15 research outputs found
The Eclipse Mapping Null Space: Comparing Theoretical Predictions with Observed Maps
High-precision exoplanet eclipse light curves, like those possible with JWST,
enable flux and temperature mapping of exoplanet atmospheres. These eclipse
maps will have unprecedented precision, providing an opportunity to constrain
current theoretical predictions of exoplanet atmospheres. However, eclipse
mapping has unavoidable mathematical limitations because many map patterns are
unobservable. This ``null space'' has implications for making comparisons
between predictions from general circulation models (GCMs) and the observed
planet maps, and, thus, affects our understanding of the physical processes
driving the observed maps. We describe the eclipse-mapping null space and show
how GCM forward models can be transformed to their observable modes for more
appropriate comparison with retrieved eclipse maps, demonstrated with
applications to synthetic data of an ultra-hot Jupiter and a cloudy warm
Jupiter under JWST-best-case- and extreme-precision observing scenarios. We
show that the effects of the null space can be mitigated and manipulated
through observational design, and JWST exposure times are short enough to not
increase the size of the null space. Furthermore, we show the mathematical
connection between the null space and the ``eigenmapping'' method,
demonstrating how eigenmaps can be used to understand the null space in a
model-independent way. We leverage this connection to incorporate null-space
uncertainties in retrieved maps, which increases the uncertainties to now
encompass the ground truth for synthetic data. The comparisons between observed
maps and forward models that are enabled by this work, and the improved
eclipse-mapping uncertainties, will be critical to our interpretation of
multidimensional aspects of exoplanets in the JWST era.Comment: 20 pages, 12 figures. Accepted for publication in The Astronomical
Journal. Note that PDF readers may blur figures 1 and 3, which can be fixed
by zooming i
Exoplanets: Correlated Noise and Cautionary Tales
Transiting exoplanets provide the best opportunity for planetary characterization, and thus the search for life outside the Solar System. These planets orbit such that they pass in front ( transit\u27\u27) and behind ( eclipse\u27\u27) their host star, and a spectrum of the lost flux constrains the atmospheric properties of the planet. In transits, the flux modulation scales with the cross-sectional area of the planet, and the spectrum includes signatures of molecules in the upper atmosphere of the planet\u27s terminator, which the host star\u27s light passes through on the way to the observer. With eclipses, the lost flux is the direct emission of the planet, a spectrum of which contains emission and absorption features of molecules in the atmosphere depending on atmospheric thermal structure. These signals scale with the size and brightness of the planet and are so dwarfed by the brightness of the host star that only \u3e 1000 K Jupiter-sized planets are observable with current instrumentation. In this work, I develop new techniques and compare existing data analysis methods to extract weak planetary signals. Chapter 2 describes a new elliptical photometry data analysis approach to disentangle exoplanet observations from telescope vibrations. Chapter 3 describes an analysis of Spitzer Space Telescope observations of eclipses of the planet WASP-29b using elliptical photometry and two different light curve modeling methods, and addresses the differences between results. In Chapter 4, I analyze two similar observations of WASP-34b using a grazing eclipse light-curve model. Finally, in Chapter 5 I reanalyze all Spitzer eclipse observations of the Neptune-sized GJ 436b, applying the lessons learned from my earlier works, and comparing my results with the literature
Planet Eclipse Mapping with Long-Term Baseline Drifts
High precision lightcurves combined with eclipse mapping techniques can
reveal the horizontal and vertical structure of a planet's thermal emission and
the dynamics of hot Jupiters. Someday, they even may reveal the surface maps of
rocky planets. However, inverting lightcurves into maps requires an
understanding of the planet, star and instrumental trends because they can
resemble the gradual flux variations as the planet rotates (ie. partial phase
curves). In this work, we simulate lightcurves with baseline trends and assess
the impact on planet maps. Baseline trends can be erroneously modeled by
incorrect astrophysical planet map features, but there are clues to avoid this
pitfall in both the residuals of the lightcurve during eclipse and sharp
features at the terminator of the planet. Models that use a Gaussian process or
polynomial to account for a baseline trend successfully recover the input map
even in the presence of systematics but with worse precision for the m=1
spherical harmonic terms. This is also confirmed with the ThERESA eigencurve
method where fewer lightcurve terms can model the planet without correlations
between the components. These conclusions help aid the decision on how to
schedule observations to improve map precision. If the m=1 components are
critical, such as measuring the East/West hotspot shift on a hot Jupiter,
better characterization of baseline trends can improve the m=1 terms'
precision. For latitudinal North/South information from the remaining mapping
terms, it is preferable to obtain high signal-to-noise at ingress/egress with
more eclipses.Comment: AJ, accepted, 22 page
Latitudinal Asymmetry in the Dayside Atmosphere of WASP-43b
We present two-dimensional near-infrared temperature maps of the canonical hot Jupiter WASP-43b using a phase-curve observation with JWST NIRSpec/G395H. From the white-light planetary transit, we improve constraints on the planet’s orbital parameters and measure a planet-to-star radius ratio of 0.15883−0.00053+0.00056 . Using the white-light phase curve, we measure a longitude of maximum brightness of 6.9−0.°5+0.°5 east of the substellar point and a phase-curve offset of 10.0−0.°8+0.°8 . We also find a ≈4σ detection of a latitudinal hotspot offset of −13.4−1.°7+3.°2 , the first significant detection of a nonequatorial hotspot in an exoplanet atmosphere. We show that this detection is robust to variations within planetary parameter uncertainties, but only if the transit is used to improve constraints, showing the importance of transit observations to eclipse mapping. Maps retrieved from the NRS1 and NRS2 detectors are similar, with hotspot locations consistent between the two detectors at the 1σ level. Our JWST data show brighter (hotter) nightsides and a dimmer (colder) dayside at the shorter wavelengths relative to fits to Spitzer 3.6 and 4.5 μm phase curves. Through comparison between our phase curves and a set of general circulation models, we find evidence for clouds on the nightside and atmospheric drag or high metallicity reducing the eastward hotspot offset
Proxima Centauri b is not a transiting exoplanet
We report Spitzer Space Telescope observations during predicted transits of
the exoplanet Proxima Centauri b. As the nearest terrestrial habitable-zone
planet we will ever discover, any potential transit of Proxima b would place
strong constraints on its radius, bulk density, and atmosphere. Subsequent
transmission spectroscopy and secondary-eclipse measurements could then probe
the atmospheric chemistry, physical processes, and orbit, including a search
for biosignatures. However, our photometric results rule out planetary transits
at the 200~ppm level at 4.5, yielding a 3 upper radius limit
of 0.4~R_\rm{\oplus} (Earth radii). Previous claims of possible transits from
optical ground- and space-based photometry were likely correlated noise in the
data from Proxima Centauri's frequent flaring. Follow-up observations should
focus on planetary radio emission, phase curves, and direct imaging. Our study
indicates dramatically reduced stellar activity at near-to-mid infrared
wavelengths, compared to the optical. Proxima b is an ideal target for
space-based infrared telescopes, if their instruments can be configured to
handle Proxima's brightness.Comment: 8 pages, 3 figures, 2 tables, accepted for publication in MNRA
A broadband thermal emission spectrum of the ultra-hot Jupiter WASP-18b
Close-in giant exoplanets with temperatures greater than 2,000 K (''ultra-hot
Jupiters'') have been the subject of extensive efforts to determine their
atmospheric properties using thermal emission measurements from the Hubble and
Spitzer Space Telescopes. However, previous studies have yielded inconsistent
results because the small sizes of the spectral features and the limited
information content of the data resulted in high sensitivity to the varying
assumptions made in the treatment of instrument systematics and the atmospheric
retrieval analysis. Here we present a dayside thermal emission spectrum of the
ultra-hot Jupiter WASP-18b obtained with the NIRISS instrument on JWST. The
data span 0.85 to 2.85 m in wavelength at an average resolving power of
400 and exhibit minimal systematics. The spectrum shows three water emission
features (at 6 confidence) and evidence for optical opacity,
possibly due to H, TiO, and VO (combined significance of 3.8).
Models that fit the data require a thermal inversion, molecular dissociation as
predicted by chemical equilibrium, a solar heavy element abundance
(''metallicity'', M/H = 1.03 solar), and a
carbon-to-oxygen (C/O) ratio less than unity. The data also yield a dayside
brightness temperature map, which shows a peak in temperature near the
sub-stellar point that decreases steeply and symmetrically with longitude
toward the terminators.Comment: JWST ERS bright star observations. Uploaded to inform JWST Cycle 2
proposals. Manuscript under review. 50 pages, 14 figures, 2 table
Nightside clouds and disequilibrium chemistry on the hot Jupiter WASP-43b
Hot Jupiters are among the best-studied exoplanets, but it is still poorly understood how their chemical composition and cloud properties vary with longitude. Theoretical models predict that clouds may condense on the nightside and that molecular abundances can be driven out of equilibrium by zonal winds. Here we report a phase-resolved emission spectrum of the hot Jupiter WASP-43b measured from 5-12 μm with JWST's Mid-Infrared Instrument (MIRI). The spectra reveal a large day-night temperature contrast (with average brightness temperatures of 1524±35 and 863±23 Kelvin, respectively) and evidence for water absorption at all orbital phases. Comparisons with three-dimensional atmospheric models show that both the phase curve shape and emission spectra strongly suggest the presence of nightside clouds which become optically thick to thermal emission at pressures greater than ~100 mbar. The dayside is consistent with a cloudless atmosphere above the mid-infrared photosphere. Contrary to expectations from equilibrium chemistry but consistent with disequilibrium kinetics models, methane is not detected on the nightside (2σ upper limit of 1-6 parts per million, depending on model assumptions)
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Nightside clouds and disequilibrium chemistry on the hot Jupiter WASP-43b
Hot Jupiters are among the best-studied exoplanets, but it is still poorly understood how their chemical composition and cloud properties vary with longitude. Theoretical models predict that clouds may condense on the nightside and that molecular abundances can be driven out of equilibrium by zonal winds. Here we report a phase-resolved emission spectrum of the hot Jupiter WASP-43b measured from 5 μm to 12 μm with the JWST’s Mid-Infrared Instrument. The spectra reveal a large day–night temperature contrast (with average brightness temperatures of 1,524 ± 35 K and 863 ± 23 K, respectively) and evidence for water absorption at all orbital phases. Comparisons with three-dimensional atmospheric models show that both the phase-curve shape and emission spectra strongly suggest the presence of nightside clouds that become optically thick to thermal emission at pressures greater than ~100 mbar. The dayside is consistent with a cloudless atmosphere above the mid-infrared photosphere. Contrary to expectations from equilibrium chemistry but consistent with disequilibrium kinetics models, methane is not detected on the nightside (2σ upper limit of 1–6 ppm, depending on model assumptions). Our results provide strong evidence that the atmosphere of WASP-43b is shaped by disequilibrium processes and provide new insights into the properties of the planet’s nightside clouds. However, the remaining discrepancies between our observations and our predictive atmospheric models emphasize the importance of further exploring the effects of clouds and disequilibrium chemistry in numerical models.Peer reviewe
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