41 research outputs found
Exploring Kepler Giant Planets in the Habitable Zone
The Kepler mission found hundreds of planet candidates within the habitable
zones (HZ) of their host star, including over 70 candidates with radii larger
than 3 Earth radii () within the optimistic habitable zone (OHZ)
(Kane et al. 2016). These giant planets are potential hosts to large
terrestrial satellites (or exomoons) which would also exist in the HZ. We
calculate the occurrence rates of giant planets (~3.0--25~) in
the OHZ and find a frequency of for G stars, for K stars, and for M stars. We compare this with
previously estimated occurrence rates of terrestrial planets in the HZ of G, K
and M stars and find that if each giant planet has one large terrestrial moon
then these moons are less likely to exist in the HZ than terrestrial planets.
However, if each giant planet holds more than one moon, then the occurrence
rates of moons in the HZ would be comparable to that of terrestrial planets,
and could potentially exceed them. We estimate the mass of each planet
candidate using the mass-radius relationship developed by Chen & Kipping
(2016). We calculate the Hill radius of each planet to determine the area of
influence of the planet in which any attached moon may reside, then calculate
the estimated angular separation of the moon and planet for future imaging
missions. Finally, we estimate the radial velocity semi-amplitudes of each
planet for use in follow up observations.Comment: 19 Pages, 16 Figures, 5 Table
Demarcating circulation regimes of synchronously rotating terrestrial planets within the habitable zone
We investigate the atmospheric dynamics of terrestrial planets in synchronous rotation within the habitable zone of low-mass stars using the Community Atmosphere Model (CAM). The surface temperature contrast between day and night hemispheres decreases with an increase in incident stellar flux, which is opposite the trend seen on gas giants. We define three dynamical regimes in terms of the equatorial Rossby deformation radius and the Rhines length. The slow rotation regime has a mean zonal circulation that spans from day to night side, with both the Rossby deformation radius and the Rhines length exceeding planetary radius, which occurs for planets around stars with effective temperatures of 3300 K to 4500 K (rotation period > 20 days). Rapid rotators have a mean zonal circulation that partially spans a hemisphere and with banded cloud formation beneath the substellar point, with the Rossby deformation radius is less than planetary radius, which occurs for planets orbiting stars with effective temperatures of less than 3000 K (rotation period < 5 days). In between is the Rhines rotation regime, which retains a thermally-direct circulation from day to night side but also features midlatitude turbulence-driven zonal jets. Rhines rotators occur for planets around stars in the range of 3000 K to 3300 K (rotation period ⌠5 to 20 days), where the Rhines length is greater than planetary radius but the Rossby deformation radius is less than planetary radius. The dynamical state can be observationally inferred from comparing the morphology of the thermal emission phase curves of synchronously rotating planets
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
Exploring Kepler Giant Planets in the Habitable Zone
The Kepler mission found hundreds of planet candidates within the Habitable Zones (HZ) of their host star, including over 70 candidates with radii larger than three Earth radii (Râ) within the optimistic HZ (OHZ). These giant planets are potential hosts to large terrestrial satellites (or exomoons) which would also exist in the HZ. We calculate the occurrence rates of giant planets (R_p = 3.0â25 Râ) in the OHZ, and find a frequency of (6.5 ± 1.9)% for G stars, (11.5 ± 3.1)% for K stars, and (6 ± 6)% for M stars. We compare this with previously estimated occurrence rates of terrestrial planets in the HZ of G, K, and M stars and find that if each giant planet has one large terrestrial moon then these moons are less likely to exist in the HZ than terrestrial planets. However, if each giant planet holds more than one moon, then the occurrence rates of moons in the HZ would be comparable to that of terrestrial planets, and could potentially exceed them. We estimate the mass of each planet candidate using the massâradius relationship developed by Chen & Kipping. We calculate the Hill radius of each planet to determine the area of influence of the planet in which any attached moon may reside, then calculate the estimated angular separation of the moon and planet for future imaging missions. Finally, we estimate the radial velocity semi-amplitudes of each planet for use in follow-up observations
The K2-3 system revisited: testing photoevaporation and core-powered mass loss with three small planets spanning the radius valley
Multi-planet systems orbiting M dwarfs provide valuable tests of theories of
small planet formation and evolution. K2-3 is an early M dwarf hosting three
small exoplanets (1.5-2.0 Earth radii) at distances of 0.07-0.20 AU. We measure
the high-energy spectrum of K2-3 with HST/COS and XMM-Newton, and use
empirically-driven estimates of Ly-alpha and extreme ultraviolet flux. We use
EXOFASTv2 to jointly fit radial velocity, transit, and SED data. This
constrains the K2-3 planet radii to 4% uncertainty and the masses of K2-3b and
c to 13% and 30%, respectively; K2-3d is not detected in RV measurements. K2-3b
and c are consistent with rocky cores surrounded by solar composition envelopes
(mass fractions of 0.36% and 0.07%), H2O envelopes (55% and 16%), or a mixture
of both. However, based on the high-energy output and estimated age of K2-3, it
is unlikely that K2-3b and c retain solar composition atmospheres. We pass the
planet parameters and high-energy stellar spectrum to atmospheric models.
Dialing the high-energy spectrum up and down by a factor of 10 produces
significant changes in trace molecule abundances, but not at a level detectable
with transmission spectroscopy. Though the K2-3 planets span the small planet
radius valley, the observed system architecture cannot be readily explained by
photoevaporation or core-powered mass loss. We instead propose 1) the K2-3
planets are all volatile-rich, with K2-3d having a lower density than typical
of super-Earths, and/or 2) the K2-3 planet architecture results from more
stochastic processes such as planet formation, planet migration, and impact
erosion.Comment: 15 pages, 7 figure, published in AJ, HLSPs at
https://archive.stsci.edu/hlsp/mstarpanspe
Detecting Earth-like Biosignatures on Rocky Exoplanets around Nearby Stars with Ground-based Extremely Large Telescopes
As we begin to discover rocky planets in the habitable zone of nearby stars with missions like TESS and CHEOPS, we will need quick advancements on instrumentation and observational techniques that will enable us to answer key science questions, such as What are the atmospheric characteristics of habitable zone rocky planets? How common are Earth-like biosignatures in rocky
planets?} How similar or dissimilar are those planets to Earth? Over the next decade we expect to have discovered several Earth-analog candidates, but we will not have the tools to study the atmospheres of all of them in detail. Ground-based ELTs can identify biosignatures in the spectra of these candidate exo-Earths and understand how the planets' atmospheres compare to the Earth at different epochs. Transit spectroscopy, high-resolution spectroscopy, and reflected-light direct imaging on ELTs can identify multiple biosignatures for habitable zone, rocky planets around M stars at optical to near-infrared wavelengths. Thermal infrared direct imaging can detect habitable zone, rocky planets around AFGK stars, identifying ozone and motivating reflected-light
follow-up observations with NASA missions like HabEx/LUVOIR. Therefore, we recommend that the Astro2020 Decadal Survey Committee support: (1) the search for Earth-like biosignatures on rocky planets around nearby stars as a key science case; (2) the construction over the next decade of ground-based Extremely Large Telecopes (ELTs), which will provide the large aperture and
spatial resolution necessary to start revealing the atmospheres of
Earth-analogues around nearby stars; (3) the development of instrumentation that optimizes the detection of biosignatures; and (4) the generation of accurate line lists for potential biosignature gases, which are needed as model templates to detect those molecules
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