39 research outputs found
Physically-motivated basis functions for temperature maps of exoplanets
Thermal phase curves of exoplanet atmospheres have revealed temperature maps as a function of planetary longitude, often by sinusoidal decomposition of the phase curve. We construct a framework for describing two-dimensional temperature maps of exoplanets with mathematical basis functions derived for a fluid layer on a rotating, heated sphere with drag/friction, which are generalisations of spherical harmonics. These basis functions naturally produce physically-motivated temperature maps for exoplanets with few free parameters. We investigate best practices for applying this framework to temperature maps of hot Jupiters by splitting the problem into two parts: (1) we constrain the temperature map as a function of latitude by tuning the basis functions to reproduce general circulation model outputs, since disk-integrated phase curve observations do not constrain this dimension; and (2) we infer the temperature maps of real hot Jupiters using original reductions of several Spitzer phase curves, which directly constrain the temperature variations with longitude. The resulting phase curves can be described with only three free parameters per bandpass – an efficiency improvement over the usual five or so used to describe sinusoidal decompositions of phase curves. Upon obtaining the hemispherically averaged day side and night side temperatures, the standard approach would be to use zero-dimensional box models to infer the Bond albedo and redistribution efficiency. We elucidate the limitation of these box models by demonstrating that negative Bond albedos may be obtained due to a choice of boundary condition on the night side temperature. We propose generalized definitions for the Bond albedo and heat redistribution efficiency for use with two-dimensional (2D) temperature maps. Open-source software called kelp is provided to efficiently compute the 2D temperature maps, phase curves, albedos and redistribution efficiencies
Water Vapor and Clouds on the Habitable-Zone Sub-Neptune Exoplanet K2-18b
Results from the Kepler mission indicate that the occurrence rate of small
planets ( ) in the habitable zone of nearby low-mass stars may be
as high as 80%. Despite this abundance, probing the conditions and atmospheric
properties on any habitable-zone planet is extremely difficult and has remained
elusive to date. Here, we report the detection of water vapor and the likely
presence of liquid and icy water clouds in the atmosphere of the
habitable-zone planet K2-18b. The simultaneous detection of water
vapor and clouds in the mid-atmosphere of K2-18b is particularly intriguing
because K2-18b receives virtually the same amount of total insolation from its
host star ( W m) as the Earth receives from the Sun
(1361 W m), resulting in the right conditions for water vapor to
condense and explain the detected clouds. In this study, we observed nine
transits of K2-18b using HST/WFC3 in order to achieve the necessary sensitivity
to detect the water vapor, and we supplement this data set with Spitzer and K2
observations to obtain a broader wavelength coverage. While the thick
hydrogen-dominated envelope we detect on K2-18b means that the planet is not a
true Earth analog, our observations demonstrate that low-mass habitable-zone
planets with the right conditions for liquid water are accessible with
state-of-the-art telescopes.Comment: Published in ApJL, includes important updates to stellar and planet
parameter
APPleSOSS: A Producer of ProfiLEs for SOSS. Application to the NIRISS SOSS Mode
The SOSS mode of the NIRISS instrument is poised to be one of the workhorse
modes for exoplanet atmosphere observations with the newly launched James Webb
Space Telescope. One of the challenges of the SOSS mode, however, is the
physical overlap of the first two diffraction orders of the G700XD grism on the
detector. Recently, the ATOCA algorithm was developed and implemented as an
option in the official JWST pipeline, as a method to extract SOSS spectra by
decontaminating the detector -- that is, separating the first and second
orders. Here, we present APPleSOSS (A Producer of ProfiLEs for SOSS), which
generates the spatial profiles for each diffraction order upon which ATOCA
relies. We validate APPleSOSS using simulated SOSS time series observations of
WASP-52b, and compare it to ATOCA extractions using two other spatial profiles
(a best and worst case scenario on-sky), as well as a simple box extraction
performed without taking into account the order contamination. We demonstrate
that APPleSOSS traces retain a high degree of fidelity to the true underlying
spatial profiles, and therefore yield accurate extracted spectra. We further
confirm that the effects of the order contamination for relative measurements
(e.g., exoplanet transmission or emission observations) is small -- the
transmission spectrum obtained from each of our four tests, including the
contaminated box extraction, deviates by 0.1 from the
atmosphere model input into our noiseless simulations. We further confirm via a
retrieval analysis that the atmosphere parameters (metallicity and C/O)
obtained from each transmission spectrum are consistent at the 1 level
with the true underlying values.Comment: 12 pages, 9 figures. Submitted to PAS
Water Vapor and Clouds on the Habitable-zone Sub-Neptune Exoplanet K2-18b
Results from the Kepler mission indicate that the occurrence rate of small planets (<3 R⊕) in the habitable zone of nearby low-mass stars may be as high as 80%. Despite this abundance, probing the conditions and atmospheric properties on any habitable-zone planet is extremely difficult and has remained elusive to date. Here, we report the detection of water vapor and the likely presence of liquid and icy water clouds in the atmosphere of the 2.6 R ⊕ habitable-zone planet K2-18b. The simultaneous detection of water vapor and clouds in the mid-atmosphere of K2-18b is particularly intriguing because K2-18b receives virtually the same amount of total insolation from its host star (1368^(+114)_(-107) W m⁻²) as the Earth receives from the Sun (1361 W m⁻²), resulting in the right conditions for water vapor to condense and explain the detected clouds. In this study we observed nine transits of K2-18b using Hubble Space Telescope/WFC3 in order to achieve the necessary sensitivity to detect the water vapor, and we supplement this data set with Spitzer and K2 observations to obtain a broader wavelength coverage. While the thick hydrogen-dominated envelope we detect on K2-18b means that the planet is not a true Earth analog, our observations demonstrate that low-mass habitable-zone planets with the right conditions for liquid water are accessible with state-of-the-art telescopes
Characterizing the Near-infrared Spectra of Flares from TRAPPIST-1 During JWST Transit Spectroscopy Observations
We present the first analysis of JWST near-infrared spectroscopy of stellar
flares from TRAPPIST-1 during transits of rocky exoplanets. Four flares were
observed from 0.6--2.8 m with NIRISS and 0.6--3.5 m with NIRSpec
during transits of TRAPPIST-1b, f, and g. We discover P and Br
line emission and characterize flare continuum at wavelengths from 1--3.5
m for the first time. Observed lines include H,
P-P, Br, He I 0.7062m, two Ca II
infrared triplet (IRT) lines, and the He I IRT. We observe a reversed Paschen
decrement from P-P alongside changes in the light curve shapes
of these lines. The continuum of all four flares is well-described by blackbody
emission with an effective temperature below 5300 K, lower than temperatures
typically observed at optical wavelengths. The 0.6--1 m spectra were
convolved with the TESS response, enabling us to measure the flare rate of
TRAPPIST-1 in the TESS bandpass. We find flares of 10 erg large enough
to impact transit spectra occur at a rate of 3.6 flare
d, 10 higher than previous predictions from K2. We measure
the amount of flare contamination at 2 m for the TRAPPIST-1b and f
transits to be 500450 and 2100400 ppm, respectively. We find up to
80% of flare contamination can be removed, with mitigation most effective from
1.0--2.4 m. These results suggest transits affected by flares may still be
useful for atmospheric characterization efforts.Comment: 29 pages, 17 figures, 3 tables, accepted to The Astrophysical Journa
ATOCA: an algorithm to treat order contamination. Application to the NIRISS SOSS mode
After a successful launch, the James Webb Space Telescope is preparing to
undertake one of its principal missions, the characterization of the
atmospheres of exoplanets. The Single Object Slitless Spectroscopy (SOSS) mode
of the Near Infrared Imager and Slitless Spectrograph (NIRISS) is the only
observing mode that has been specifically designed for this objective. It
features a wide simultaneous spectral range (0.6--2.8\,\micron) through two
spectral diffraction orders. However, due to mechanical constraints, these two
orders overlap slightly over a short range, potentially introducing a
``contamination'' signal in the extracted spectrum. We show that for a typical
box extraction, this contaminating signal amounts to 1\% or less over the
1.6--2.8\,\micron\ range (order 1), and up to 1\% over the 0.85--0.95\,\micron\
range (order 2). For observations of exoplanet atmospheres (transits, eclipses
or phase curves) where only temporal variations in flux matter, the
contamination signal typically biases the results by order of 1\% of the
planetary atmosphere spectral features strength. To address this problem, we
developed the Algorithm to Treat Order ContAmination (ATOCA). By constructing a
linear model of each pixel on the detector, treating the underlying incident
spectrum as a free variable, ATOCA is able to perform a simultaneous extraction
of both orders. We show that, given appropriate estimates of the spatial trace
profiles, the throughputs, the wavelength solutions, as well as the spectral
resolution kernels for each order, it is possible to obtain an extracted
spectrum accurate to within 10\,ppm over the full spectral range.Comment: Submitted to PASP. 22 pages, 12 figure
Atmospheric Reconnaissance of TRAPPIST-1 b with JWST/NIRISS: Evidence for Strong Stellar Contamination in the Transmission Spectra
TRAPPIST-1 is a nearby system of seven Earth-sized, temperate, rocky
exoplanets transiting a Jupiter-sized M8.5V star, ideally suited for in-depth
atmospheric studies. Each TRAPPIST-1 planet has been observed in transmission
both from space and from the ground, confidently rejecting cloud-free,
hydrogen-rich atmospheres. Secondary eclipse observations of TRAPPIST-1 b with
JWST/MIRI are consistent with little to no atmosphere given the lack of heat
redistribution. Here we present the first transmission spectra of TRAPPIST-1 b
obtained with JWST/NIRISS over two visits. The two transmission spectra show
moderate to strong evidence of contamination from unocculted stellar
heterogeneities, which dominates the signal in both visits. The transmission
spectrum of the first visit is consistent with unocculted starspots and the
second visit exhibits signatures of unocculted faculae. Fitting the stellar
contamination and planetary atmosphere either sequentially or simultaneously,
we confirm the absence of cloud-free hydrogen-rich atmospheres, but cannot
assess the presence of secondary atmospheres. We find that the uncertainties
associated with the lack of stellar model fidelity are one order of magnitude
above the observation precision of 89 ppm (combining the two visits). Without
affecting the conclusion regarding the atmosphere of TRAPPIST-1 b, this
highlights an important caveat for future explorations, which calls for
additional observations to characterize stellar heterogeneities empirically
and/or theoretical works to improve model fidelity for such cool stars. This
need is all the more justified as stellar contamination can affect the search
for atmospheres around the outer, cooler TRAPPIST-1 planets for which
transmission spectroscopy is currently the most efficient technique.Comment: 26 pages, 11 figures, accepted for publication in The Astrophysical
Journal Letter
WASP-107b’s Density Is Even Lower: A Case Study for the Physics of Planetary Gas Envelope Accretion and Orbital Migration
With a mass in the Neptune regime and a radius of Jupiter, WASP-107b presents a challenge to planet formation theories. Meanwhile, the planet's low surface gravity and the star's brightness also make it one of the most favorable targets for atmospheric characterization. Here, we present the results of an extensive 4 yr Keck/HIRES radial-velocity (RV) follow-up program of the WASP-107 system and provide a detailed study of the physics governing the accretion of the gas envelope of WASP-107b. We reveal that WASP-107b's mass is only 1.8 Neptune masses (M_b = 30.5 ± 1.7 M_⊕). The resulting extraordinarily low density suggests that WASP-107b has a H/He envelope mass fraction of >85% unless it is substantially inflated. The corresponding core mass of <4.6 M_⊕ at 3σ is significantly lower than what is traditionally assumed to be necessary to trigger massive gas envelope accretion. We demonstrate that this large gas-to-core mass ratio most plausibly results from the onset of accretion at gsim1 au onto a low-opacity, dust-free atmosphere and subsequent migration to the present-day a_b = 0.0566 ± 0.0017 au. Beyond WASP-107b, we also detect a second, more massive planet (M_c sin i = 0.36 ± 0.04MJ ) on a wide eccentric orbit (e _c = 0.28 ± 0.07) that may have influenced the orbital migration and spin–orbit misalignment of WASP-107b. Overall, our new RV observations and envelope accretion modeling provide crucial insights into the intriguing nature of WASP-107b and the system's formation history. Looking ahead, WASP-107b will be a keystone planet to understand the physics of gas envelope accretion