences in how the various codes handle the
discretization of the atmospheres (e.g., sub-layering), the treatment of
molecular opacities (e.g., correlated-k, line-by-line) and the default
spectroscopic repositories generally used by each model (e.g., HITRAN, HITEMP,
ExoMol)
Transmission Spectroscopy of the Habitable Zone Exoplanet LHS 1140 b with JWST/NIRISS
LHS 1140 b is the second-closest temperate transiting planet to Earth with an equilibrium temperature low enough to support surface liquid water. At 1.730 ± 0.025 R ⊕, LHS 1140 b falls within the radius valley separating H2-rich mini-Neptunes from rocky super-Earths. Recent mass and radius revisions indicate a bulk density significantly lower than expected for an Earth-like rocky interior, suggesting that LHS 1140 b could be either a mini-Neptune with a small envelope of hydrogen (∼0.1% by mass) or a water world (9%–19% water by mass). Atmospheric characterization through transmission spectroscopy can readily discern between these two scenarios. Here we present two JWST/NIRISS transit observations of LHS 1140 b, one of which captures a serendipitous transit of LHS 1140 c. The combined transmission spectrum of LHS 1140 b shows a telltale spectral signature of unocculted faculae (5.8σ), covering ∼20% of the visible stellar surface. Besides faculae, our spectral retrieval analysis reveals tentative evidence of residual spectral features, best fit by Rayleigh scattering from a N2-dominated atmosphere (2.3σ), irrespective of the consideration of atmospheric hazes. We also show through Global Climate Models (GCMs) that H2-rich atmospheres of various compositions (100×, 300×, 1000× solar metallicity) are ruled out to >10σ. The GCM calculations predict that water clouds form below the transit photosphere, limiting their impact on transmission data. Our observations suggest that LHS 1140 b is either airless or, more likely, surrounded by an atmosphere with a high mean molecular weight. Our tentative evidence of a N2-rich atmosphere provides strong motivation for future transmission spectroscopy observations of LHS 1140 b
CAMEMBERT: A Mini-Neptunes GCM Intercomparison, Protocol Version 1.0. A CUISINES Model Intercomparison Project
With an increased focus on the observing and modelling of mini-Neptunes,
there comes a need to better understand the tools we use to model their
atmospheres. In this paper, we present the protocol for the CAMEMBERT
(Comparing Atmospheric Models of Extrasolar Mini-neptunes Building and
Envisioning Retrievals and Transits) project, an intercomparison of general
circulation models (GCMs) used by the exoplanetary science community to
simulate the atmospheres of mini-Neptunes. We focus on two targets well studied
both observationally and theoretically with planned JWST Cycle 1 observations:
the warm GJ~1214b and the cooler K2-18b. For each target, we consider a
temperature-forced case, a clear sky dual-grey radiative transfer case, and a
clear sky multi band radiative transfer case, covering a range of complexities
and configurations where we know differences exist between GCMs in the
literature. This paper presents all the details necessary to participate in the
intercomparison, with the intention of presenting the results in future papers.
Currently, there are eight GCMs participating (ExoCAM, Exo-FMS, FMS PCM,
Generic PCM, MITgcm, RM-GCM, THOR, and the UM), and membership in the project
remains open. Those interested in participating are invited to contact the
authors.Comment: Accepted to PS
A transmission spectrum of the sub-Earth planet L98-59~b in 1.1-1.7 m
With the increasing number of planets discovered by TESS, the atmospheric
characterization of small exoplanets is accelerating. L98-59 is a M-dwarf
hosting a multi-planet system, and so far, four small planets have been
confirmed. The innermost planet b is smaller and lighter
than Earth, and should thus have a predominantly rocky composition. The Hubble
Space Telescope observed five primary transits of L98-59b in m,
and here we report the data analysis and the resulting transmission spectrum of
the planet. We measure the transit depths for each of the five transits and, by
combination, we obtain a transmission spectrum with an overall precision of
ppm in for each of the 18 spectrophotometric channels. With this level
of precision, the transmission spectrum does not show significant modulation,
and is thus consistent with a planet without any atmosphere or a planet having
an atmosphere and high-altitude clouds or haze. The scenarios involving an
aerosol-free, H-dominated atmosphere with HO or CH are inconsistent
with the data. The transmission spectrum also disfavors, but does not rules
out, an HO-dominated atmosphere without clouds. A spectral retrieval
process suggests that an H-dominated atmosphere with HCN and clouds or haze
may be the preferred solution, but this indication is non-conclusive. Future
James Webb Space Telescope observations may find out the nature of the planet
among the remaining viable scenarios.Comment: 17 pages, 5 figures, 7 tables, accepted for publication in A
Impact of Clouds and Hazes on the Simulated JWST Transmission Spectra of Habitable Zone Planets in the TRAPPIST-1 System
The TRAPPIST-1 system, consisting of an ultra-cool host star having seven
known Earth-size planets will be a prime target for atmospheric
characterization with JWST. However, the detectability of atmospheric molecular
species may be severely impacted by the presence of clouds and/or hazes. In
this work, we perform 3-D General Circulation Model (GCM) simulations with the
LMD Generic model supplemented by 1-D photochemistry simulations at the
terminator with the Atmos model to simulate several possible atmospheres for
TRAPPIST-1e, 1f and 1g: 1) modern Earth, 2) Archean Earth, and 3) CO2-rich
atmospheres. JWST synthetic transit spectra were computed using the GSFC
Planetary Spectrum Generator (PSG). We find that TRAPPIST-1e, 1f and 1g
atmospheres, with clouds and/or hazes, could be detected using JWST's NIRSpec
prism from the CO2 absorption line at 4.3 um in less than 15 transits at 3
sigma or less than 35 transits at 5 sigma. However, our analysis suggests that
other gases would require hundreds (or thousands) of transits to be detectable.
We also find that H2O, mostly confined in the lower atmosphere, is very
challenging to detect for these planets or similar systems if the planets'
atmospheres are not in a moist greenhouse state. This result demonstrates that
the use of GCMs, self-consistently taking into account the effect of clouds and
sub-saturation, is crucial to evaluate the detectability of atmospheric
molecules of interest as well as for interpreting future detections in a more
global (and thus robust and relevant) approach.Comment: 36 pages, 19 figure