15 research outputs found
The Eccentricity Distribution of Short-Period Planet Candidates Detected by Kepler in Occultation
We characterize the eccentricity distribution of a sample of ~50 short-period
planet candidates using transit and occultation measurements from NASA's Kepler
Mission. First, we evaluate the sensitivity of our hierarchical Bayesian
modeling and test its robustness to model misspecification using simulated
data. When analyzing actual data assuming a Rayleigh distribution for
eccentricity, we find that the posterior mode for the dispersion parameter is
. We find that a two-component Gaussian
mixture model for and provides a better model
than either a Rayleigh or Beta distribution. Based on our favored model, we
find that of planet candidates in our sample come from a population
with an eccentricity distribution characterized by a small dispersion
(), and come from a population with a larger dispersion
(). Finally, we investigate how the eccentricity distribution
correlates with selected planet and host star parameters. We find evidence that
suggests systems around higher metallicity stars and planet candidates with
smaller radii come from a more complex eccentricity distribution.Comment: Accepted for publication in Ap
Sensitivity Analyses of Exoplanet Occurrence Rates from Kepler and Gaia
We infer the number of planets per star as a function of orbital period and planet size using Kepler archival data products with updated stellar properties from the Gaia Data Release 2. Using hierarchical Bayesian modeling and Hamiltonian Monte Carlo, we incorporate planet radius uncertainties into an inhomogeneous Poisson point process model. We demonstrate that this model captures the general features of the outcome of the planet formation and evolution around GK stars and provides an infrastructure to use the Kepler results to constrain analytic planet distribution models. We report an increased mean and variance in the marginal posterior distributions for the number of planets per GK star when including planet radius measurement uncertainties. We estimate the number of planets per GK star between 0.75 and 2.5 R⊕ and with orbital periods of 50–300 days to have a 68% credible interval of 0.49–0.77 and a posterior mean of 0.63. This posterior has a smaller mean and a larger variance than the occurrence rate calculated in this work and in Burke et al. for the same parameter space using the Q1−Q16 (previous Kepler planet candidate and stellar catalog). We attribute the smaller mean to many of the instrumental false positives at longer orbital periods being removed from the DR25 catalog. We find that the accuracy and precision of our hierarchical Bayesian model posterior distributions are less sensitive to the total number of planets in the sample, and more so for the characteristics of the catalog completeness and reliability and the span of the planet parameter space
Transmission Spectra of Transiting Planet Atmospheres: Model Validation and Simulations of the Hot Neptune GJ 436b for JWST
We explore the transmission spectrum of the Neptune-class exoplanet GJ 436b,
including the possibility that its atmospheric opacity is dominated by a
variety of non- equilibrium chemical products. We also validate our
transmission code by demonstrating close agreement with analytic models that
use only Rayleigh scattering or water vapor opacity. We find broad disagreement
with radius variations predicted by another published model. For GJ 436b, the
relative coolness of the planet's atmosphere, along with its implied high
metallicity, may make it dissimilar in character compared to "hot Jupiters."
Some recent observational and modeling efforts suggest low relative abundances
of H2O and CH4 present in GJ 436b's atmosphere, compared to calculations from
equilibrium chemistry. We include these characteristics in our models and
examine the effects of absorption from methane-derived higher order
hydrocarbons. Significant absorption from HCN and C2H2 are found throughout the
infrared, while C2H4 and C2H6 are less easily seen. We perform detailed
simulations of JWST observations, including all likely noise sources, and find
that we will be able to constrain chemical abundance regimes from this planet's
transmission spectrum. For instance, the width of the features at 1.5, 3.3, and
7 microns indicates the amount of HCN versus C2H2 present. The NIRSpec prism
mode will be useful due to its large spectral range and the relatively large
number of photo-electrons recorded per spectral resolution element. However,
extremely bright host stars like GJ 436 may be better observed with a higher
spectroscopic resolution mode in order to avoid detector saturation. We find
that observations with the MIRI low resolution spectrograph should also have
high signal-to-noise in the 5 - 10 micron range due to the brightness of the
star and the relatively low spectral resolution (R ~ 100) of this mode.Comment: 33 pages, 12 figures, Accepted to Ap
Probing potassium in the atmosphere of HD 80606b with tunable filter transit spectrophotometry from the Gran Telescopio Canarias
We report observations of HD 80606 using the 10.4-m Gran Telescopio Canarias
(GTC) and the OSIRIS tunable filter imager. We acquired very-high-precision,
narrow-band photometry in four bandpasses around the K I absorption feature
during the January 2010 transit of HD 80606b and during out-of-transit
observations conducted in January and April of 2010. We obtained differential
photometric precisions of \sim 2.08e-4 for the in-transit flux ratio measured
at 769.91-nm, which probes the K I line core. We find no significant difference
in the in-transit flux ratio between observations at 768.76 and 769.91 nm. Yet,
we find a difference of \sim 8.09 \pm 2.88e-4 between these observations and
observations at a longer wavelength that probes the K I wing (777.36 nm). While
the presence of red noise in the transit data has a non-negligible effect on
the uncertainties in the flux ratio, the 777.36-769.91 nm colour during transit
shows no effects from red noise and also indicates a significant colour change,
with a mean value of \sim 8.99\pm0.62e-4. This large change in the colour is
equivalent to a \sim 4.2% change in the apparent planetary radius with
wavelength, which is much larger than the atmospheric scale height. This
implies the observations probed the atmosphere at very low pressures as well as
a dramatic change in the pressure at which the slant optical depth reaches
unity between \sim770 and 777 nm. We hypothesize that the excess absorption may
be due to K I in a high-speed wind being driven from the exoplanet's exosphere.
We discuss the viability of this and alternative interpretations, including
stellar limb darkening, starspots, and effects from Earth's atmosphere. We
strongly encourage follow-up observations of HD 80606b to confirm the signal
measured here. Finally, we discuss the future prospects for exoplanet
characterization using tunable filter spectrophotometry.Comment: Accepted to MNRAS; revised version includes some major updates; now
21 pages, with 14 figures and 9 table
Planetary Candidates Observed by Kepler. VIII. A Fully Automated Catalog With Measured Completeness and Reliability Based on Data Release 25
We present the Kepler Object of Interest (KOI) catalog of transiting
exoplanets based on searching four years of Kepler time series photometry (Data
Release 25, Q1-Q17). The catalog contains 8054 KOIs of which 4034 are planet
candidates with periods between 0.25 and 632 days. Of these candidates, 219 are
new and include two in multi-planet systems (KOI-82.06 and KOI-2926.05), and
ten high-reliability, terrestrial-size, habitable zone candidates. This catalog
was created using a tool called the Robovetter which automatically vets the
DR25 Threshold Crossing Events (TCEs, Twicken et al. 2016). The Robovetter also
vetted simulated data sets and measured how well it was able to separate TCEs
caused by noise from those caused by low signal-to-noise transits. We discusses
the Robovetter and the metrics it uses to sort TCEs. For orbital periods less
than 100 days the Robovetter completeness (the fraction of simulated transits
that are determined to be planet candidates) across all observed stars is
greater than 85%. For the same period range, the catalog reliability (the
fraction of candidates that are not due to instrumental or stellar noise) is
greater than 98%. However, for low signal-to-noise candidates between 200 and
500 days around FGK dwarf stars, the Robovetter is 76.7% complete and the
catalog is 50.5% reliable. The KOI catalog, the transit fits and all of the
simulated data used to characterize this catalog are available at the NASA
Exoplanet Archive.Comment: 61 pages, 23 Figures, 9 Tables, Accepted to The Astrophysical Journal
Supplement Serie
The Occurrence of Rocky Habitable-zone Planets around Solar-like Stars from Kepler Data
We present the occurrence rates for rocky planets in the habitable zones (HZs) of main-sequence dwarf stars based on the Kepler DR25 planet candidate catalog and Gaia-based stellar properties. We provide the first analysis in terms of star-dependent instellation flux, which allows us to track HZ planets. We define η⊕ as the HZ occurrence of planets with radii between 0.5 and 1.5 R⊕ orbiting stars with effective temperatures between 4800 and 6300 K. We find that η⊕ for the conservative HZ is between 0.37^(+0.48)_(−0.21) (errors reflect 68% credible intervals) and 0.60^(+0.90)_(−0.36) planets per star, while the optimistic HZ occurrence is between 0.58^(+0.73)_(−0.33) and 0.88^(+1.28)_(−0.51) planets per star. These bounds reflect two extreme assumptions about the extrapolation of completeness beyond orbital periods where DR25 completeness data are available. The large uncertainties are due to the small number of detected small HZ planets. We find similar occurrence rates between using Poisson likelihood Bayesian analysis and using Approximate Bayesian Computation. Our results are corrected for catalog completeness and reliability. Both completeness and the planet occurrence rate are dependent on stellar effective temperature. We also present occurrence rates for various stellar populations and planet size ranges. We estimate with 95% confidence that, on average, the nearest HZ planet around G and K dwarfs is ~6 pc away and there are ~4 HZ rocky planets around G and K dwarfs within 10 pc of the Sun
The Occurrence of Rocky Habitable Zone Planets Around Solar-Like Stars from Kepler Data
We present occurrence rates for rocky planets in the habitable zones (HZ) of
main-sequence dwarf stars based on the Kepler DR25 planet candidate catalog and
Gaia-based stellar properties. We provide the first analysis in terms of
star-dependent instellation flux, which allows us to track HZ planets. We
define as the HZ occurrence of planets with radius between 0.5
and 1.5 orbiting stars with effective temperatures between 4800 K
and 6300 K. We find that for the conservative HZ is between
(errors reflect 68\% credible intervals) and
planets per star, while the optimistic HZ occurrence is
between and planets per star.
These bounds reflect two extreme assumptions about the extrapolation of
completeness beyond orbital periods where DR25 completeness data are available.
The large uncertainties are due to the small number of detected small HZ
planets. We find similar occurrence rates using both a Poisson likelihood
Bayesian analysis and Approximate Bayesian Computation. Our results are
corrected for catalog completeness and reliability. Both completeness and the
planet occurrence rate are dependent on stellar effective temperature. We also
present occurrence rates for various stellar populations and planet size
ranges. We estimate with confidence that, on average, the nearest HZ
planet around G and K dwarfs is about 6 pc away, and there are about 4 HZ rocky
planets around G and K dwarfs within 10 pc of the Sun.Comment: To appear in The Astronomical Journa
DOPPLER SIGNATURES OF THE ATMOSPHERIC CIRCULATION ON HOT JUPITERS
The meteorology of hot Jupiters has been characterized primarily with thermal measurements, but recent observations suggest the possibility of directly detecting the winds by observing the Doppler shift of spectral lines seen during transit. Motivated by these observations, we show how Doppler measurements can place powerful constraints on the meteorology. We show that the atmospheric circulation—and Doppler signature—of hot Jupiters splits into two regimes. Under weak stellar insolation, the day-night thermal forcing generates fast zonal jet streams from the interaction of atmospheric waves with the mean flow. In this regime, air along the terminator (as seen during transit) flows toward Earth in some regions and away from Earth in others, leading to a Doppler signature exhibiting superposed blueshifted and redshifted components. Under intense stellar insolation, however, the strong thermal forcing damps these planetary-scale waves, inhibiting their ability to generate jets. Strong frictional drag likewise damps these waves and inhibits jet formation. As a result, this second regime exhibits a circulation dominated by high-altitude, day-to-night airflow, leading to a predominantly blueshifted Doppler signature during transit. We present state-of-the-art circulation models including non-gray radiative transfer to quantify this regime shift and the resulting Doppler signatures; these models suggest that cool planets like GJ 436b lie in the first regime, HD 189733b is transitional, while planets hotter than HD 209458b lie in the second regime. Moreover, we show how the amplitude of the Doppler shifts constrains the strength of frictional drag in the upper atmospheres of hot Jupiters. If due to winds, the ~2 km s[superscript –1] blueshift inferred on HD 209458b may require drag time constants as short as 10[superscript 4]-10[superscript 6] s, possibly the result of Lorentz-force braking on this planet's hot dayside